Glossary

A

Abaxial muscles@@Muscles derived from the lateral portions of the myotome.

Acetylation@@See Histone acetylation.

Achondroplasia@@Condition wherein chondrocytes stop proliferating too early, resulting in short limbs (achondroplasic dwarfism). Often caused by mutations that activate the FgfR3 gene prematurely.

Acron@@Anterior region of the body of an arthropod such as an insect. Includes the brain.

Acrosomal process@@A fingerlike process in the sperm head extended by the polymerization of actin filaments during the early stages of fertilization in sea urchins and many other species. It contains surface molecules for species-specific recognition between sperm and egg.

Acrosome (acrosomal vesicle)@@Caplike organelle that, together with the sperm nucleus, forms the sperm head. Contains proteolytic enzymes that can digest the extracellular coats surrounding the egg, allowing the sperm to gain access to the egg cell membrane, to which it fuses.

Acrosome reaction@@The Ca²⁺-dependent fusion of the sperm acrosome with the sperm cell membrane. Results in exocytosis and the release of proteolytic enzymes that allow the sperm to penetrate the egg’s extracellular matrix and fertilize the egg.

Actinomyosin contractions@@Contractile forces within a cell caused by myosin attaching to and moving along filamentous actin. Examples include muscle cell contractions and the apical constriction of neural plate cells at the hinge points.

Activins@@Members of the TGF-β superfamily of proteins; with Nodal, important in specifying the different regions of the mesoderm and for distinguishing the left and right body axes of vertebrates.

Adenohypophyseal placode@@An outpocketing of the ectoderm in the roof of the oral region that develops into a structure known as Rathke’s pouch before differentiating into the anterior lobe of the vertebrate pituitary gland.

Adepithelial cells@@Cells that migrate into the imaginal discs early in the development of the holometabolous insect larva; these cells give rise to muscles and nerves in the pupal stage.

Adult stem cells@@Multipotent cells in an adult organism capable of regenerating numerous cell types of the adult. Example: hematopoietic stem cells, which generate all the different types of  blood cells.

Adult stem cell niche@@Niche that houses adult stem cells and regulates stem cell self-renewal, survival, and differentiation of the progeny that leave the niche.

Allantois@@In amniote species, extraembryonic membrane that stores urinary wastes and helps mediate gas exchange. It is derived from splanchnopleure at the caudal end of the primitive streak. In mammals, the size of the allantois depends on how well nitrogenous wastes can be removed by the chorionic placenta. In reptiles and birds, the allantois becomes a large sac, as there is no other way to keep the toxic by-products of metabolism away from the developing embryo.

Alternation of generations@@In plants, a life cycle in which a haploid multicellular gamete-producing stage (the gametophyte) alternates with a diploid multicellular spore-producing stage (the sporophyte).

Alternative pre-mRNA splicing@@A means of producing multiple different proteins from a single gene by splicing together different sets of exons to generate different types of mRNAs.

Ametabolous@@A pattern of insect development in which there is no larval stage and the insect undergoes direct development to a small adult form following a transitory pronymph stage.

Amnion@@“Water sac.” A membrane enclosing and protecting the embryo and its surrounding amniotic fluid. Derived from the two layers of the somatopleure: ectoderm that supplies epithelial cells, and mesoderm that generates the connective tissue.

Amniote egg@@Egg that develops extraembryonic membranes (the amnion, chorion, allantois, and yolk sac) that provide nourishment and other environmental needs to the developing embryo. Characteristic of the amniote vertebrates: the reptiles and birds, in which the egg typically develops in a shell outside the mother’s body; and the mammals, where the egg has become modified to develop inside the mother.

Amniotes@@The groups of vertebrates in which the embryo develops an amnion (water sac) that surrounds the body of the embryo. Includes reptiles, birds, and mammals. Compare with Anamniote.

Amniotic fluid@@A secretion that serves as a “shock absorber” for the developing embryo while preventing it from drying out.

Ampulla@@Latin, “flask.” The segment of the mammalian oviduct distal to the uterus and near the ovary, where fertilization takes place.

Anagen@@The growth phase of a hair follicle, during which the hair grows in length.

Analogous@@Structures and/or their respective components whose similarity arises from their performing a similar function rather than their arising from a common ancestor (e.g., the wing of a butterfly versus the wing of a bird). Compare with Homologous.

Anamniotes@@The fish and amphibians; i.e., the vertebrate groups that do not form an amnion during embryonic development. Compare with Amniotes.

Anchor cell@@The cell connecting the overlying gonad to the vulval precursor cells in C. elegans. If the anchor cell is destroyed, the VPCs will not form a vulva, but instead become part of the hypodermis.

Androgen insensitivity syndrome@@Intersex condition in which an XY individual has a mutation in the gene encoding the androgen receptor protein that binds testosterone. This results in a female external phenotype, lack of a uterus and oviducts and presence of abdominal testes.

Androgen@@Masculinizing substance, usually a steroid hormone such as testosterone.

Anencephaly@@A congenital defect (almost always lethal) resulting from failure to close the anterior neuropore. The forebrain remains in contact with the amniotic fluid and subsequently degenerates, so the vault of the skull fails to form.

Aneuploidy@@Condition in which one or more chromosome(s) is either lacking or present in multiple copies.

Angiogenesis@@Process by which the primary network of blood vessels created by vasculogenesis is remodeled and pruned into a distinct capillary bed, arteries, and veins.

Angiopoietins@@Paracrine factors that mediate the interaction between endothelial cells and pericytes.

Animal cap@@In amphibians, the roof of the blastocoel (in the animal hemisphere).

Animal pole@@The pole of the egg or embryo where the concentration of yolk is relatively low; opposite end of the egg from the vegetal pole.

Anoikis@@Rapid apoptosis that occurs when epithelial cells lose their attachment to the extracellular matrix.

Anorectal junction@@The meeting of endoderm and ectoderm at the anus in vertebrate embryos.

Antennapedia complex@@A region of Drosophila chromosome 3 containing the homeotic genes labial (lab), Antennapedia (Antp), sex combs reduced (scr), deformed (dfd), and proboscipedia (pb), which specify head and thoracic segment identities.

Anterior intestinal portal (AIP)@@The posterior opening of the developing foregut region of the primitive gut tube; it opens into the future midgut region which is contiguous with the yolk sac at this stage.

Anterior visceral endoderm (AVE)@@Mammalian equivalent to the chick hypoblast and similar to the head portion of the amphibian organizer, it creates an anterior region by secreting antagonists of Nodal.

Anterior-posterior axis (AP)@@The primary body axis. Also referred to as anteroposterior axis, it defines the head versus the tail (or mouth versus anus). Sometimes used in reference to the limb to refer to the thumb (anterior)-pinkie (posterior) axis.

Anther@@A pollen-producing organ on the stamen (male portion) of a flower.

Anti-Müllerian hormone (AMH)@@TGF-β family paracrine factor secreted by the embryonic testes that induces apoptosis of the epithelium and destruction of the basal lamina of the Müllerian duct, preventing formation of the uterus and oviducts. Also known as anti-Müllerian factor, or AMF. Sometimes called Müllerian-inhibiting factor (MIF).

Anticlinal divisions@@In plants, cell divisions in which the new cell walls are laid down perpendicular to the surface of the plant. Compare with Periclinal divisions.

Anurans@@Frogs and toads. Compare with Urodeles.

Aorta-gonad-mesonephros region (AGM)@@A mesenchymal area in the lateral plate splanchnopleure near the ventral aorta that produces the hematopoietic stem cells.

Apical constriction@@Constriction of the apical end of a cell, caused by localized contraction of actinomyosin complexes at the apical border.

Apical ectodermal ridge (AER)@@A ridge along the distal margin of the limb bud that will become a major signaling center for the developing limb. Its roles include (1) maintaining the mesenchyme beneath it in a plastic, proliferating state that enables the linear (proximal-distal) growth of the limb; (2) maintaining the expression of those molecules that generate the anterior-posterior axis; and (3) interacting with the proteins specifying the anterior-posterior and dorsal-ventral axes so that each cell is given instructions on how to differentiate.

Apical epidermal cap (AEC)@@Forms in the wound epidermis of an amputated salamander limb and acts similarly to the apical ectodermal ridge during normal limb development.

Apoptosis@@Programmed cell death. Apoptosis is an active process that prunes unneeded structures (e.g., frog tails, male mammary tissue), controls the number of cells in particular tissues, and sculpts complex organs (e.g., digits, heart). See also Anoikis.

Archenteron@@The primitive gut of an embryo. In the sea urchin, it is formed by invagination of the vegetal plate into the blastocoel.

Area opaca@@The peripheral ring of avian blastoderm cells that have not shed their deep cells.

Area pellucida@@A 1-cell-thick area in the center of the avian blastoderm (following shedding of most of the deep cells) that forms most of the actual embryo.

Assisted reproductive technology (ART)@@Fertility treatments in which eggs or embryos are removed from a woman's body. These include in vitro fertilization, ICSI, and other technologies.

Autocrine interaction@@The same cells that secrete paracrine factors also respond to them.

Autonomous specification@@A mode of cell commitment in which the blastomere inherits a determinant, usually a set of transcription factors from the egg cytoplasm, and these transcription factors regulate gene expression to direct the cell into a particular path of development.

Autopod@@The distal bones of a vertebrate limb: carpals and metacarpals (forelimb), tarsals and metatarsals (hindlimb), and phalanges (“digits”; the fingers and toes).

Axis specification@@The process that establishes regional patterns of differentiation, thus defining different cell types along each of the axes of a developing embryo.

Axoneme@@The portion of a cilium or flagellum consisting of two central microtubules surrounded by a row of 9 doublet microtubules. The motor protein dynein attached to the doublet microtubules provides the force for ciliary and flagellar function.

B

B cells@@See Neural stem cells.

Bacteroid@@A special state of a bacterium, which has become differentiated as part of a symbiotic complex with legume roots.

Basal disc@@The “foot” of a hydra; enables the animal to stick to rocks or the undersides of pond plants.

Basal lamina@@Specialized, closely knit sheets of extracellular matrix that underlie epithelia, composed largely of laminin and type IV collagen. Epithelial cells adhere to the basal lamina in part via binding between integrins and laminin. Sometimes called the basement membrane.

Basal layer@@The inner layer of both the embryonic and adult epidermis. This layer contains epidermal stem cells attached to a basement membrane. Also called stratum germinativum.

Basal transcription factors@@Transcription factors that specifically bind to the CpG-rich sites, forming a “saddle” that can recruit RNA polymerase II and position it for transcription.

Basic fibroblast growth factor (Fgf2)@@One of three growth factors required for the generation of hemangioblasts from the splanchnic mesoderm. See also Angiopoietins.

Bicoid@@Anterior morphogen critical for establishing anterior-posterior polarity in the Drosophila embryo. Functions as a transcription factor to activate anterior-specific gap genes and as a translational repressor to suppress posterior-specific gap genes.

Bilaminar germ disc@@An amniote embryo prior to gastrulation; consists of epiblast and hypoblast layers.

Bilateral holoblastic cleavage@@Cleavage pattern, found primarily in tunicates, in which the first cleavage plane establishes the right-left axis of symmetry in the embryo and each successive division orients itself to this plane of symmetry. Thus the half-embryo formed on one side of the first cleavage plane is the mirror image of the other side.

Bilaterians (triploblasts)@@Those animals characterized by bilaterian body symmetry and the presence of three germ layers (endoderm, ectoderm, and mesoderm). Includes all animal groups except the sponges, cnidarians, ctenophores, and placozoans.

Bindin@@A 30,500-Da protein on the acrosomal process of sea urchin sperm that mediates the species-specific recognition between the sperm and the egg vitelline envelope during fertilization.

Bioelectric signaling@@Communication between cells based on changes in the transmembrane voltage potential.

Bipotential gonad@@Common precursor tissue derived from the genital ridge in mammals, from which the male and female gonads diverge.

Bithorax complex@@The region of Drosophila chromosome 3 containing the homeotic gene Ultrabithorax (Ubx), which is required for the identity of the third thoracic segment, and the abdominal A (abdA) and Abdominal B (AbdB) genes, which are responsible for the segmental identities of the abdominal segments.

Blastema@@A group of undifferentiated progenitor cells that form in some organisms at the site of an amputation; the blastema is able to grow and differentiate to replace the amputated tissue.

Blastocoel@@A fluid-filled cavity of the blastula stage of an embryo.

Blastocyst@@A mammalian blastula. The blastocoel is expanded and the inner cell mass is positioned on one side of the ring of trophoblast cells.

Blastoderm@@The layer of cells formed during cleavage at the animal pole in telolecithal eggs, as in fish, reptiles, and birds. Because the yolk concentrated in the vegetal region of the egg impedes cleavage, only the small amount of yolk-free cytoplasm at the animal pole is able to divide in these eggs. During development, the blastoderm spreads around the yolk as it forms the embryo.

Blastodisc@@Small region at the animal pole of the telolecithal eggs of fish, birds, and reptiles, containing the yolk-free cytoplasm where cleavage can occur and that gives rise to the embryo. Following cleavage, the blastodisc becomes the blastoderm.

Blastomere@@A cleavage-stage cell resulting from mitosis.

Blastopore@@The invagination point where gastrulation begins. In deuterostomes, this marks the site of the anus. In protostomes, this marks the site of the mouth.  

Blastula@@Early-stage embryo consisting of a sphere of cells surrounding an inner fluid-filled cavity, the blastocoel.

Blood islands@@Aggregations of hemangioblasts in the splanchnic mesoderm. It is generally thought that the inner cells of these blood islands become blood progenitor cells, while the outer cells become angioblasts.

BMP family@@See Bone morphogenetic proteins.

Bone marrow-derived stem cells (BMDCs)@@See Mesenchymal stem cells.

Bone morphogenetic proteins (BMPs)@@Members of the TGF-β superfamily of proteins. Originally identified by their ability to induce bone formation, BMPs are extremely multifunctional, having been found to regulate cell division, apoptosis, cell migration, and differentiation.

Bulge@@A region of the hair follicle that is a niche for adult stem cells.

C

<note prime symbol after 5 in two places>

5¢ cap@@A uniquely altered nucleotide on the 5¢ end of a precursor mRNA that stabilizes its attachment and is typically necessary for initiating transcription of a gene.

Cadherins@@Calcium-dependent adhesion molecules. Transmembrane proteins that interact with other cadherins on adjacent cells and are critical for establishing and maintaining intercellular connections, spatial segregation of cell types, and the organization of animal form.

Cajal-Retzius cells@@Reelin-secreting cells in the neocortex just under the pial surface. Reelin directs the migration of newly born neurons toward the pial surface.

Calcium wave@@A progressive amplitude of calcium ions traversing a newly fertilized egg, starting at the point of sperm entry, activating calcium-dependent processes developmental processes.

Callus@@In plants, a growing mass of unorganized and undifferentiated cells that cover a wound; these cells can be induced to form a plant meristem and develop into shoots and/or roots.

Cambrian explosion@@The rapid diversification of life in the Cambrian period (approximately 541 million years ago) during which most major animal groups appeared, including those with species living today.

Canalization model@@In plants, a model for explaining the polarized transport of auxin along the apical to basal axis during vascular development and regeneration. In this model, the vasculature apical to the developing tissue or healing wound serves as the auxin source, and the row of increasingly basal developing or regenerating vascular cells serve as the auxin sink.

Cap sequence@@See Transcription initiation site.

Capacitation@@The set of physiological changes by which mammalian sperm become capable of fertilizing an egg.

Cardiac neural crest@@Subregion of the cranial neural crest that extends from the otic (ear) placodes to the third somites. Cardiac neural crest cells develop into melanocytes, neurons, cartilage, and connective tissue. Cardiac neural crest also contributes to the muscular-connective tissue wall of the large arteries (the “outflow tracts”) of the heart, as well as contributing to the septum that separates pulmonary circulation from the aorta.

Cardiogenic mesoderm@@See Heart fields.

Carpels@@The female reproductive organs of an angiosperm flower, containing stigma, an ovary, and often a style. Sometimes called a pistil.

Catagen@@Regression phase of the hair follicle regeneration cycle.

Catenins@@A complex of proteins that anchor cadherins inside the cell. The cadherin-catenin complex forms the classic adherens junctions that help hold epithelial cells together and, by binding to the actin (microfilament) cytoskeleton of the cell, integrate the epithelial cells into a mechanical unit. One of them, β-catenin, can also be a transcription factor.

β-Catenin@@A protein that can act as an anchor for cadherins or as a transcription factor (induced by the Wnt pathway). It is important in the specification of germ layers throughout the animal phyla.

CatSper channels@@Cation (usually, Ca²⁺) channels that seem to be specific to sperm. They are critical for sperm cell locomotion and guidance.

Caudal intestinal portal (CIP)@@The anterior opening of the developing hindgut region of the primitive gut tube; it opens into the future midgut region, which is contiguous with the yolk sac at this stage.

Cavitation@@In mammalian embryos, a process whereby the trophoblast cells secrete fluid into the morula to create a blastocoel. The membranes of trophoblast cells pump sodium ions (Na⁺) into the central cavity, drawing in water osmotically and thus creating and enlarging the blastocoel.

Cell adhesion molecules@@Adhesion molecules that hold cells together. The major group of these is the cadherins. See also Cadherins.

Cellular blastoderm@@Stage of Drosophila development in which all the cells are arranged in a single-layered jacket around the yolky core of the egg.

Cellularization@@The process of creating individual cells out of a multinucleated cell (a syncytium) by separating the nuclei by cell membranes. Example: During early development in the Drosophila embryo, the syncytial blastoderm is turned into a cellular blastoderm when cell membranes grow inward between the peripheral nuclei, separating off individual cells from the inner yolky cytoplasm.

Central nervous system (CNS)@@The brain and spinal cord of vertebrates.

Centrolecithal@@Type of egg, such as those of insects, that has yolk in the center and undergoes superficial cleavage.

Centrosome-attracting body (CAB)@@Cellular structure that, in some invertebrate blastomeres, positions the centrosomes asymmetrically and recruits particular mRNAs so that the resulting daughter cells are different sizes and have different properties.

Cephalic furrow@@A transverse furrow formed during gastrulation in Drosophila that separates the future head region (procephalon) from the germ band, which will form the thorax and abdomen.

Cerebellum@@A region of the hindbrain that is particularly important in motor control; above and behind the medulla.

Cerebral cortex@@The cerebral cortex is derived from the embryonic forebrain and constitutes the “front” (and in humans the largest) region of the brain. Also called the cerebrum.

Cerebrum@@The cerebrum is derived from the embryonic forebrain and constitutes the “front” (and in humans the largest) region of the brain. It is commonly referred to as the cerebral cortex.

Cervix@@An inner muscular entrance to the uterus, secretes mucus that regulates sperm entry into the uterus. During pregnancy, it functions as a muscular band that holds the fetus in the uterus until delivery.

Chemoaffinity hypothesis@@Hypothesis put forth by Sperry in 1965 suggesting that nerve cells in the brain acquire individual chemical tags that distinguish them from one another and that these guide the assembly and organization of the neural circuits in the brain.

Chimeric embryo@@Embryo made from tissues of more than one genetic source.

ChIP-Seq@@Chromatin immunoprecipitation sequencing. A lab protocol used to identify the precise DNA sequences bound by particular transcription factors or nucleosomes containing specific modified histones.

Choanocyte@@Found in sponges, it is a type of cell that contains a central flagellum surrounded by a collar of microvilli; choanocytes power the unidirectional flow of water through the sponge and also function as multipotent stem cells during sponge regeneration.

Choanoflagellates@@A group of unicellular and colonial free-living eukaryotes. The cells have the same basic structure as choanocytes of sponges, with a central flagellum surrounded by a collar of microvilli. Ancient choanoflagellates are considered to have been the common ancestor of all metazoans.

Chordamesoderm@@Axial mesoderm in a chordate embryo that produces the notochord.  

Chorion@@An extraembryonic membrane essential for gas exchange in amniote embryos. It is generated from the extraembryonic somatopleure. The chorion adheres to the shell in birds and reptiles, allowing the exchange of gases between the egg and the environment. It forms the embryonic/fetal portion of the placenta in mammals.

Choroid plexus@@A network of blood vessels located in each of the brain ventricles and functions to produce the cerebrospinal fluid.

Chromatin@@The complex of DNA and protein in which eukaryotic genes are contained.

<Note Cis s/b italic>

Cis-regulatory elements (CREs)@@Regulatory elements (promoters and enhancers) that reside on the same stretch of DNA as the gene they regulate.

Cleavage@@A series of rapid mitotic cell divisions following fertilization in many early embryos; cleavage divides the embryo without increasing its mass.

Cleavage furrow@@A groove formed in the cell membrane in a dividing cell due to tightening of the microfilamentous ring.

Cleft of Brachet@@In amphibian gastrulation, it is a region of extracellular matrix that separates the ectoderm from the involuting mesendoderm.

Clitoris@@The sensitive erectile tissue at the anterior of the vulva in the female genitalia.

Cloaca@@Latin, “sewer.” An endodermally lined chamber at the caudal end of the embryo that will become the receptacle for waste from the intestine and the kidneys and products from the gonads. Amphibians, reptiles, and birds retain this organ and use it to void gametes and both liquid and solid wastes. In mammals, the cloaca becomes divided by a septum into the urogenital sinus and the rectum.

Cloacal membrane@@At the caudal end of the hindgut formed by closely apposed endoderm and ectoderm; future site of the anus.

Clock-wavefront model@@A current theory to explain synchronized somite formation. Posits that two converging systems to regulate where the somite boundary can form (a morphogen-derived wavefront) and (2) when this boundary formation should occur (a “clock” mediated by the interactions of Notch and Delta proteins.).

Clonogenic neoblasts (cNeoblasts)@@Pluripotent stem cells in flatworms that migrate to a wound site and regenerate the tissue; form the regeneration blastema of flatworms.

Cochlea@@In amniotes, the portion of the inner ear involved in hearing. The mammalian cochlea is derived from the otic placode. It is spiral in shape and contains the organ of Corti, the sensory structure for hearing.

Cochleovestibular ganglion@@The ganglion adjacent to the otic vesicle. It forms the major neural connection between the brain and the inner ear structures.

Coelom@@Space between the somatic mesoderm and splanchnic mesoderm that becomes the body cavity. In mammals, the coelom becomes subdivided into the pleural, pericardial, and peritoneal cavities, enveloping the thorax, heart, and abdomen, respectively.

Collective cell migration@@The movement of a sheet of cells, wherein the cells at the leading edge of the sheet in part provide the locomotory force, using pseudopodia to pull the rest of the cells forward. Cells behind the leading edge, being surrounded by other cells, are prevented from forming locomotory pseudopodia due to contact inhibition of cell movement. However, during chemoattractive responses the cells at the rear of the cluster can form actomyosin arrays that squeeze these cells forward.

Collective migration@@Migration of self-propelled cells exerting directionally coordinated forces upon one another, as opposed to individually migrating cells or the movement of a group of cells caused by tissue pushing due to proliferation or intercalation.

Collinearity@@A spatial correspondence between the 3´ to 5´ order of Hox genes on a chromosome and the temporal correspondence of activation of those same genes along a given embryonic axis.

Commitment@@Describes a state in which a cell’s developmental fate has become restricted even though it is not yet displaying overt changes in cellular biochemistry and function.

Compaction@@A unique feature of mammalian cleavage, mediated by the cell adhesion molecule E-cadherin. The cells in the early (around eight-cell) embryo change their adhesive properties and become tightly attached to each other.

Compensatory regeneration@@Form of regeneration in which the differentiated cells divide but maintain their differentiated functions (e.g., mammalian liver).

Competence@@The ability of cells or tissues to respond to a specific inductive signal.

Conditional specification@@The ability of cells to achieve their respective fates by interactions with other cells. What a cell becomes is in large measure specified by paracrine factors secreted by its neighbors.

Conceptus@@The product of fertilization, from the zygote through birth.

Congenital adrenal hyperplasia@@A condition in which the presence of excess testosterone results in  female pseudohermaphroditism.

Congenital anomalies@@Any defects that an individual is born with. Congenital anomalies can be hereditary. or they can have an environmental cause (e.g., exposure to teratogenic plants, drugs, chemicals, radiation, etc.). They can also be idiopathic (i.e., cause is unknown).

Conjoined twins@@Monozygotic twins that share some part of their bodies; they may even share a vital organ, such as a heart or liver.

Consensus sequence@@When referring to an intron, these are located at the 5′ and 3′ ends of the introns that signal the “splice sites” of the intron.

Contact inhibition@@The mechanism whereby cells are prohibited from forming locomotory pseudopodia along contact surfaces with other cells. These interactions with the cell membranes of other cells prevent “backward” migration over other cells and result in “forward” migration of the leading edge of cells.

Context dependency@@The meaning or role of an individual component of a system (such as a transcription factor) is dependent on its context. For example, in the formation of tetrapod limb joints, the same BMPs can induce either cell death or cell differentiation, depending on the stage of the responding cell.

Convergent evolution@@The independent evolution of similar traits among organisms not closely related but adapting to a similar environment.

Convergent extension@@A phenomenon wherein cells intercalate to narrow the tissue and at the same time move it forward. Mechanism used for elongation of the archenteron in the sea urchin embryo, notochord of the tunicate embryo, and involuting mesoderm of the amphibian. This movement is reminiscent of traffic on a highway when several lanes must merge to form a single lane.

Cornified layer@@The outer layer of the epidermis, consisting of keratinocytes that are now dead, flattened sacs of keratin protein with their nuclei pushed to one edge of the cell. These cells are continually shed throughout life and are replaced by new cells. Also called stratum corneum.

Corona radiata@@The innermost layer of cumulus cells around a mammalian egg, immediately adjacent to the zona pellucida.

Corpora allata@@Insect glands that secrete juvenile hormone (JH) during larval molts.

Corpus luteum@@ The progesterone-secreting remains of an ovarian follicle after its oocyte and its surrounding cumulus cells have been ovulated.

Cortex@@An outer structure (in contrast with medulla, an inner structure).

Cortical flash@@A momentary influx of calcium ions into the rim of the oocyte, observed as the sperm and egg contact one another, especially echinoderms. It may help initiate egg activation and polyspermy events.

Cortical granule reaction@@The basis of the slow block to polyspermy in many animal species, including sea urchins and most mammals. A mechanical block to polyspermy that in sea urchins becomes complete about a minute after successful sperm-egg fusion, in which enzymes from the egg’s cortical granules contribute to the formation of a fertilization envelope that blocks further sperm entry. See Slow block to polyspermy.

Cortical granules@@Membrane-bound, Golgi-derived structures located in the egg cortex; contain enzymes and other components. The exocytosis of these granules at fertilization is homologous to the exocytosis of the acrosome in sperm in the acrosome reaction.

Cortical plate@@The layer of cells in the developing cerebrum of mammals formed by neurons in the ventricular zone migrating outward along radial glial processes to a position near the outer surface of the brain, where they will set up the six layers of the neocortex.

Cortical rotation@@In Xenopus, refers to the ~30 degree rotation of the outer layer of the oocyte cortex that occurs shortly after fertilization.

CpG islands@@Regions of DNA rich in the CpG sequence: a cytosine and a guanosine connected by a normal phosphate bond. Promoters often contain such islands, and transcription is often initiated nearby, possibly because they bind the basal transcription factors that recruit RNA polymerase II.

Cranial (cephalic) neural crest cells@@Neural crest cells in the future head region that migrate to produce the craniofacial mesenchyme, which differentiates into the cartilage, bone, cranial neurons, glia, and connective tissues of the face. These cells also enter the pharyngeal arches and pouches to give rise to thymic cells, the odontoblasts of the tooth primordia, and the bones of the middle ear and jaw.

Cranial (sensory) placodes@@Ectodermal thickenings that form in the cranial region of the vertebrate embryo; includes the olfactory (nasal), otic (ear), and lens (eye) placodes, and placodes that give rise to sensory neurons of various cranial nerves. Also called cranial ectodermal placodes.

Craniorachischisis@@The failure of the entire neural tube to close.

Cranium@@The vertebrate skull, composed of the neurocranium (skull vault and base) and the viscerocranium (jaws and other pharyngeal arch derivatives).

Cre-lox@@A site-specific recombinase technology that allows control over the spatial and temporal pattern of a gene knockout or gene misexpression.

CRISPR@@Clustered Regularly Interspaced Short Palindromic Repeat. Stretches of DNA in prokaryotes that when transcribed into RNA serve as guides for recognizing segments of viral DNA. Used in association with Cas9 (CRISPR-associated enzyme 9) in a relatively fast and inexpensive method of gene editing.

Crossing over@@The exchange of genetic material during meiosis, whereby genes from one chromatid are exchanged with homologous genes from another.

Crown cells@@Cells neighboring the nodal cells, critical to setting up the left-right axis in the mammalian embryo. Crown cells each have a single immobile cilium that senses the left-to-right movement of fluids caused by the motile cilia on node cells. This sets up a cascade of events within the crown cells that serve to maintain Nodal expression on the left side, where it can activate Pitx1 genes, which determine the left and right-sidedness.

Crypt@@A deep tubular recess or pit. Example: intestinal crypts between the intestinal villi.

Cumulus@@A layer of cells surrounding the mammalian egg, made up of ovarian follicular (granulosa) cells that nurture the egg until it is released from the ovary. The innermost layer of cumulus cells, the corona radiate, is released with the egg at ovulation.

Cumulus-oocyte complex (COC)@@The consortium of a single oocyte arrested in metaphase II, surrounded by the cumulus cells and embedded within a web of extracellular matrix.

Cutaneous appendages@@Species-specific epidermal modifications that include hairs, scales, scutes, feathers, hooves, claws, and horns.

Cyclin B@@The larger subunit of mitosis-promoting factor, shows the cyclical behavior that is key to mitotic regulation, accumulating during S and being degraded after the cells have reached M. Cyclin B regulates the small subunit of MPF, the cyclin-dependent kinase.

Cytokines@@Paracrine factors important in cell signaling and the immune response. During blood formation, they are collected and concentrated by the extracellular matrix of the stromal (mesenchymal) cells at the sites of hematopoiesis and are involved in blood cell and lymphocyte formation.

Cytonemes@@Specialized filopodial projections that extend out from a cell (sometimes more than 100 µm) to make contact with another cell producing a paracrine factor. Paracrine factors may be delivered to target cells by attaching to receptors on the tips of cytonemes and traveling down the length of the cytonemes to the body of the target cells. A cytoneme can also extend from a cell producing a paracrine factor to make contact with a target cell.

Cytoplasmic determinants@@Factors within the egg cytoplasm that determine cell fate; these are molecules, often transcription factors, that regulate gene expression. In autonomous specification, these are apportioned to different blastomeres of the early embryo.

Cytotrophoblast@@Mammalian extraembryonic epithelium composed of the original trophoblast cells, it adheres to the endometrium through adhesion molecules and, in species with invasive placentation such as the mouse and human, secretes proteolytic enzymes that enable the cytotrophoblast to enter the uterine wall and remodel the uterine blood vessels so that the maternal blood bathes fetal blood vessels.

D

<The following Dazl term s/b italic>

Dazl@@”Deleted in Azospermia” gene whose protein product is found in the nuclei and cytoplasm of germ cells. Mutations of the gene impede spermatogenesis in human males and block both oogenesis and spermatogenesis in mice.

De novo regeneration@@Regeneration of a structure anew (de novo) from cells that are reprogrammed to form the structure rather than from differentiated cells of that structure. For example, in certain plants de novo regeneration of whole organs is possible from single cells.

Decidua@@The maternal portion of the placenta, made from the endometrium of the uterus.

Decidualization@@ The alterations of the uterine tissue in response female hormones and the implanting embryo. These include the induction of uterine decidual cells and the vascular changes to maternal arteries.

Deep homology@@Signal transduction pathways composed of homologous proteins arranged in a homologous manner that are used for the same function in both protostomes and deuterostomes.

Definitive endoderm@@The endoderm that enters the interior of the amniote embryo through the primitive streak during gastrulation, replacing the visceral endoderm, which is primarily forming the yolk sac and allantois along with the splanchnic lateral plate mesoderm.

Delamination@@The splitting of one cellular sheet into two more or less parallel sheets.

Dendritic arbor@@Extensive branching found in the dendrites of some neurons, such as Purkinje neurons.

Dental lamina@@A broad epidermal thickening in the jaw that later resolves into separate placodes, which together with the underlying mesenchyme form teeth.

Dermal tissue@@In animals, the tissue (the dermis) that underlies the epidermis; together they make up the skin. In plants, the tissue that makes up the outer layer (the epidermis) of the plant; epidermal cells and guard cells that surround stomata are examples of cell types found in dermal tissue.

Dermomyotome@@Dorsolateral portion of the somite that contains skeletal muscle progenitor cells (including those that migrate into the limbs) and the cells that generate the dermis of the back.

Determination front@@Equivalent to the “wavefront” of the “clock-wavefront” model for somite formation; where boundaries of somites form, determined by a caudal^HIGH-to-rostral^LOW gradient of FGF in the presomitic mesoderm.

Determination@@The stage of commitment following specification; the determined stage, assumed irreversible, is when a cell or tissue is capable of differentiating autonomously even when placed into a non-neutral environment.

Deuterostomes@@In the deuterostome animal groups (including echinoderms, tunicates, cephalochordates, and vertebrates), during embryonic development the first opening (i.e., the blastopore) becomes the anus while the second opening becomes the mouth (hence, deutero stoma, “mouth second”). Compare with Protostomes.

Development@@The process of progressive and continuous change that generates a complex multicellular organism from a single cell. Development occurs throughout embryogenesis, maturation to the adult form, and continues into senescence.

Developmental constraints@@In evolution, the limitation of the number and forms of possible phenotypes that can be created by the interactions that are possible among molecules and between modules in the developing organism.

Developmental plasticity@@The ability of an embryo or larva to react to an environmental input with a change in form, state, movement, or rate of activity (i.e., phenotypic change).

Dextral coiling@@Right-coiling. In a snail, having its coils open to the right of its shell. See also Sinistral coiling.

Dictyate resting stage@@The prolonged diplotene stage of the first meiotic division in mammalian primary oocytes. They remain in this stage until just prior to ovulation, when they complete meiosis I and are ovulated as secondary oocytes.

Diencephalon@@The caudal subdivision of the prosencephalon that will form the optic vesicles, retinas, pineal gland, and the thalamic and hypothalamic brain regions, which receive neural input from the retina.

Differential adhesion hypothesis@@A model explaining patterns of cell sorting based on thermodynamic principles. Cells interact so as to form an aggregate with the smallest interfacial free energy and therefore, the most thermodynamically stable pattern.

Differential affinity@@In the context of a developing embryo, this pertains to the biophysical strength of molecular and cellular adhesion that can power the separation of cell populations.

Differential gene expression@@A basic principle of developmental genetics: In spite of the fact that all the cells of an individual body contain the same genome, the specific proteins expressed by the different cell types are widely diverse. Differential gene expression, differential nRNA processing, differential mRNA translation, and differential protein modification all work to allow the extensive differentiation of cell types.

Differential pre-mRNA processing@@Also known as alternative splice, this refers to the selective removal and pairing of introns and exons respectively, which can result in the building of distinct proteins.

Differentiation@@The process by which an unspecialized cell becomes specialized into one of the many cell types that make up the body.

Digestive tube@@The primitive gut of the embryo, which extends the length of the body from the pharynx to the cloaca. Buds from the digestive tube form the thyroid, thymus, and parathyroid glands, lungs, liver, gallbladder, and pancreas.

5α-Dihydrotestosterone (DHT)@@A steroid hormone derived from testosterone by the action of the enzyme 5α-ketosteroid reductase 2. DHT is required for masculinization of the male urethra, prostate, penis, and scrotum.

Diploblasts@@“Two-layer” animals; they possess endoderm and ectoderm but most species lack true mesoderm. Includes the ctenophores (comb jellies) and cnidarians (jellyfish, corals, hydra, sea anemones). Compare with Bilaterians.

Discoidal cleavage@@Meroblastic cleavage pattern for telolecithal eggs, in which the cell divisions occur only in the small blastodisc, as in birds, reptiles, and fish.

Discoidal meroblastic cleavage@@See Discoidal cleavage.

Direct developers@@Animals in which juveniles are essentially immature versions of the adult. Compare with Indirect developers.

Disruption@@Abnormality or congenital defect caused by exogenous agents (teratogens) such as plants, chemicals, viruses, radiation, or hyperthermia.

DNA-binding domain@@Transcription factor domain that recognizes a particular DNA sequence.

Dormancy@@In seed plants, a prolonged period of quiescence that a seed can undergo prior to germination.

Dorsal blastopore lip@@Location of the involuting marginal zone cells of amphibian gastrulation. Migrating marginal cells sequentially become the dorsal lip of the blastopore, turn inward and travel along the inner surface of the outer animal cap cells (i.e., the blastocoel roof).

Dorsal closure@@A process that brings together the two sides of the epidermis of the Drosophila embryo at the dorsal surface.

Dorsal mesentery@@A derivative of the splanchnic mesoderm, this fibrous membrane connects the endoderm to the body wall. Involved in the looping of the developing intestines.

Dorsal root ganglia@@Sensory spinal ganglia derived from the trunk neural crest that migrate along the ventral pathway and stay in the sclerotome. Sensory neurons of the DRG connect centrally with neurons in the dorsal horn of the spinal cord.

Dorsal-ventral (dorsosventral) axis (DV)@@The plane defining the back (dorsum) versus the belly (ventrum). When referring to the limb, this axis refers to the knuckles (dorsal) and palms (ventral).

Dorsolateral hinge points (DLHPs)@@In the formation of the avian and mammalian neural tube, two hinge regions in the lateral sides of the neural plate that bend the two sides of the plate inward toward each other after the medial hinge point (MHP) has bent the plate along its midline.

Dorsolateral pathway@@Pathway taken by trunk neural crest cells traveling dorsolaterally beneath the ectoderm to become melanocytes.

Double fertilization@@In angiosperms, a process wherein one sperm nucleus combines with the egg nucleus to produce a zygote, while the other sperm combines with somatic cells to produce the triploid endosperm.

Double-negative gate@@A mechanism whereby a repressor locks the genes of specification, and these genes can be unlocked by the repressor of that repressor. (In other words, activation by the repression of a repressor.)

Doublesex (Dsx)@@Protein product of a Drosophila gene that is active in both males and females, but whose RNA transcript is spliced in a sex-specific manner to produce sex-specific transcription factors: the female-specific transcription factor activates female-specific genes and inhibits male development; the male-specific transcription factor inhibits female traits and promotes male traits.

Duplication and divergence@@Gene duplications resulting from replication errors. Once replicated, the gene copies can diverge by random mutation, developing different expression patterns and new functions.

E

E-cadherin@@A type of cadherin expressed in epithelial tissues as well as all early mammalian embryonic cells (the E stands for epithelial). See Cadherins.

Ecdysone@@Insect steroid hormone, secreted by the prothoracic glands, that is modified in peripheral tissues to become the active molting hormone 20-hydroxyecdysone. Crucial to insect metamorphosis.

Ecdysone receptors (EcRs)@@Nuclear proteins that bind to ecdysone in insects; when bound EcR forms an active complex with another protein that binds to DNA, inducing transcription of ecdysone-responsive genes. Evolutionarily related to, and almost identical in structure to, thyroid hormone receptors.

Ecdysozoans@@One of the two major protostome groups; characterized by exoskeletons that periodically molt. The arthropods (including insects and crustaceans) and the nematodes (roundworms, including the model organism C. elegans) are two prominent groups. See also Lophotrochozoans.

Ecological evolutionary developmental biology (eco-evo-devo)@@The science studying the ways by which developmental changes initiated by the environment can affect evolution. It deals primarily with the evolutionary aspects of developmental symbiosis, developmental plasticity, and niche construction.

Ecological developmental biology (eco-devo)@@The science studying the ways by which developmental changes initiated by the environment can affect evolution. It deals primarily with the evolutionary aspects of developmental symbiosis, developmental plasticity, and niche construction.

Ecomorph@@See Morph.

Ectoderm@@Greek ektos, “outside.” The cells that remain on either the outside (amphibian) or dorsal (avian, mammalian) surface of the embryo following gastrulation. Of the three germ layers, the ectoderm forms the nervous system from the neural tube and neural crest; also generates the epidermis covering the embryo.

Ectodermal appendages@@Structures that form from specific regions of epidermal ectoderm (placodes) and underlying mesenchyme through a series of interactive inductions; includes hairs, scales, scutes (such as the coverings of turtle shells), teeth, sweat glands, mammary glands, and feathers.

Ectopic pregnancy@@A potentially fatal, condition in which the early mammalian embryo implants and begins to grow within an area of the body outside the uterus. When the embryo implants into an oviduct, it is often called a tubal pregnancy.

Efflux transport@@A process found whereby transporter proteins move a substance from the inside to the outside of a cell. Example: PIN proteins in plants use efflux transport to move auxin from inside to outside the cell.

Egg chamber@@An ovariole or egg tube (over a dozen per ovary) in which the Drosophila oocyte will develop, containing 15 interconnected nurse cells and a single oocyte.

Embryo@@A developing organism prior to birth or hatching. In humans, the term embryo generally refers to the early stages of development, starting with the fertilized egg until the end of organogenesis (first 8 weeks of gestation). After this, the developing human is called a fetus until its birth.

Embryo sac@@The female gametophyte of an angiosperm. Found inside the ovule, it consists of eight or fewer cells formed by the division of the haploid nucleus of the megaspore cell.

Embryogenesis@@The stages of development between fertilization and hatching (or birth).

Embryoid bodies@@Spherical aggregates derived from embryonic stem cells that show markers for all three germ layers of the embryo.

Embryology@@The study of animal development from fertilization to hatching or birth.

Embryonic period@@In human development, the first 8 weeks in utero prior to the fetal period; the time during which most organ systems form.

Embryonic shield@@A localized thickening on the future dorsal side of the fish embryo; functionally equivalent to the dorsal blastopore lip of amphibians.

Embryonic stem cells (ESCs)@@Pluripotent stem cells derived from cultures of the mammalian inner cell mass blastomeres that are capable of generating all the cell types of the body.

Embryophytes@@Land plants. These are named embryophytes because they all undergo embryogenesis.

EMT@@See Epithelial-to-mesenchymal transition.

Enamel knot@@The signaling center for tooth development, a group of cells induced in the epithelium by the neural crest-derived mesenchyme that secretes paracrine factors that pattern the cusp of the tooth.

Endocrine disruptors@@Hormonally active compounds in the environment (e.g., DES; BPS; aromatase) that can have major detrimental effects on development, particularly of the gonads. Many endocrine disruptors are also obesogens (cause increased production of fat cells and fat accumulation).

Endoderm@@Greek endon, “within.” The innermost germ layer; forms the epithelial lining of the respiratory tract, the gastrointestinal tract, and the accessory organs (e.g., liver, pancreas) of the digestive tract. In the amphibian embryo, the yolk-containing cells of the vegetal hemisphere become endoderm. In amniote embryos, the endoderm is the most ventral of the three germ layers, and also forms the epithelium of the yolk sack and allantois.

Endomesoderm@@The combination of endodermal and mesodermal cells.

Endometrium@@The epithelial lining of the uterus.

Endosperm@@A triploid seed tissue found in angiosperms, providing nutrition for the developing embryo.

Endosymbiosis@@Greek, “living within.” Describes the situation in which one cell lives inside another cell or one organism lives within another.

Endothelins@@Small peptides secreted by blood vessels that have a role in vasoconstriction and can direct the migration of certain neural crest cells as well as the extension of certain sympathetic axons that have endothelin receptors, e.g. targeting of neurons from the superior cervical ganglia to the carotid artery.

Energids@@In Drosophila, the nuclei at the periphery of the syncytial blastoderm and their associated cytoplasmic islands of cytoskeletal proteins.

Enhancer@@A DNA sequence that controls the efficiency and rate of transcription from a specific promoter. Enhancers bind specific transcription factors that activate the gene by (1) recruiting enzymes (such as histone acetyltransferases) that break up the nucleosomes in the area or (2) stabilizing the transcription initiation complex.

Enterocoely@@The embryonic process of forming the coelom by extending mesodermal pouches from the gut. Typical of most deuterostomes. See also Schizocoely.

Enveloping layer (EVL)@@A cell population in the zebrafish embryo at the mid-blastula transition made up of the most superficial cells from the blastoderm, which form an epithelial sheet a single cell layer thick. The EVL is an extraembryonic protective covering that is sloughed off during later development.

Ependyma@@Epithelial lining of the spinal cord canal and the ventricles of the brain.

Ependymal cells@@Epithelial cells that line the ventricles of the brain and canal of the spinal cord; they secrete cerebrospinal fluid.

Eph receptors@@Receptor for ephrin ligands.

Ephrin ligands@@Primarily function as membrane tethered ligands that induce signaling through their Eph receptor binding partners.

Ephrins@@Juxtacrine ligands. Binding between an ephrin ligand on one cell and an Eph receptor on an adjacent cell results in signals being sent to both cells. These signals are often those of either attraction or repulsion, and ephrins are often seen directing cell migration and defining where cell boundaries are to form. As well as directing neural crest cell migration, ephrins and Eph receptors function in the formation of blood vessels, neurons, and somites.

Epialleles@@Variants of chromatin structure that can be inherited between generations. In most known cases, epialleles are differences in DNA methylation patterning that are able to affect the germ line and thereby be transmitted to offspring.

Epiblast@@The outer layer of the thickened margin of the epibolizing blastoderm in the gastrulating fish embryo or the upper layer of the bilaminar gastrulating embryonic disc in amniotes (reptiles, birds and mammals. The epiblast contains ectoderm precursors in fish and all three germ layer precursors of the embryo proper (plus the amnion) in amniotes. It also forms the avian chorion and allantois.

Epiboly@@The movement of epithelial sheets (usually of ectodermal cells) that spread as a unit (rather than individually) to enclose the deeper layers of the embryo. Epiboly can occur by the cells dividing, by the cells changing their shape, or by several layers of cells intercalating into fewer layers. Often, all three mechanisms are used.

Epibranchial placodes@@A subgroup of cranial placodes that form in the pharyngeal region of the vertebrate embryo. Gives rise to the sensory neurons of three cranial nerves: facial (VII), glossopharyngeal (IX), and vagus (X).

Epidermal placodes@@The thickenings of epidermal ectoderm associated with ectodermal appendages. See Ectodermal appendages.

Epididymis@@Derived from the Wolffian duct, the tube adjacent to the testis that links the efferent tubules to the ductus deferens.

Epigenesis@@The view supported by Aristotle and William Harvey that the organs of the embryo are formed de novo (“from scratch”) at each generation.

Epigenetics@@The study of mechanisms that act on the phenotype without changing the nucleotide sequence of the DNA. Specifically, these changes work “outside the gene” (i.e., epigenetically) by altering gene expression rather than by altering the gene sequence as mutation does. Epigenetic changes can sometimes be transmitted to future generations, a phenomenon referred to as epigenetic inheritance.

Epimorphosis@@Form of regeneration observed when adult structures undergo dedifferentiation to form a relatively undifferentiated mass of cells that then redifferentiates to form the new structure (e.g., amphibian limb regeneration).

Epistasis@@The interaction of two or more different alleles on the phenotype. Often, it refers to the suppression of the effects of one gene by another.

Epithelial cells@@Cells that are tightly linked together to form a sheet or tube (an epithelium) with little extracellular matrix.

Epithelial-to-mesenchymal transition (EMT)@@An orderly series of events whereby the cells of an epithelial sheet or structures are transformed into migratory mesenchymal cells. In this transition, a polarized stationary epithelial cell, which normally interacts with basement membrane through its basal surface, becomes a migratory mesenchymal cell that can invade tissues and form organs in new places. The converse is the mesenchymal-to-epithelial transition (MET), which occurs when mesenchymal cells coalesce and from an epithelial structures (as when presomitic mesoderm forms somites.)

Equatorial region@@The junction between the inner acrosomal membrane and the sperm cell membrane in mammals. It is exposed by the acrosomal reaction and is where membrane fusion between sperm and egg begins.

Equivalence group@@In the development of C. elegans, the group of six vulval precursor cells, each of which is competent to become induced by the anchor cell.

Erythropoietin@@A hormone that acts on erythroid progenitor cells to produce proerythroblasts, which will generate red blood cells.

Euchromatin@@The comparatively open state of chromatin that contains most of the organism’s genes, most of which are capable of being transcribed. Compare with Heterochromatin.

Evolutionary developmental biology (“evo-devo”)@@A model of evolution that integrates developmental genetics and population genetics to explain and the origin of biodiversity.

Exine@@The outer coating of a pollen grain or spore. It is extremely resistant to decay.

Exon@@In a gene, the region or regions of DNA that encode the protein. Compare with Intron.

Extended evolutionary synthesis@@A model of evolution emphasizing developmental plasticity, epigenetic inheritance, niche constriction, and the reciprocal interactions between the organism and its environment.

External granular layer@@A germinal zone of cerebellar neuroblasts that migrate from the germinal neuroepithelium to the outer surface of the developing cerebellum.

Extracellular matrix (ECM)@@Macromolecules secreted by cells into their immediate environment, forming a region of noncellular material in the interstices between the cells. Extracellular matrices are made up of collagen, proteoglycans, and a variety of specialized glycoprotein molecules such as fibronectin and laminin.

Eye field@@Region in the anterior portion of the neural tube that will develop into the neural and pigmented retinas.

F

Fasciculation@@In neural development, the process of one axon adhering to and using another axon for growth.

Fast block to polyspermy@@Mechanism by which additional sperm are prevented from fusing with a fertilized sea urchin egg by changing the electric potential to a more positive level. Has not been demonstrated in mammals. Compare with Slow block to polyspermy.

Fate map@@Diagrams based on having followed cell lineages from specific regions of the embryo in order to “map” larval or adult structures onto the region of the embryo from which they arose. The superimposition of a map of “what is to be” onto a structure that has yet to develop into these organs.

Fertilization cone@@An extension from the surface of the egg where the egg and sperm have fused during fertilization. Caused by polymerization of actin, it provides a connection that widens the cytoplasmic bridge between egg and sperm, allowing the sperm nucleus and proximal centriole to enter the egg.

Fertilization envelope@@Forms from the vitelline envelope of the sea urchin egg following cortical granule release. Glycosaminoglycans released by the cortical granules absorb water to expand the space between the cell membrane and fertilization envelope.

Fertilization@@Fusion of male and female gametes followed by fusion of the haploid gamete nuclei to restore the full complement of chromosomes characteristic of the species and initiation in the egg cytoplasm of those reactions that permit development to proceed.

Fetal alcohol spectrum disorder (FASD)@@See Fetal alcohol syndrome.

Fetal alcohol syndrome (FAS)@@Condition of babies born to alcoholic mothers, characterized by small head size, specific facial features, and small brain that often shows defects in neuronal and glial migration. FAS is the most prevalent congenital mental retardation syndrome. Another term, fetal alcohol spectrum disorder (FASD) has been coined to encompass the less visible behavioral effects on children exposed prenatally to alcohol.

Fetal period@@In human development, the period following the embryonic period, from the end of 8 weeks to birth; the period after organ systems have mostly formed and generally growth and modeling is occurring.

Fetus@@The stage in mammalian development between the embryonic stage and birth, characterized by growth and modeling. In humans, from the ninth week of gestation to birth.

Fibroblast growth factor receptors (FGFRs)@@A set of receptor tyrosine kinases that are activated by FGFs, resulting in activation of the dormant kinase and phosphorylation of certain proteins (including other FGF receptors) within the responding cell.

Fibroblast growth factors (FGFs)@@A family of paracrine factors that regulate cell proliferation and differentiation.

Fibronectin@@A very large (460 kDa) glycoprotein dimer synthesized by numerous cell types and secreted into the extracellular matrix. Functions as a general adhesive molecule, linking cells to one another and to other substrates such as collagen and proteoglycans, and provides a substrate for cell migration.

Filopodia@@Long, thin processes containing microfilaments; cells can move by extending, attaching, and then contracting filopodia. Produced, for example, by migrating mesenchyme cells in sea urchin embryos, growth cones for nerve outgrowth, tip cells in blood vessel formation.

First polar body@@The smaller cell produced when a primary oocyte goes through its first meiotic division, producing one large cell, the secondary oocyte, which retains most of the cytoplasm, and a tiny cell, the first polar body, which is ultimately lost. Both cells are haploid.

Floor plate@@Ventral region of the neural tube important in establishing dorsal-ventral polarity. Induced to form by Sonic hedgehog secreted from the adjacent notochord. It becomes a secondary signaling center that also secretes Sonic hedgehog, establishing a gradient that is highest ventrally.

Floral organ identity genes@@Angiosperm genes (sets A, B, C, D, and E) that determine the locations and fates of floral meristem cells.

Fluorescent dye@@Compounds, such as fluorescein and green fluorescent protein (GFP), that emit bright light at a specific wavelength when excited with ultraviolet light.

Focal adhesions@@Where the cell membrane adheres to the extracellular matrix in migrating cells, mediated by connections between actin, integrin and the extracellular matrix.

Follicle@@A small group of cells around a cavity. E.g., mammalian ovarian follicle, composed of a single ovum surrounded by granulosa cells and thecal cells; hair follicle, feather follicle, where a hair or feather is produced.

Follicle-stimulating hormone (FSH)@@A peptide hormone secreted by the mammalian pituitary that promotes ovarian follicle development and spermatogenesis.

Foot activation gradient@@A gradient, highest at the basal disc, that appears to be present in Hydra that permits the basal disc to form only in one place.

Forebrain@@Also called the prosencephalong; the most anterior vesicle of the developing vertebrate brain. Will form two secondary brain vesicles: the telencephalon and the diencephalon.

Forward genetics@@Genetic technique of exposing an organism to an agent that causes random mutations and screening for particular phenotypes. Compare with Reverse genetics.

Fractones@@Condensed branched or bulbar structures of extracellular matrix that have been shown in the neural stem cell niche of the ventricular/subventricular zone to function as collecting locations for signaling molecules.

Frontonasal process@@Cranial prominence formed by neural crest cells from the midbrain and rhombomeres 1 and 2 of the hindbrain that forms the forehead, the middle of the nose, and the primary palate.

Fruit@@In flowering plants, a ripened and mature seed-containing ovary or ovaries.

G

GAL4/UAS@@An experimental method using validated promoter sequences to manipulate specific gene activity with spatial and temporal control. GAL4 is the transcriptional activator expressed under a tissue specific promoter, and UAS is the GAL4-responsive sequence that will enable expression of a target gene (e.g., GFP).

Gamete@@A specialized reproductive cell through which sexually reproducing parents pass chromosomes to their offspring; an egg or a sperm.

Gametogenesis@@The production of gametes.

Gametophyte@@The haploid stage of the alternating life cycle in plants and algae that produces gametes (eggs and sperm); the sexual phase. Compare with Sporophyte.

Ganglia (sing. ganglion)@@Clusters of neuronal cell bodies whose axons form a nerve.

Gap genes@@Drosophila zygotic genes expressed in broad (about three segments wide), partially overlapping domains. Gap mutants lacked large regions of the body (several contiguous segments).

Gastrula@@A stage of the embryo following gastrulation that contains the three germ layers that will interact to generate the organs of the body.

Gastrulation@@A process involving movement of the blastomeres of the embryo relative to one another resulting in the formation of the three germ layers of the embryo.  

Gastruloid@@Gastrula-like, multicellular structures derived from 3D culturing of stem cells in vitro.

Gene regulatory network (GRN)@@Pattern generated by the interactions among transcription factors and their enhancers that help define the course that development follows.

Generative cell@@In angiosperms, the cell of a pollen grain that divides to produce the two male gamete nuclei.

Genetic assimilation@@The process by which a phenotypic character initially produced only in response to some environmental influence becomes, through a process of selection, taken over by the genotype so that it is formed even in the absence of the environmental influence that had first been necessary.

Genetic heterogeneity@@The production of similar phenotypes by mutations in different genes.

Genital ridge@@A thickening of the splanchnic mesoderm and of the underlying intermediate mesodermal mesenchyme on the medial edge of the mesonephros; it forms the testis or ovary. Also called the germinal ridge.

Genital tubercle@@A structure cranial to the cloacal membrane during the indifferent stage of differentiation of the mammalian external genitalia. It will form either the clitoris in the female fetus or the penis in the male.

Genome@@The complete DNA sequence of an individual organism.

Genomic equivalence@@The theory that every cell of an organism has the same genome as every other cell.

Germ band@@A collection of cells along the ventral midline of the Drosophila embryo that forms during gastrulation by convergence and extension that includes all the cells that will form the trunk of the embryo and the thorax and abdomen of the adult.

Germ cell lineage@@The cells that form the gametes, including the primordial germ cells, the developing sperm and eggs, and the mature gametes.

Germ cells@@A group of cells set aside for reproductive function; germ cells become the cells of the gonads (ovary and testis) that undergo meiotic cell divisions to generate the gametes. Compare with Somatic cells.

Germ layer@@One of the three layers of the embryo, ectoderm, mesoderm, and endoderm, in triploblastic organisms, or of the two layers, ectoderm and endoderm, in diploblastic organisms, generated by the process of gastrulation, that will form all of the tissues of the body except for the germ cells.

Germ line@@The line of cells that become germ cells, separate from the somatic cells, found in many animals, including insects, roundworms, and vertebrates. Specification of the germ line can occur autonomously from determinants found in cytoplasmic regions of the egg, or can occur later through induction by neighboring cells.

Germ ring@@A thickened ring of cells in the margin of the deep cells that appears in a fish embryo once the blastoderm has covered about half of the yolk cell. Composed of a superficial layer, the epiblast, and an inner layer, the hypoblast.

Germinal crescent@@A region in the anterior portion of the avian and reptilian blastoderm area pellucida containing the hypoblasts displaced by migrating endodermal cells. It contains the primordial germ cells (precursors of the germ cells), which later migrate through the blood vessels to the gonads.

Glia@@Supportive cells of the central nervous system, derived from the neural tube, and of the peripheral nervous system, derived from neural crest.

Glial-derived neurotrophic factor (GDNF)@@A paracrine factor that binds to the Ret receptor tyrosine kinase. It is produced by the gut mesenchyme that attracts vagal and sacral neural crest cells, and it is produced by the metanephrogenic mesenchyme to induce the formation and branching of the ureteric buds.

Glycogen synthase kinase 3 (GSK3)@@Targets β-catenin for destruction.

Gonadal sex determination@@See Primary sex determination.

Gonocytes@@Mammalian primordial germ cells (PGCs) that have arrived at the genital ridge of a male embryo and have become incorporated into the sex cords.

Granule cells@@Derived from neuroblasts of the external granule layer of the developing cerebellum. Granule neurons migrate back toward the ventricular (ependymal) zone, where they produce a region called the internal granule layer.

Granulosa cells@@Cortical epithelial cells of the fetal ovary, granulosa cells surround individual germ cells that will become the ova and will form, with thecal cells, the follicles that envelop the germ cells and secrete steroid hormones. The number of granulosa cells increase and form concentric layers around the oocyte as the oocyte matures prior to ovulation.

Gray crescent@@A band of inner gray cytoplasm that appears following a rotation of the cortical cytoplasm with respect to the internal cytoplasm in the marginal region of the single-cell amphibian embryo. Gastrulation starts in this location.

Gray matter@@Regions of the brain and spinal cord rich in neuronal cell bodies. Compare with White matter.

Green fluorescent protein (GFP)@@A protein that occurs naturally in certain jellyfish. It emits bright green fluorescence when exposed to ultraviolet light. The GFP gene is widely used as a transgenic label for cells in developmental and other research, since cells that express GFP are easily identified by a bright green glow.

Ground tissue@@In plants, all tissue that is not dermal or vascular; functions primarily in storage, support, and photosynthesis, and includes filler tissue called parenchyma and the more supportive collenchyma and sclerenchyma.

Growth cone@@The motile tip of a neuronal axon; leads nerve outgrowth.

Gynandromorph@@Greek gynos, “female”; andros, “male.” An animal in which some body parts are male and others are female. Compare with Hermaphrodite.

H

Hair cells@@Sensory receptors that transform the movement of fluid into electrical signals. In the inner ear, they are in the organ of Corti of the cochlea for hearing and the semicircular canals for balance. They are also in the lateral line organs of fish and amphibians for detecting movement and changes in pressure in the water.

Hair follicle stem cells@@See Follicular stem cells.

Halteres@@A pair of balancers on the third thoracic segment of two-winged flies, such as Drosophila.

Hatched blastula@@Free-swimming sea urchin embryo, after the cells of the animal hemisphere synthesize and secrete a hatching enzyme that digests the fertilization envelope.

Head activation gradient@@A morphogenetic gradient in Hydra that is highest at the hypostome and permits the head to form.

Head mesoderm@@Mesoderm located anterior to the trunk mesoderm, consisting of the unsegmented paraxial mesoderm and prechordal mesoderm. This region provides the head mesenchyme that forms much of the connective tissues and musculature of the head.

Head process@@In avian embryos, the anterior portion of the chordamesoderm that passes through Hensen’s node and migrates anteriorly, ahead of the notochordal mesoderm, to come to lie underneath cells that will form forebrain and midbrain.

Heart fields (cardiogenic mesoderm)@@In vertebrates, two regions of splanchnic mesoderm, one on each side of the body, that become specified for heart development. In amniotes, the cardiac cells of the heart field migrate through the primitive streak during gastrulation such that the medial-lateral arrangement of these early cells will become the anterior-posterior (rostral-caudal) axis of the developing heart tube.

Heart tube@@Linear (anterior-to-posterior) structure formed at the midline of the heart fields; will become the atria, ventricles, and the base of the aorta and pulmonary arteries.

Heat shock proteins@@Intercellular proteins induced by stress, they help other proteins to fold correctly and maintain their functions.

Hedgehog@@A family of paracrine factors used by the embryo to induce particular cell types and to create boundaries between tissues. Hedgehog proteins must become complexed with a molecule of cholesterol in order to function. Vertebrates have at least three homologues of the Drosophila hedgehog gene: sonic hedgehog (shh), desert hedgehog (dhh), and indian hedgehog (ihh).

Hemangioblasts@@Rapidly dividing cells, usually stem cells, that form blood vessels and blood cells.

Hematopoiesis@@The generation of blood cells.

Hematopoietic inductive microenvironments (HIMs)@@Cell regions that induce different sets of transcription factors in multipotent hematopoietic stem cells; these transcription factors specify the developmental path taken by descendants of those cells.

Hematopoietic stem cell (HSC)@@A pluripotent stem cell type that generates a series of intermediate progenitor cells whose potency is restricted to certain blood cell lineages. These lineages are then capable of producing all the blood cells and lymphocytes of the body.

Hemimetabolous@@A form of insect metamorphosis that includes pronymph, nymph, and imago (adult) stages.

Hemogenic endothelial cell@@Primary endothelial cells of the dorsal aorta, in the ventral area, derived from the lateral plate. They give rise to the hematopoietic stem cells (HSCs) that migrate to the liver and bone marrow and become the adult hematopoietic stem cells.

Hensen’s node@@In avian embryos, a regional thickening of cells at the anterior end of the primitive streak. The center of Hensen’s node contains a funnel-shaped depression (sometimes called the primitive pit) through which cells can enter the embryo to form the notochord and prechordal plate. Hensen’s node is the functional equivalent of the dorsal lip of the amphibian blastopore (i.e., the organizer) and the fish embryonic shield.

Hepatic diverticulum@@The liver precursor, a bud of endoderm that extends out from the foregut into the surrounding mesenchyme.

Hepatocyte growth factor (HGF)@@Paracrine factor secreted by the stellate cells of the liver that allows the hepatocytes to re-enter the cell cycle during compensatory regeneration. Also called scatter factor.

Hermaphrodite@@An individual in which both ovarian and testicular tissues exist, having either ovotestes (gonads containing both ovarian and testicular tissue) or an ovary on one side and a testis on the other. Compare with Gynandromorph.

Heterochromatin@@Chromatin that remains condensed throughout most of the cell cycle and replicates later than most of the other chromatin. Usually transcriptionally inactive. Compare with Euchromatin.

Heterochrony@@Greek, “different time.” A shift in the relative timing of two developmental processes as a mechanism to generate phenotypic variation available for natural selection. One module changes its time of expression or growth rate relative to the other modules of the embryo.

Heterometry@@Greek, “different measure.” A change in the amount of a gene product as a mechanism to generate phenotypic variation available for natural selection.

Heterophilic binding@@Binding between different molecules, as when a receptor in the membrane of one cell binds to a different type of receptor in the membrane of another cell.

Heterotopy@@Greek, “different place.” The spatial alteration of gene expression (e.g., transcription factors or paracrine factors) as a mechanism to generate phenotypic variation available for natural selection.

Heterotypy@@Greek, “different kind.” The alteration of the actual coding region of the gene, changing the functional properties of the protein being synthesized, as a mechanism to generate phenotypic variation available for natural selection.

High CpG-content promoters (HCPs)@@Promoters with many CpG islands; these promoters often regulate developmental genes required for the construction of the organism; their default state is “on.” Hindbrain@@The most caudal vesicle of the developing vertebrate brain; will form two secondary brain vesicles, the metencephalon and myelencephalon. Also called the rhombencephalon.

Histone acetylation@@The addition of negatively charged acetyl groups to histones, which neutralizes the basic charge of lysine and loosens the histones, thus activating transcription.

Histone acetyltransferases@@Enzymes that place acetyl groups on histones (especially on lysines in histones H3 and H4). Acetyltransferases destabilize the nucleosomes so that they come apart easily, thus facilitating transcription.

Histone deacetylases@@Enzymes that remove acetyl groups from histones, stabilizing the nucleosomes and preventing transcription.

Histone methylation@@The addition of methyl groups to histones. Can either activate or further repress transcription, depending on the amino acid that is methylated and the presence of other methyl or acetyl groups in the vicinity.

Histone methyltransferases@@Enzymes that add methyl groups to histones and either activate or repress transcription.

Histone@@Positively charged proteins that are the major protein component of chromatin. See also Nucleosome.

Holobiont@@Term for the composite organism of a host and its persistent symbionts.

Holometabolous@@The type of insect metamorphosis found in flies, beetles, moths, and butterflies. There is no pronymph stage. The insect hatches as a larva (a caterpillar, grub, or maggot) and progresses through instar stages as it gets bigger between larval molts, a metamorphic molt to become a pupa, an imaginal molt and finally the emergence (eclose) of the adult (imago).

Homeotic complex (Hom-C)@@The region of Drosophila chromosome 3 containing both the Antennapedia complex and the bithorax complex.

Homeotic mutants@@Result from mutations of homeotic selector genes, in which one structure is replaced by another (as where an antenna is replaced by a leg).

Homeotic selector genes@@A class of Drosophila genes regulated by the protein products of the gap, pair-rule, and segment polarity genes whose transcription determines the developmental fate of each segment.

Homeotic transformations@@The replacement of one structure by another during development due to a homeotic mutation. See also Homeotic mutants.

Homologous@@Structures and/or their respective components whose similarity arises from their being derived from a common ancestral structure. For example, the wing of a bird and the forelimb of a human. Compare with Analogous.

Homophilic binding@@Binding between like molecules, as when a receptor in the membrane of one cell binds to the same type of receptor in the cell membrane of another cell.

Horizontal transmission@@When a host that is born free of symbionts but subsequently becomes infected, either by its environment or by other members of the species. Can also refer to the transfer of genes from one organism to another without involving reproduction, as can occur in bacteria. Compare with Vertical transmission.

Host@@The larger organism in a symbiotic relationship in which one of the organisms involved is much larger than the other, and the smaller organism may live on the surface or inside the body of the larger. Also refers to the organism receiving a graft from a donor in a tissue transplant.

Hox gene family@@The large family of related genes that dictate (at least in part) regional identity in the embryo, particularly along the anterior-posterior axis. Hox genes encode transcription factors that regulate the expression of other genes. All known mammalian genomes contain four copies of the Hox complex per haploid set, located on four different chromosomes (Hoxa through Hoxd in the mouse, HOXA through HOXD in humans). The mammalian Hox/HOX genes are numbered 1–13, starting from that end of each complex that is expressed most anteriorly.

20-Hydroxyecdysone (20E)@@An insect hormone, the active form of ecdysone, that initiates and coordinates each molt, regulates the changes in gene expression that occur during metamorphosis, and signals imaginal disc differentiation.

Hyperactivation@@The increased and more forceful motility displayed by capacitated sperm of some mammalian species. Hyperactivation has been proposed to help detach capacitated sperm from the oviductal epithelium, allow sperm to travel more effectively through viscous oviductal fluids, and facilitate penetration of the extracellular matrix of the cumulus cells.

Hypermorphosis@@The extension of development beyond its ancestral state; it is an evolutionary mechanism whereby the total developmental time is extended without altering the rate of development. Example: In humans, the extension of the fetal rate of brain growth beyond birth.

Hypoblast@@The inner layer of the thickened margin of the epibolizing blastoderm in the gastrulating fish embryo or the lower layer of the bilaminar embryonic blastoderm in birds and mammals. The hypoblast in fish (but not in birds and mammals) contains the precursors of both the endoderm and mesoderm. In birds and mammals, it contains precursors to the extraembryonic endoderm of the yolk sac.

Hypomorphic mutations@@Mutations that reduce gene function, as opposed to a “null” mutation that results in the loss of a protein’s function.

Hypostome@@A conical region of the “head” of a hydra that contains the mouth.

I

Imaginal cells@@Cells carried around in the larva of the holometabolous insect that will form the structures of the adult. During the larval stages, these cells increase in number, but do not differentiate until the pupal stage; include imaginal discs, histoblasts, and clusters of imaginal cells within each larval organ.

Imaginal molt@@Final molt in a holometabolous insect when the adult (imago) cuticle forms beneath the pupal cuticle, and the adult later emerges from the pupal case at adult eclosion.

Imago@@A winged and sexually mature adult insect.

Implantation@@When the embryo attaches to the uterus and burrows inside the uterine epithelium, remodeling the uterine blood supply and forming the placenta.

Imprinted genes@@A phenomenon in mammals whereby only the sperm-derived or only the egg-derived allele of the gene is expressed, sometimes due to inactivation of one allele by DNA methylation during spermatogenesis or oogenesis.

In situ hybridization@@A molecular procedure used to visualize the spatial expression of a given transcript within a tissue or whole embryo.

in vitro fertilization (IVF)@@A series of procedures wherein oocytes are collected from ovaries and fertilized by sperm in the laboratory. After a period of growth in the laboratory, resulting embryos can be transferred into the uterus.

Indirect developers@@Animals for which embryonic development includes a larval stage with characteristics very different from those of the adult organism, which emerges only after a period of metamorphosis. Compare with Direct developers.

Induced pluripotent stem cells (iPSCs)@@Adult cells that have been converted to cells with the pluripotency of embryonic stem cells. Usually accomplished by the activation of certain transcription factors.

Inducer@@Tissue that produces a signal (or signals) that induces a cellular behavior in some other tissue.

Induction@@The process by which one cell population influences the development of neighboring cells via interactions at close range.

Inflorescence meristem@@The meristem that develops from the shoot apical meristem when the plant begins flowering; the inflorescence meristem produces the floral meristems, which form the carpels, stamens, petals and sepals of each flower. See Meristem.

Ingression@@Migration of individual cells from the surface layer into the interior of the embryo. The cells become mesenchymal (i.e., they separate from one another) and migrate independently.

Initial cells@@Totipotent stem cells generated by the early plant embryo; there are two clusters of these: the shoot apical meristem and the root apical meristem.

Inner cell mass (ICM)@@A small group of internal cells within a mammalian blastocyst that will eventually develop into the embryo proper and its associated yolk sac, allantois, and amnion.

Inositol 1,4,5-trisphosphate (IP3)@@A second messenger generated by the phospholipase C enzyme that releases intracellular Ca²⁺ stores. Important in the initiation of both cortical granule release and sea urchin development.

Instars@@The stages between larval molts in holometabolous insects. During these stages, the larva (caterpillar, grub, or maggot) feeds and grows larger between each molt, until the end of the final instar stage, when the larva is transformed into a pupa.

Instructive interaction@@A mode of inductive interaction in which a signal from the inducing cell is necessary for initiating new gene expression in the responding cell.

Insulin-like growth factors (IGFs)@@Growth factors that initiate an FGF-like signal transduction cascade that interferes with the signal transduction pathways of both BMPs and Wnts. IGFs are required for the formation of the anterior neural tube, including the brain and sensory placodes of amphibians.

Integrins@@A family of receptor proteins, named for the fact that they integrate extracellular and intracellular scaffolds, allowing them to work together. On the extracellular side, integrins bind to sequences found in several adhesive proteins in extracellular matrix, including fibronectin, vitronectin (in the basal lamina of the eye), and laminin. On the cytoplasmic side, integrins bind to talin and α-actinin, two proteins that connect to actin microfilaments. This dual binding enables the cell to move by using myosin to contract the actin microfilaments against the fixed extracellular matrix.

Integuments@@An outer protective layer such as the skin or the seed coats of plants.

Intercalation@@The merging movement of cells from two (or more) adjacent populations into fewer (or one) layers.

Interkinetic nuclear migration@@The movement of nuclei within certain cells as they go through the cell cycle; seen in the germinal neuroepithelium in which nuclei translocate from the basal end to the apical end of the cells near the ventricular surface, where they undergo mitosis, after which they slowly migrate basally again.

Intermediate mesoderm@@Mesoderm immediately lateral to the paraxial mesoderm. It forms the outer (cortical) portion of the adrenal gland and the urogenital system, consisting of the kidneys, gonads, and their associated ducts.

Intermediate progenitor cells (IP cells)@@Neuron precursor cells of the subventricular zone; derived from radial glial cells.

Internal granular layer@@A layer in the cerebellum that is formed by the migration of granule cells from the external granular layer back toward the ventricular zone.

Interstitial stem cell@@A type of stem cell found within the ectodermal layer of Hydra that generates neurons, secretory cells, nematocytes, and gametes.

Intestinal stem cell (ISC) niche@@A microenvironment of cells within the crypt of the intestinal villius that houses the intestinal stem cells and functions to generate all the differentiated cells types of the epithelium.

Intine@@The inner wall of a pollen grain, composed mostly of cellulose.

Intramembranous bones@@Bones formed by intramembranous ossification.

Introns@@Non-protein-coding regions of DNA within a gene. Compare with Exon.

Invagination@@The infolding of a region of cells, much like the indenting of a soft rubber ball when it is poked.

Invariant cell lineage@@When each cell of an embryo gives rise to the same number and type of cells in every embryo of that species, as seen in the embryos of the roundworm Caenorhabditis elegans.

Involuting marginal zone (IMZ)@@Cells that involute during Xenopus gastrulation, includes precursors of the pharyngeal endoderm, head mesoderm, notochord, somites, and heart, kidney, and ventral mesoderm.

Involution@@Inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.

Isolecithal@@Greek, “equal yolk.” Describes eggs with sparse, equally distributed yolk particles, as in sea urchins, mammals, and snails.

Izumo@@A protein on the equatorial region of mature mammalian sperm that binds to Juno, on the cell membrane of the oocyte. These proteins help stabilize sperm-egg binding.

J

JAK@@Janus kinase proteins. Linked to FGF receptors in the JAK-STAT cascade.

Juno@@A protein found anchored to the mammalian oocyte cell membrane that is critical in binding to Izumo, a protein on mammalian sperm.

Juvenile hormone (JH)@@A lipid hormone in insects that prevents the ecdysone-induced changes in gene expression that are necessary for metamorphosis. Thus, its presence during a molt ensures that the result of that molt is another larval instar, not a pupa or an adult.

Juxtacrine interactions@@When cell membrane proteins on one cell surface interact with receptor proteins on adjacent (juxtaposed) cell surfaces.

Juxtacrine signaling@@Signaling between cells that are juxtaposed, i.e., in direct contact with one another.

K

Keratinocytes@@Differentiated epidermal cells that are bound tightly together and produce a water-impermeable seal of lipid and protein.

Koller’s sickle@@See Primitive streak.

Kozak consensus sequence@@A short sequence of nucleic acids that represent a common motif that serves to signal to the ribosome to initiate translation.

Kupffer’s vesicle@@Transient fluid-filled organ housing the cilia that control left-right asymmetry in zebrafish.

L

Labioscrotal folds@@Folds surrounding the cloacal membrane in the indifferent stage of differentiation of mammalian external genitalia. They will form the labia majora in the female and the scrotum in the male. Also called urethral folds or genital swellings.

Laminae@@Latin for “layers.” In the brain, neurons are organized into laminae and clusters of cells called nuclei (not to be confused with the nucleus of a single cell).

Laminar identity@@The six distinct layers of the cortex of the human brain; each layer houses a different and distinctive population of neurons.

Laminin@@A large glycoprotein and major component of the basal lamina, plays a role in assembling the extracellular matrix, promoting cell adhesion and growth, changing cell shape, and permitting cell migration.

Larva@@The sexually immature stage of an organism, often of significantly different appearance than the adult and frequently the stage that lives the longest and is used for feeding or dispersal.

Larval settlement@@Ability of marine larvae to suspend development until they experience a particular environmental cue for settlement.

Laryngotracheal groove@@An outpouching of endodermal epithelium in the center of the pharyngeal floor, between the fourth pair of pharyngeal pouches, that extends ventrally. The laryngotracheal groove then bifurcates into the branches that form the paired bronchi and lungs.

Last eukaryotic common ancestor (LECA)@@The hypothetical ancestor to all plants, animals and fungi, thought to have been a unicellular protist that had flagella and mitochondria.

Last universal common ancestor (LUCA)@@The ancient common ancestor to all life on Earth.

Lateral inhibition@@The inhibition of a cell by the activity of a neighboring cell.

Lateral line placodes@@Paired ectodermal placodes that form as posterior cranial placodes in amphibians and fish. They generate neuromasts, which contain mechanosensing hair cells and their innervating neurons; function to detect the flow of water, weak bioelectric fields, and changes in pressure. See Hair cells.

Lateral plate mesoderm@@Mesodermal sheet lateral to the intermediate mesoderm. Gives rise to appendicular bones, connective tissues of the limb buds, circulatory system (heart, blood vessels, and blood cells), muscles and connective tissues of the digestive and respiratory tracts, and lining of coelom and its derivatives. It also helps form a series of extraembryonic membranes that are important for transporting nutrients to the embryo.

Lateral somitic frontier@@The boundary between the primaxial and abaxial muscles and between the somite-derived and lateral plate-derived dermis.

<PROD: Note prime symbol in following XR2>

Leader sequence@@See 5′ Untranslated region.

Leghemoglobin@@An oxygen-carrying protein, similar to hemoglobin, found in the nitrogen-fixing root nodules of legumes. It is a plant protein whose genes are induced by symbiotic bacteria. This protein protects the nitrogenase enzyme from being inactivated by oxygen.

Lens placode@@Paired epidermal thickenings induced by the underlying optic cups to invaginate to form the lens vesicles, which differentiate into the adult transparent eye lenses that allow light to impinge on the retinas.

Ligand@@A molecule secreted by a cell that elicits a response in another cell by binding to a receptor on that cell.

Limb bud@@A circular bulge that will form the future limb. The limb bud is formed by the proliferation of mesenchyme cells from the somatic layer of the limb field lateral plate mesoderm (the limb skeletal precursor cells) and from the somites (the limb muscle precursor cells).

Limb field@@An area of the embryo containing all of the cells capable of forming a limb.

Lineage tracing@@Tracking the development of cell maturation over time. Embryonic cells can be labeled and tracked to see what they become in the larva or adult organism; this allows the development of a fate map. See Fate map.

Lophotrochozoans@@One of two major protostome groups, many of which are characterized spiral cleavage and by the larval form known as the trochophore. A diverse group that includes the annelids (segmented worms such as earthworms), molluscs (e.g., snails), and flatworms (e.g., Planaria). See also Ecdysozoans.

Low CpG-content promoters (LCPs)@@These promoters are usually found in the genes whose products characterize mature, fully differentiated cells. The CpG sites are usually methylated and their default state is “off,” although they can be activated by specific transcription factors.

Luteinizing hormone (LH)@@A hormone secreted by the mammalian pituitary that stimulates the production of steroid hormones, such as estrogen from the ovarian follicle cells and testosterone from the testicular Leydig cells. A surge in LH levels causes the primary oocyte to complete meiosis I and prepares the follicle for ovulation.

 

M

Macromeres@@Larger cells generated by asymmetrical cleavage, e.g., the four large cells generated by the fourth cleavage when the vegetal tier of the sea urchin embryo undergoes an unequal equatorial cleavage.

MADS-box transcription factors@@A family of proteins that all share a conserved motif in their DNA-binding domains; they are found in diverse groups of eukaryotes.

Malformation@@Abnormalities caused by genetic events such as gene mutations, chromosomal aneuploidies, and translocations.

Mammalian gut-associated lymphoid tissue (GALT)@@Lymphoid tissue that mediates mucosal immunity and oral immune tolerance, allowing mammals to eat food without creating an immune response to it. Intestinal microbes are critical for the maturation of GALT.

Mantle zone (intermediate zone)@@Second layer of the developing spinal cord and medulla that forms around the original neural tube. Because it contains neuronal cell bodies and has a grayish appearance grossly, it will form the gray matter.

Marginal zone@@(1) The third and outer zone of the developing spinal cord and medulla composed of a cell-poor region composed of axons extending from neurons residing in the mantle zone. Will form the white matter as glial cells cover the axons with myelin sheaths, which have a whitish appearance. (2) In amphibian gastrula, where gastrulation begins, the region surrounding the equator of the blastula, where the animal and vegetal hemispheres meet. (3) In bird and reptile gastrulae (= marginal belt), a thin layer of cells between the area pellucida and the area opaca, important in determining cell fate during early development.  

Maternal contributions@@The stored mRNAs and proteins within the cytoplasm of the egg, produced from the maternal genome during the primary oocyte stage. See also Maternal-to-zygotic transition.

Maternal effect@@An effect occurring during embryonic development that is controlled by gene products that were stored in the egg while it was in the ovary; these gene products were made using the maternal genome through gene transcription within the egg itself, prior to its undergoing meiosis, or within nurse cells that transport gene products into the egg.

Maternal effect genes@@Genes belonging to the maternal genome that are used to make messenger RNAs or proteins that are localized to different regions of an egg and affect embryonic development, as seen in the Drosophila egg. See Maternal effect.

Maternal-to-zygotic transition (MZT)@@The moment in embryonic time when control of gene expression shifts from being derived from stored mRNAs within the oocyte (maternal contributions) to being controlled by new transcription from the zygotic genome. Seen in many different animal groups. Sometimes called the oocyte-to-embryo transition.

Mechanical anisotropy@@Having a difference in a mechanical property, such as stretchability, along different axes.

Medial hinge point (MHP)@@In birds and mammals, formed by the cells at the midline of the neural plate. MHP cells become anchored to the notochord beneath them and form a hinge, which forms a furrow at the dorsal midline and helps bend the neural plate as it forms a neural tube.

Medulla@@The posterior region of the hindbrain. Can also refer to the inner portion of a structure.

Megasporangium@@The structure in which megaspores form.

Meiosis@@A unique division process that in animals occurs only in germ cells, to reduce the number of chromosomes to a haploid complement. All other cells divide by mitosis. Meiosis differs from mitosis in that (1) meiotic cells undergo two cell divisions without an intervening period of DNA replication, and (2) homologous chromosomes (each consisting of two sister chromatids joined at a kinetochore) pair together and recombine genetic material.

Melanocyte stem cells@@Adult stem cells derived from trunk neural crest cells that form melanoblasts and come to reside in the bulge niche of the hair or feather follicle and which give rise to the pigment of the skin, hair, and feathers.

Menstrual cycle@@In human females, the cyclical monthly hormonal changes that coordinate ovulation of the oocyte with the receptivity of the uterus for the developing embryo.

Meristem@@Tissue in plants containing undifferentiated and actively dividing cells. This is where production of new plant tissue occurs. Different types of meristems give rise to different structures of the plant. The two main meristems are the shoot and root apical meristems.

Meroblastic cleavage@@Greek meros, “part.” Refers to the cell division (cleavage) pattern in zygotes containing large amounts of yolk, wherein only a portion of the cytoplasm is cleaved. The cleavage furrow does not penetrate the yolky portion of the cytoplasm because the yolk platelets impede membrane formation there. Only part of the egg is destined to become the embryo, while the other portion—the yolk—serves as nutrition for the embryo, as in insects, fish, reptiles, and birds.

Mesencephalon@@The midbrain, the middle vesicle of the developing vertebrate brain; major derivatives include optic tectum and tegmentum. Its lumen becomes the cerebral aqueduct.

Mesenchymal cells@@Unconnected or loosely connected cells that can act as independent migratory units. Compare with Epithelial cells.

Mesenchymal stem cells (MSCs)@@Also called bone marrow-derived stem cells, or BMDCs. Multipotent stem cells that originate in the bone marrow, MSCs are able to give rise to numerous bone, cartilage, muscle, and fat lineages.

Mesendoderm@@The combined mesoderm and endoderm populations.

Mesentoblast@@In snail embryos, the 4d blastomere whose progeny give rise to most of the mesodermal (heart, kidney and muscles) and endodermal (gut tube) structures.

Mesoderm@@Greek mesos, “between.” The middle of the three embryonic germ layers, lying between the ectoderm and the endoderm. The mesoderm gives rise to muscles and skeleton, connective tissue, the urogenital system (kidneys, gonads, and ducts), blood and blood vessels, and most of the heart.

Mesodermal mantle@@The cells that involute through the ventral and lateral blastopore lips during amphibian gastrulation and will form the heart, kidneys, bones, and parts of several other organs.

Mesomeres@@The eight cells generated in the sea urchin embryo by the fourth cleavage when the four cells of the animal tier divide meridionally into eight blastomeres, each with the same volume.

Mesonephric kidney (mesonephros)@@The second kidney of the amniote embryo, induced in the adjacent mesenchyme by the middle portion of the nephric (Wolffian) duct. It functions briefly in urine filtration in some mammalian species, and mesonephric tubules form the tubes that transport sperm from the testes to the urethra (the epididymis and vas deferens). Forms the adult kidney of anamniotes (fish and amphibians). Compare with Pronephros.

Mesp1@@A gene for a transcription factor that is critical in the regulatory network that specifies heart cells. These networks are conserved across vertebrates. See also Nkx2-5.

Messenger RNA (mRNA)@@RNA that codes for a protein and leaves the nucleus after being processed from nuclear RNA in a manner that excises non-coding domains and protects the ends of the strand.

Metamorphic climax@@When the major metamorphic changes, such as tail and gill resorption, and intestinal remodeling, occur in the amphibian. The concentration of T4 rises dramatically and TRβ levels peak.

Metamorphic molt@@Pupal molt; in holometabolous insects, the molt at the end of the final instar stage, when the larva becomes a pupa.

Metamorphosis@@Changing from one form to another, such as the transformation of an insect larva to a sexually mature adult or a tadpole to a frog.

Metanephric mesenchyme@@An area of mesenchyme, derived from posterior regions of the intermediate mesoderm, involved in mesenchymal-epithelial interactions that generate the metanephric kidney and will form the secretory nephrons. Also called metanephrogenic mesenchyme.

Micromeres@@Small cells created by asymmetrical cleavage, e.g., four small cells generated by the fourth cleavage at the vegetal pole when the vegetal tier of the sea urchin embryo undergoes an unequal equatorial cleavage.

Micropyle@@The only place where Drosophila sperm can enter the egg, at the future dorsal anterior region of the embryo, a tunnel in the chorion (eggshell) that allows sperm to pass through it one at a time.

MicroRNAs (miRNAs)@@Small (about 22 nucleotide) RNAs complementary to a portion of a particular mRNA that regulates translation of a specific message. MicroRNAs usually bind to the 3′ UTR of mRNAs and inhibit their translation.

Microspikes@@Essential for neuronal pathfinding, microfilament-containing pointed filopodia of the growth cone that elongate and contract to allow axonal migration. Microspikes also sample the microenvironment and send signals back to the soma.

Microsporangia@@The places within the anther where the microspores are produced.

Mid-blastula transition (MBT)@@The transition from the early rapid biphasic (only M and S phases) mitoses of the embryo to a stage characterized by (1) mitoses that include the “gap” stages (G1 and G2) of the cell cycle, (2) loss of synchronicity of cell division, and (3) transcription of new (zygotic) mRNAs needed for gastrulation and cell specification.

Midbrain@@The middle vesicle of the developing vertebrate brain; major derivatives include optic tectum and tegmentum. Its lumen becomes the cerebral aqueduct. Also called the Mesencephalon.

MITF@@Microphthalmia-associated transcription factor. A transcription factor necessary for melanoblast specification and pigment production. Its name comes from the fact that a mutation in the gene for this transcription factor causes small eyes (microphthalmia) in mice.

Mitosis-promoting factor (MPF)@@Consists of cyclin B and a cyclin-dependent kinase (CDK), required to initiate entry into the mitotic (M) phase of the cell cycle in both meiosis and mitosis.

Modularity@@A principle of the theoretical systems approach. The organism develops as a system of discrete and interacting modules.

Molecular parsimony@@The principle that development in all lineages uses the same types of molecules (the “small toolkit”). The “toolkit” includes transcription factors, paracrine factors, adhesion molecules, and signal transduction cascades that are remarkably similar from one phylum to another.

Monospermy@@Only one sperm enters the egg, and a haploid sperm nucleus and a haploid egg nucleus combine to form the diploid nucleus of the fertilized egg (zygote), thus restoring the chromosome number appropriate for the species.

Morph@@One of several different potential phenotypes that result from environmental conditions. Also called an ecomorph.

Morphallaxis@@Type of regeneration that occurs through the repatterning of existing tissues with little new growth (e.g., Hydra).

Morphogenesis@@The organization of the cells of the body into functional structures via coordinated cell growth, cell migration, and cell death.

Morphogens@@Greek, “form-givers.” Diffusible biochemical molecules that can determine the fate of a cell by their concentrations, in that cells exposed to high levels of a morphogen will activate different genes than those exposed to lower levels.

Morpholino@@An antisense oligonucleotide against an mRNA; used to experimentally inhibit protein expression.

Morula@@Latin, “mulberry.” Vertebrate embryo of 16–64 cells; precedes the blastula or blastocyst stage. Mammalian morula occurs at the 16-cell stage, consists of a small group of internal cells (that will form the inner cell mass) surrounded by a larger group of external cells (that will form the trophoblast).

Multicellularity@@Consisting of multiple cells.

Multipotent@@Refers to the ability of a stem cell to generate different cell types with restricted specificity for the tissue in which they reside. Example: Most adult stem cells in organs of animals are multipotent.

Multipotent cardiac precursor cells@@Precursor cells of the heart field that form cardiomyocytes, endocardium, epicardium, and the Purkinje fibers of the heart.

Mutualism@@A form of symbiosis in which the relationship benefits both partners.

Mycorrhizae@@Fungi that form symbiotic relationships with plants to extend their roots. While the plant supplies the fungus with sugars, the fungus absorbs water and mineral nutrients from the soil.

Myoblasts@@Muscle precursor cells.

Myoepithelia@@Epithelia whose cells possess characteristics of both epithelial and muscle cells, e.g., the two epithelial layers of Hydra.

Myogenic regulatory factors (MRFs)@@Basic helix-loop-helix transcription factors (such as MyoD, Myf5 and myogenin) that are critical regulators of muscle development.

Myotome@@Portion of the somite that gives rise to all skeletal muscles of the vertebrate body except for those in the head. The myotome has two components: the primaxial component, closest to the neural tube, which forms the musculature of the back and rib cage, and the abaxial component, away from the neural tube, which forms the muscles of the limbs and ventral body wall.

N

N-cadherin@@A type of cadherin that is highly expressed on cells of the developing central nervous system (the N stands for Neural). May play roles in mediating neural signals. See also Cadherins.

Naïve@@Unaffected, lacking experience.

Naïve ESC@@An embryonic stem cell (ESC) that is the most immature, undifferentiated ESC with the greatest potential for pluripotency. Compare with Primed ESC.

Neocortex@@A layer of gray matter in the cerebrum that is a distinguishing feature of the mammalian brain; it stratifies into six layers of neuronal cell bodies, each with different functional properties.

Nephric duct@@See Wolffian duct.

Nephron@@Functional unit of the kidney.

Netrin-1@@Paracrine factor found in a gradient that guide axonal growth cones. Netrins are important in commissural axon migration and retinal axon migration. Netrin-1 is secreted by the floor plate; netrin-2 is secreted by the lower region of the spinal cord.

Netrin-2@@Paracrine factor found in a gradient that guide axonal growth cones. Netrins are important in commissural axon migration and retinal axon migration. Netrin-1 is secreted by the floor plate; netrin-2 is secreted by the lower region of the spinal cord.

Neural crest@@A transient band of cells, arising from the lateral edges of the neural plate, that joins the neural tube to the epidermis. It gives rise to a cell population—the neural crest cells—that detach during formation of the neural tube and migrate to form a variety of cell types and structures, including sensory neurons, enteric neurons, glia, pigment cells, and (in the head) bone and cartilage.  

Neural crest specifiers@@A set of transcription factors (e.g., FoxD3, Sox9, Id, Twist, and Snail) induced by the neural plate border-specifying transcription factors, that specify the cells that are to become the neural crest.

Neural folds@@Thickened edges of the neural plate that move upward during neurulation and migrate toward the midline and eventually fuse to form the neural tube.

Neural groove@@U-shaped groove that forms in the center of the neural plate during primary neurulation.

Neural keel@@A band of neural precursor cells that are brought into the dorsal midline during convergence and extension movements in the epiblast of the fish embryo. It extends over the axial and paraxial mesoderm and eventually forms a rod of tissue that separates from the epidermal ectoderm and develops a slit-like lumen to become the neural tube.

Neural plate border specifiers@@A set of transcription factors (e.g., Distalless-5, Pax3, and Pax7), induced by the neural plate inductive signals, that collectively confer upon the border region the ability to form neural crest and dorsal neural tube cell types. Induce expression of neural crest specifiers.

Neural plate@@The region of the dorsal ectoderm that is specified to be neural ectoderm. It later folds upward to become the neural tube.

Neural restrictive silencer element (NRSE)@@A regulatory DNA sequence found in several mouse genes that prevents a promoter’s activation in any tissue except neurons, limiting the expression of these genes to the nervous system.

Neural restrictive silencer factor (NRSF)@@A zinc finger transcription factor that binds the NRSE and is expressed in every cell that is not a mature neuron.

Neural retina@@Derived from the inner layer of the optic cup, composed of a layered array of cells that include the light and color-sensitive photoreceptor cells (rods and cones), the cell bodies of the ganglion cells, bipolar interneurons that transmit electric stimuli from the rods and cones to the ganglion cells, Müller glial cells that maintain its integrity, amacrine neurons (which lack large axons), and horizontal neurons that transmit electric impulses in the plane of the retina.

Neural stem cells (NSCs)@@Stem cells of the central nervous system capable of neurogenesis throughout life. In vertebrates, NSCs retain much of the characteristics of their embryonic progenitor cell, the radial glial cell.

Neural tube@@The embryonic precursor to the central nervous system (brain and spinal cord).

Neural tube defects (NTDs)@@Malformations during embryonic development that result in a failure of some portion of the neural tube to close, NTDs include anancephaly (lack of closure of the forebrain) and spina bifida (lack of closure of the spinal cord.

Neurocranium@@The vault and base of the skull.

Neuromesoderm progenitors (NMPs)@@In vertebrate embryos, a population of multipotent progenitor cells with the potential to contribute to both the neural tube and paraxial (somitic) mesoderm, found in the posteriormost region (caudal end) of the embryo.

Neurons@@Nerve cells; cells specialized for the conduction and transmission of information via electrical and chemical signals.

Neuropores@@The two open ends (anterior neuropore and posterior neuropore) of the neural tube that later close.

Neurotransmitters@@Molecules (e.g., acetylcholine, GABA, serotonin) secreted at the ends of axons. These molecules cross the synaptic cleft and are received by the adjacent neuron, thus relaying the neural signal. See Synapse.

Neurotrophins/neurotropins@@Neurotrophic (Greek, “nourishing”) refers to a factor’s ability to keep the neuron alive, usually by supplying growth factors. Neurotropic (Latin, “turning”) refers to a substance that attracts or repulses neurons. Because many factors have both properties, both terms are used; in the recent literature, neurotrophin appears to be preferred.

Neurula@@Refers to an embryo during neurulation (i.e., while the neural tube is forming).

Neurulation@@Process of folding of the neural plate and closing of the cranial and caudal neuropores to form the neural tube.

Nieuwkoop center@@The dorsalmost vegetal blastomeres of the amphibian blastula, formed as a consequence of the cortical rotation initiated by the sperm entry; an important signaling center on the dorsal side of the embryo. One of its main functions is to induce the Organizer.

Nkx2-5@@ A transcription factor that is critical in the regulatory network that specifies the heart cells. These networks are conserved across vertebrates. See also Mesp1.

Nodal@@A paracrine factor and member of the TGF-β family involved in establishing left-right asymmetry in vertebrates and invertebrates.

Node@@The mammalian homologue of Hensen’s node.

Non-skeletogenic mesenchyme@@Formed from the veg2 layer of the 60-cell sea urchin embryo, it generates pigment cells, immunocytes, and muscle cells. Also called secondary mesenchyme.

Notch protein@@Transmembrane protein that is a receptor for Delta, Jagged, or Serrate, participants in juxtacrine interactions. Ligand binding causes Notch to undergo a conformational change that enables a part of its cytoplasmic domain to be cut off by the presenilin-1 protease. The cleaved portion enters the nucleus and binds to a dormant transcription factor of the CSL family. When bound to the Notch protein, the CSL transcription factors activate their target genes.

Notochord@@A transient mesodermal rod in the most dorsal portion of the embryo that plays an important role in inducing and patterning the nervous system. Characteristic feature of chordates.  

Nucleosome@@The basic unit of chromatin structure, composed of an octamer of histone proteins (two molecules each of histones H2A, H2B, H3, and H4) wrapped with two loops containing approximately 147 base pairs of DNA.

Nucleus (pl. nuclei)@@(1) The membrane-enclosed organelle housing the eukaryotic chromosomes. (2) An organized cluster of the cell bodies of neurons in the brain with specific functions and connections.

Nurse cells@@Cells that provides nourishment to a developing egg. In Drosophila ovarioles, fifteen interconnected nurse cells generate mRNAs and proteins that are transported to a single developing oocyte.

O

Obligate mutualism@@Symbiosis in which the species involved are interdependent with one another to such an extent that neither partner could survive without the other.

Olfactory placode@@Paired epidermal thickening that forms the nasal epithelium (smell receptors) as well as the ganglia for the olfactory nerves.

Oocyte@@A developing egg. A primary oocyte is in a stage of growth, has not gone through meiosis, and has a diploid nucleus. A secondary oocyte has completed its first meiotic division but not the second, and is haploid.

Oocyte-to-embryo transition@@See Maternal-to-zygote transition.

Oogenesis@@The development of the egg (ovum), including meiotic divisions and maturation.

Oogonia (sing. oogonium)@@In animals, the female germ line cells that divide mitotically to become oocytes.

Optic nerve@@Cranial nerve (CN II) that forms from axons of the neural retina that grow back to the brain by traveling down the optic stalk.

Optic vesicle@@Extend from the diencephalon and activate the head ectoderm’s latent lens-forming ability.

Oral plate@@A region where the ectoderm of the stomodeum meets the endoderm of the primitive gut. It later breaks open to form the oral opening.

Organ of Corti@@The receptor organ in the cochlea of the inner ear. It contains hair cells that reside in a fluid-filled chamber. Pressure waves from the movement of fluid in this chamber are transformed by the hair cells into action potentials that are sent to the brain by the auditory nerve and interpreted as sound.

Organizer@@In amphibians, the dorsal lip cells of the blastopore and their derivatives (notochord and head endomesoderm). Functionally equivalent to Hensen’s node in chick, the node in mammals, and the shield in fish. Organizer action establishes the basic body plan of the early embryo. Also known as the Spemann Organizer or (more correctly) the Spemann-Mangold organizer.

Organogenesis@@Interactions between, and rearrangement of, cells of the three germ layers to produce tissues and organs.

Organoids@@Rudimentary organs, usually the size of a pea, grown in culture from pluripotent stem cells.

Orthologues@@Genes from different species that are similar in DNA sequence because those genes were inherited from a common ancestor. Compare with Paralogues.

Otic cup@@The structure formed during the morphogenesis of the inner ear when the otic placode invaginates to the point of forming the shape of a cup. This stage of inner ear development comes after the otic pit stage. Once the edges of the otic cup come together and fuse, the structure is called the otic vesicle (otocyst).

Otic pit@@The structure formed during the morphogenesis of the inner ear when the otic placode starts to invaginate, creating an indentation.

Otic placode@@Paired epidermal thickening that invaginates to form the inner ear labyrinth, whose neurons form the acoustic ganglia that enable us to hear.

Otic vesicle@@Also known as the auditory vesicle, this neural ectodermal structure forms by closing otic cup into a sac, and will ultimately develop into the structures of the inner ear.

Outer radial glia (oRG)@@Progenitor cells that reside in the subventricular zone of the cerebrum and give rise to intermediate progenitor (IP) cells.

Ovarian follicles@@Fluid-filled tissue, made of follicle cells, wherein an oocyte develops prior to ovulation.

Ovary@@The structure that produces an ovum, the female gamete. In mammals, there is a pair of ovaries in the abdomen, which propel eggs into the oviducts. In angiosperms, the ovary is part of the carpel that contains the ovule(s).

Ovastacin@@A protease released by the cortical granules of mammalian eggs after fertilization to digest ZP2 and thereby prevent further sperm from entering the egg.

Oviducts@@The Fallopian tubes (uterine tubes) that connect the uterus with the ovary. They capacitate sperm and brings the early embryo into the uterus.

Oviparity@@Young hatch from eggs ejected by the mother, as in birds, amphibians, and most invertebrates.

Ovoviviparity@@Young hatch from eggs held within the mother’s body where they continue to develop for a period of time, as in certain reptiles and sharks. Compare with Viviparity.

Ovulation@@Release of the egg from the ovary.

Ovules@@In angiosperms, the structure comprising the megasporangium and the integument, which, after fertilization, develops into a seed.

Ovum@@The mature egg (at the stage of meiosis at which it is fertilized). Plural, ova.

P

P-cadherin@@A type of cadherin found predominantly on the placenta, where it helps the placenta stick to the uterus (the P stands for placenta). See also Cadherins.

Pair-rule genes@@Drosophila zygotic genes, regulated by gap gene proteins. Pair-rule genes are each expressed in seven stripes that divide the embryo into transverse bands perpendicular to the anterior-posterior axis. Pair-rule mutants lack portions of every other segment.

PAR proteins@@Found in the cytoplasm of oocytes of the nematode C. elegans; involved in determining the anterior-posterior axis of the embryo following fertilization.

Paracrine factor@@A secreted, diffusible protein that provides a signal that interacts with and changes the cellular behavior of neighboring cells and tissues.

Paracrine interaction@@An interaction whereby proteins synthesized by one cell diffuse over a distance to induce changes in neighboring cells.

Paracrine signaling@@Signaling between cells that occurs across long distances through the secretion of paracrine factors into the extracellular matrix.

Paralogues@@Genes that are similar in sequence because they are the result of gene duplication events in an ancestral species. Compare with Orthologues.

Parasegments@@“Transegmental” units in Drosophila that include the posterior compartment of one segment and the anterior compartment of the immediately posterior segment; appear to be the fundamental units of embryonic gene expression.

Parasitism@@Type of symbiosis in which one partner benefits at the expense of the other.

Parasympathetic (enteric) ganglia@@Ganglia of the parasympathetic (“rest and digest”) nervous system derived from vagal and sacral neural crest cells.

Paraxial mesoderm (somitic mesoderm)@@Thick bands of embryonic mesoderm immediately adjacent to the neural tube and notochord. In the trunk, paraxial mesoderm gives rise to somites, in the head it (along with the neural crest) gives rise to the skeleton, connective tissues and musculature of the face and skull. Not to be confused with somatic (parietal) mesoderm.

Parietal mesoderm (somatic mesoderm)@@Derived from lateral mesoderm closest to the ectoderm (dorsal) and separated from other components of lateral mesoderm (splanchnic, near endoderm, ventral) by the intraembryonic coelom. Together with the overlying ectoderm, the somatic mesoderm comprises the somatopleure, which will form the body wall. The somatic mesoderm also forms part of the lining of the coelom. Not to be confused with somitic (paraxial) mesoderm.

Pattern formation@@The set of processes by which embryonic cells form ordered spatial arrangements of differentiated tissues.

Penis@@The sensitive erectile and muscular organ of the male external genitalia that ejaculates sperm and eliminates liquid waste.

Peri-implantation@@In placental mammals, the embryonic period from when the blastocyst is free in the uterus through its first interactions with the uterine endometrium.

Pericardial cavity@@The division of the coelom that surrounds the heart.

Periclinal divisions@@In plants, cell divisions in which the new cell walls are laid down parallel to the surface of the plant. Compare with Anticlinal divisions.

Pericycle cells@@A layer of cells in the roots of plants, located between the endodermis and vascular tissue. It contains adult stem cells that can form new root apical meristems to grow lateral roots.

Pericytes@@Smooth-musclelike cells recruited to cover endothelial cells during vasculogenesis.

Periderm@@A temporary epidermis-like covering in the embryo that is shed once the inner layer differentiates to form a true epidermis.

Peripheral nervous system (PNS)@@All the nerves and neurons lying outside the CNS (central nervous system; brain and spinal cord).

Peritoneal cavity@@The division of the coelom that encloses the abdominal organs. Compare with Pericardial cavity; Pleural cavity.

Permissive interaction@@Inductive interaction in which the responding tissue has already been specified, and needs only an environment that allows the expression of these traits.

Petals@@In an angiosperm flower, a non-sexual, non-photosynthetic, modified leaf. These are frequently brightly colored and can attract pollinating insects to the flower.

Pharyngeal arches@@Paired bars of mesenchymal tissue (derived from paraxial mesoderm, lateral plate mesoderm, and neural crest cells), covered by endoderm internally and ectoderm externally. Found near the pharynx of the vertebrate embryo, the arches form gill supports in fish, and many skeletal and connective tissue structures in the face, jaw, mouth, and larynx in other vertebrates. Also called branchial arches.

Pharyngeal pouches@@Inside the pharynx, these are where the pharyngeal epithelium (endoderm) pushes out laterally to form pairs of pouches between the pharyngeal arches. These give rise to the auditory tube, wall of the tonsil, thymus gland, parathyroids and thyroid.

Pharynx@@The region of the digestive tube anterior to the point at which the respiratory tube branches off.

Phenology@@The timing of cyclical or seasonal events in the life cycles of many species. Such events include migration, flowering, and hibernation.

Phenotypic heterogeneity@@Refers to the same mutation producing different phenotypes in different individuals.

Phenotypic plasticity@@The ability of an organism to react to an environmental input with a change in form, state, movement, or rate of activity.

Phloem@@In vascular plants, the conduits that carry sugars produced by photosynthesis, along with other metabolites, from sources to sinks—primarily from the leaves to the nonphotosynthetic parts of the plant.

Phyllotaxis@@The arrangement of leaves along a plant stem.

Phylotypic stage@@The stage that typifies a phylum, such as the late neurula or pharyngula of vertebrates, and which appears to be relatively invariant and to constrain its evolution.

Pigmented epithelium@@The melanin-containing layer of the vertebrate eye that lies behind the neural retina and forms from the outer layer of the optic cup. Its black melanin pigment absorbs light coming through the neural retina. Also referred to as the pigmented retina.

PIN auxin transport proteins@@Transmembrane proteins that serve as efflux transporters that shuttle the hormone Auxin unidirectionally out of a cell.

Pioneer nerve fibers@@Axons that go ahead of other axons and serve as guides for them.

Pioneer transcription factors@@Transcription factors (e.g., Fox A1 and Pax7) that can penetrate repressed chromatin and bind to their enhancer DNA sequences, a step critical to establishing certain cell lineages.

Pistil@@See Carpels.

Placenta@@The organ in placental mammals that serves as the interface between fetal and maternal circulations and has endocrine, immune, nutritive and respiratory functions. It consists of a maternal portion (the uterine endometrium, or decidua, which is modified during pregnancy) and a fetal component (the chorion).

Placodes@@An area of ectodermal thickening. These include the cranial placodes (e.g., the olfactory, lens, and otic placodes); and the epidermal placodes of cutaneous appendages such as hair and feathers, which are formed via inductive interactions between the dermal mesenchyme and the ectodermal epithelium.

Plasmodesmata@@Cytoplasmic channels that form between adjacent plant cells, allowing for direct transport of substances between the cells.

PLCζ (Phospholipase C zeta)@@A soluble form of phospholipase C found in the head of mammalian sperm that is released during gamete fusion in fertilization. It sets off the IP₃ pathway in the egg that results in Ca²⁺ release and activation of the egg.

Pleiotropy@@The production of several effects by one gene or pair of genes.

Pleural cavity@@The division of the coelom that surrounds the lungs. Compare with Pericardial cavity; Peritoneal cavity.

Pluripotent@@Latin, “capable of many things.” A single pluripotent stem cell has the ability to give rise to different types of cells that develop from the three germ layers (mesoderm, endoderm, ectoderm) from which all the cells of the body arise. The cells of the mammalian inner cell mass (ICM) are pluripotent, as are embryonic stem cells. Each of these cells can generate any cell type in the body, but because the distinction between ICM and trophoblast has been established, it is thought that ICM cells are not able to form the trophoblast. Germ cells and germ cell tumors (such as teratocarcinomas) can also form pluripotent stem cells. Compare with Totipotent.

Pluteus larva@@Type of larva found in sea urchins and brittle stars; a planktonic larva that is bilaterally symmetrical, ciliated, and has long arms supported by skeletal spicules.

Polar lobe@@An anucleate bulb of cytoplasm extruded immediately before first cleavage, and sometimes before the second cleavage, in certain spirally cleaving embryos (mostly in the mollusc and annelid phyla). It contains the determinants for the proper cleavage rhythm and the cleavage orientation of the D blastomere.

Pole cells@@About five nuclei in the Drosophila embryo that reach the surface of the posterior pole during the ninth division cycle and become enclosed by cell membranes. The pole cells give rise to the gametes of the adult.

Pollen grains@@Male gametes of seed-bearing plants (gymnosperms and angiosperms) that are transported to the plant’s female structures by different means (e.g., wind, water, pollinating animals)

Pollination@@The process by which pollen grains are transferred from the male anther of the flower to the female stigma of that flower or another flower.

PolyA tail@@A series of adenine (A) residues that are added by enzymes to the 3′ terminus of the mRNA transcript in the nucleus. The polyA tail confers stability on the mRNA, allows the mRNA to exit the nucleus, and permits the mRNA to be translated into protein.

Polyadenylation@@The insertion of a “tail” of some 200–300 adenylate residues on the RNA transcript, about 20 bases downstream of the AAUAAA sequence. This polyA tail (1) confers stability on the mRNA, (2) allows the mRNA to exit the nucleus, and (3) permits the mRNA to be translated into protein.

Polycomb@@Family of proteins that bind to condensed nucleosomes, keeping the genes in an inactive state.

Polyphenism@@A type of phenotypic plasticity, refers to discontinuous (“either/or”) phenotypes elicited by the environment. Compare with Reaction norm.

Polyspermy@@The entrance of more than one sperm during fertilization resulting in aneuploidy (abnormal chromosome number) and either death or abnormal development. An exception, called physiological polyspermy, occurs in some organisms such as Drosophila and birds, where multiple sperm enter the egg but only one sperm pronucleus fuses with the egg pronucleus.

Polytene chromosomes@@Chromosomes in the larval cells of Drosophila (but not the imaginal cells that give rise to the adult) in which the DNA undergoes many rounds of replication without separation, forming large “puffs” that are easily visible and indicate active gene transcription.

Population asymmetry@@Mode of maintaining homeostasis in a population of stem cells in which some of the cells are more prone to producing differentiated progeny, while others divide to maintain the stem cell pool.

Positional control genes (PCGs)@@Genes that encode a variety of protein types, which all serve to promote cell type differentiation specific for the axial position as characterized during flatworm development.

Posterior marginal zone (PMZ)@@The end of the chick blastoderm where primitive streak formation begins and acts as the equivalent of the amphibian Nieuwkoop center. The cells of the PMZ initiate gastrulation and prevent other regions of the margin from forming their own primitive streaks.

Postsynaptic cell@@The target cell that receives chemical neurotransmitters from a presynaptic neuron, causing depolarization or hyperpolarization of the target cell’s membrane.

Potency@@In referring to stem cells, the power to produce different types of differentiated cells.

Pre-granulosa cells@@The cells of the primordial follicle in the ovary that develop most closely to the germ cells. These become the granulosa cells of the follicle.

Pre-mRNA@@The first product of RNA transcription that includes the entire sequence of a gene from its transcriptional initiation, all its exons and introns, and its transcriptional termination sequence.

Prechordal plate@@See Prechordal plate mesoderm.

Prechordal plate mesoderm@@Precursor of the head mesoderm. The mesoderm cells that move inward during gastrulation ahead of the chordamesoderm.

Precursor cells (precursors)@@Widely used term to denote any ancestral cell type (stem or progenitor cells) of a particular lineage (e.g., neuronal precursors; blood cell precursors).

Predator-induced polyphenism@@The ability to modulate development in the presence of predators in order to express a more defensive phenotype.

Preeclampsia@@Medical condition of pregnant women characterized by hypertension, poor renal filtration, and fetal distress. A leading cause of premature birth and both fetal and maternal deaths.

Pluripotent hematopoietic stem cell@@See Hematopoietic stem cell (HSC).

Prefrontal cortex@@The anterior part of the frontal lobe that controls our complex cognitive, emotional, and behavioral functions.

Pregnant@@A female is said to be pregnant when an embryo is embedded within the uterus; thus, pregnancy is determined by implantation, not by fertilization.

Preimplantation embryo@@Name given to the product of fertilization up until the attachment of the trophoblast to the uterine wall.

Premetamorphosis@@The first stage of metamorphosis in amphibians, in which the thyroid gland begins to mature. It is followed by prometamorphosis and the metamorphic climax.

Presomitic mesoderm (PSM)@@Mesoderm that will form the somites. Also known as the segmental plate.

Presynaptic neuron@@Neuron that transmits chemical neurotransmitters to a target cell, causing the depolarization or hyperpolarization of the target cell’s membrane.

Primary axis@@See Anterior-to-posterior axis.

Primary embryonic induction@@The process whereby the dorsal axis and central nervous system forms through interactions with the underlying mesoderm, derived from the dorsal lip of the blastopore in amphibian embryos.

Primary larvae@@Larvae that represent dramatically different body plans than the adult form and that are morphologically distinct from the adult; the plutei of sea urchins are such larvae. Compare with Secondary larvae.

Primary mesenchyme@@See Skeletogenic mesenchyme.

Primary neurulation@@The process that forms the anterior portion of the neural tube. The cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube.

Primary sex determination (gonadal sex determination)@@The determination of the gonads to form either egg-generating ovaries or sperm-generating testes. Primary sex determination is chromosomal and is not usually influenced by the environment in mammals, but can be affected by the environment in other vertebrates.

Primary spermatocytes@@Derived from mitotic division of the type B spermatogonia, these are the cells that first go through a period of growth and then enter meiosis.

Primaxial muscles@@The intercostal musculature between the ribs and the deep muscles of the back, formed from those myoblasts in the myotome closest to the neural tube.

Primed@@Prepared; in the context of embryonic stem cells, ready for differentiation.

Primed ESC@@An embryonic stem cell (ESC) that was cultured from an inner cell mass cell that already had some maturation toward the epiblast lineage. Compare with Naïve ESC. Also see Embryonic stem cells.

Primitive endoderm (PrE)@@The layer of endoderm cells created during early mammalian development when the inner cell mass splits into two layers. The lower layer, in contact with the blastocoel, is the primitive endoderm, and is homologous to the hypoblast of the avian embryo. It will form the inner lining of the yolk sac and will be used for positioning the site of gastrulation, regulating the movements of cells in the epiblast, and promoting the maturation of blood cells. It is an extraembryonic layer that does not provide cells to the body of the embryo.

Primitive groove@@A depression that forms within the primitive streak that serves as an opening through which migrating cells pass into the deep layers of the embryo.

Primitive streak@@The first morphological sign of gastrulation in amniotes, it first arises from a local thickening of the epiblast at the posterior edge of the area pellucida, called Koller’s sickle. Homologous to the amphibian blastopore.

Primordial germ cells (PGCs)@@Gamete progenitor cells, which typically arise elsewhere and migrate into the developing gonads.

Procambium@@In plants, a layer of stem cells that produces the vascular tissue; it can also give rise to the pericycle in the roots.

Proembryo@@During development in seed plants, the stage created by the asymmetrical first cleavage of the zygote. At this 2-cell stage, the smaller apical cell will give rise to all parts of the plant proper, except the tip of the root; the larger basal cell will generate the root apex and the suspensor, which connects that embryo to the nutrients in the seed.

Progamic phase@@The events of pollen development from pollination to fertilization; the period of pollen tube growth through the female pistil.

Progenitor cells@@Relatively undifferentiated cells that have the capacity to divide a few times before differentiating and, unlike stem cells, are not capable of unlimited self-renewal. They are sometimes called transit amplifying cells because they divide while migrating.

Progress zone (PZ)@@Highly proliferative limb bud mesenchyme directly beneath the apical ectodermal ridge (AER). The proximal-distal growth and differentiation of the limb bud are made possible by a series of interactions between the AER and the progress zone. Also called the undifferentiated zone.

Prometamorphosis@@The second stage in amphibian metamorphosis, during which the thyroid matures and secretes more thyroid hormones.

Promoter@@Region of a gene containing the DNA sequence to which RNA polymerase II binds to initiate transcription. See also Enhancer.

Pronephric duct@@Arises in the intermediate mesoderm, migrates caudally, and induces the adjacent mesenchyme to form the pronephros, or tubules of the initial kidney of the embryo. The pronephric tubules form functioning kidneys in fish and in amphibian larvae but are not believed to be active in amniotes. As the duct continues growing downward it induces the mesonephric mesenchyme to form tubules, at which point it is called the mesonephric duct. Also called Wolffian duct and nephric duct.

Pronephric tubules (pronephros)@@The pronephros is a functioning kidney in fish and amphibian larvae, but is not believed to be active in amniotes, and the tubules degenerate after other regions of the kidney develop. Compare with Mesonephric kidney.

Pronuclei@@The male and female haploid nuclei within a fertilized egg that fuse to form the diploid nucleus of the zygote.

Prosencephalon@@The forebrain; the most anterior vesicle of the developing vertebrate brain. Will form two secondary brain vesicles: the telencephalon and the diencephalon.

Protein-protein interaction domain@@A domain of a transcription factor that enables it to interact with other proteins on the enhancer or promoter.

Proteoglycans@@Large extracellular matrix molecules consisting of core proteins (such as syndecan) with covalently attached glycosaminoglycan polysaccharide side chains. Two of the most widespread are heparan sulfate proteoglycan and chondroitin sulfate proteoglycan.

Prothoracicotropic hormone (PTTH)@@A peptide hormone that initiates the molting process in insects when it is released by neurosecretory cells in the brain in response to neural, hormonal, or environmental signals. PTTH stimulates the production of ecdysone by the prothoracic gland.

Protocadherins@@A class of cadherins that lack the attachment to the actin skeleton through catenins. They are an important means of keeping migrating epithelia together, and they are important in separating the notochord from surrounding mesoderm during its formation.

Protostomes@@Greek, “mouth first.” Animals that form their mouth regions from the blastopore, such as molluscs. Compare with Deuterostomes.

Pupa@@A non-feeding stage of a holometabolous insect following the last instar when the organism is going through metamorphosis, being transformed from a larva into an adult (imago).

Purkinje neurons@@Large, multibranched neurons that are the major cell type of the cerebellum.

Pygostyle@@A bony plate resulting from the fusion of vertebrae at the posterior end of an avian skeleton. It often supports feathers and is important for take-off and landings.

R

R-cadherin@@A type of cadherin critical in forming the retina (the R stands for retina). See Cadherins.

R-spondin1 (Rspo1)@@Small, soluble protein that upregulates the Wnt pathway and is critical for ovary formation in mammals.

Radial glial cells (radial glia)@@Neural progenitor cells found in the ventricular zone (VZ) of the developing brain. At each division, they generate another VZ cell and a more committed cell type that leaves the VZ to differentiate.

Radial holoblastic cleavage@@Cleavage pattern in echinoderms. The cleavage planes, which divide the egg completely into separate cells (holoblastic), are parallel or perpendicular to the animal-vegetal axis of the egg.

Radial intercalation@@In fish embryos, the movement of deep epiblast cells into the more superficial epiblast layer, helping to power epiboly during gastrulation.

Reaction norm@@A type of phenotypic plasticity in which the genome encodes the potential for a continuous range of potential phenotypes; the environment the individual encounters determines which of the potential phenotypes develops. Compare with Polyphenism.

Reaction-diffusion mechanism@@Model for developmental patterning, especially that of the limb, wherein two homogeneously distributed substances (an activator, substance A, that activates itself as well as forming its own, faster-diffusing inhibitor, substance I) interact to produce stable complex patterns during morphogenesis. According to this model, set forth in the early 1950s by mathematician Alan Turing, the patterns generated by this reaction-diffusion mechanism represent regional differences in the concentrations of the two substances.

Receptor@@A protein that binds to a ligand. See also Ligand.

Receptor tyrosine kinase (RTK)@@A receptor that spans the cell membrane and has an extracellular region, a transmembrane region, and a cytoplasmic region. Ligand (paracrine factor) binding to the extracellular domain causes a conformational change in the receptor’s cytoplasmic domains, activating kinase activity that uses ATP to phosphorylate specific tyrosine residues of particular proteins.

Regeneration blastema@@A collection of relatively undifferentiated cells that are organized into new structures by paracrine factors located at the cut surface. The collection of cells may be derived from differentiated tissue near the site of amputation that dedifferentiate, go through a period of mitosis, and then redifferentiate into the lost structures, as in the regenerating salamander limb, or may be from pluripotent stem cells that migrate to the cut surface, as in flatworm regeneration.

Regeneration@@The ability to reform body structure or organ that has been damaged or destroyed by trauma or disease.

Regenerative medicine@@The therapeutic use of stem cells to correct genetic pathologies (e.g., sickle-cell anemia) or repair damaged organs.

Reporter gene@@A gene with a product that is readily identifiable and not usually made in the cells of interest. Can be fused to regulatory elements from a gene of interest, inserted into embryos, and then monitored for reporter gene expression. If the sequence contains an enhancer, the reporter gene should become active at particular times and places. The genes for green fluorescent protein (GFP) and β-galactosidase (lacZ) are common examples.

Repressor@@A DNA or RNA-binding regulatory element that actively represses the transcription of a particular gene.

Reproduction@@Biological reproduction is the process of generating offspring that are biologically similar to the parent organism.

Resact@@A 14-amino-acid peptide that has been isolated from the egg jelly of the sea urchin Arbacia punctulata that acts as a chemotactic factor and sperm-activating peptide for sperm of the same species, i.e., it is species-specific and is thereby a mechanism to ensure that fertilization is also species-specific. See also Sperm-activating peptide.

Resegmentation@@Occurs during formation of the vertebrae from sclerotomes; the rostral segment of each sclerotome recombines with the caudal segment of the next anterior sclerotome to form the vertebral rudiment and this enables the muscles of the vertebral column derived from the myotomes to coordinate the movement of the skeleton, permitting the body to move laterally.

Respiratory tube@@The future respiratory tract, which forms as an epithelial outpocketing of the pharynx, and eventually bifurcates into the two lungs.

Responder@@During induction, the tissue being induced. Cells of the responding tissue must have receptors for the inducing molecules and be competent to respond to the inducer.

Retinotectal projection@@The map of retinal connections to the optic tectum. Point-for-point correspondence between the cells of the retina and the cells of the tectum that enables the animal to see an unbroken image.

Reverse genetics@@Genetic technique of knocking out or knocking down the expression of a gene in an organism and then studying the phenotype that results. Compare with Forward genetics.

Reverse transcription PCR (RT-PCR)@@A polymerase chain reaction procedure used to amplify the sequence of a given mRNA molecule by first reverse transcribing the mRNA into complementary DNA with the help of the reverse transcriptase enzyme.

Rho GTPases@@A family of molecules including RhoA, Rac1, and Cdc42 that convert soluble actin into fibrous actin cables that anchor at the cadherins. These help mediate cell migration by lamellipodia and filopodia and the cadherin-dependent remodeling of the cytoskeleton.

Rhombencephalon@@The hindbrain, the most caudal vesicle of the developing vertebrate brain; will form two secondary brain vesicles, the metencephalon and myelencephalon.

Rhombomeres@@Periodic swellings that divide the rhombencephalon into smaller compartments, each with a different fate and different associated nerve ganglia.

Right-left axis@@The body axis that specifies the lateral sides of the body.

RNA interference (RNAi)@@Process by which miRNAs inhibit expression of specific genes by degrading their mRNAs.

RNA polymerase II@@An enzyme that binds to a promoter on DNA and, when activated, catalyzes the transcription of an RNA template from the DNA.

RNA processing@@Represents the events that result in the splicing out the intronic regions of a pre-mRNA while fusing the exonic sequences together to produce the mRNA.

RNA-induced silencing complex (RISC)@@A complex containing several proteins and a microRNA, which can then bind to the 3′ UTR of messages and inhibit their translation.

RNA-Seq (RNA sequencing)@@Using next-generation sequencing technology to sequence and quantify the RNA present in a biological sample.

Roof plate@@Dorsal region of the neural tube important in the establishment of dorsal-ventral polarity. The adjacent epidermis induces expression of BMP4 in the roof plate cells, which in turn induces a cascade of TGF-β proteins in adjacent cells of the neural tube.

Root apical meristem (RAM)@@In plants, the meristem at the tip of a growing root. See Meristem.

Rostral-caudal@@Latin, “beak-tail.” An anterior-posterior positional axis; often used when referring to vertebrate embryos or brains.

Rotational cleavage@@The cleavage pattern for mammalian and nematode embryos. In mammals, the first cleavage is a normal meridional division while in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially. In C. elegans, each asymmetrical division produces one founder cell that produces differentiated descendants; and one stem cell. The stem cell lineage always undergoes meridional division to produce (1) an anterior founder cell and (2) a posterior cell that will continue the stem cell lineage.

Roundabout proteins (Robo)@@Proteins that are receptors for slit proteins, involved in controlling the crossing of the midline of commissural axons.

S

Sacral neural crest cells@@Neural crest cells that lie posterior to the trunk neural crest and along with the vagal neural crest generate the parasympathetic (enteric) ganglia of the gut that are required for peristaltic movement in the bowels.

Satellite cells@@Populations of muscle stem cells and progenitor cells that reside alongside adult muscle fibers and can respond to injury or exercise by proliferating into myogenic cells that fuse and form new muscle fibers.

Schizocoely@@The embryonic process of forming the coelom by hollowing out a previously solid cord of mesodermal cells. Typical of protostomes. See also Enterocoely.

Sclerotomes@@Blocks of mesodermal cells in the ventromedial half of each somite that will differentiate into the vertebrae, intervertebral discs (except for the nuclei pulposi) and ribs, in addition to the meninges of the spinal cord and the blood vessels that serve the spinal cord. They are also critical in patterning the neural crest and motor neurons.

Scrotum@@The sac of the external mammalian male genitalia that contains the testes and lower sperm-carrying tubes that connect the testes to the penis.

Secondary axis@@See Dorsal-to-ventral axis.

Secondary larvae@@Larvae that possess the same basic body plan as the adult; caterpillars and tadpoles are examples. Compare with Primary larvae.

Secondary mesenchyme@@See Non-skeletal mesenchyme.

Secondary neurulation@@The process that forms the posterior portion of the neural tube by the coalescence of mesenchyme cells into a solid cord that subsequently forms cavities that coalesce to create a hollow tube.

Secondary oocyte@@The haploid oocyte following the first meiotic division (this division also generates the first polar body).

Secondary sex determination@@Developmental events, directed by hormones produced by the gonads that affect the phenotype outside the gonads. This includes the male or female duct systems and external genitalia, and, in many species, sex-specific body size, vocal cartilage, and musculature.

Seed coat@@The outer protective jacket of the embrhyonic plant (the seed). The seed coat forms from the two integumental layers of the ovule.

Segment polarity genes@@Drosophila zygotic genes, activated by the proteins encoded by the pair-rule genes, whose mRNA and protein products divide the embryo into segment-sized units, establishing the periodicity of the embryo. Segment polarity mutants showed defects (deletions, duplications, polarity reversals) in every segment.

Segmental plate@@A synonym for presomitic mesoderm, the mesoderm that will form the somites.

Segmentation genes@@Genes whose products divide the early Drosophila embryo into a repeating series of segmental primordia along the anterior-posterior axis. Include gap genes, pair-rule genes, and segment polarity genes.

Self-renewal@@The ability of a cell to divide and produce a replica of itself.

Sepals@@Outer structures of the flower, usually protective and photosynthetic, that surround the inner, fertile, portions of the flower.

Sex@@(1) the coming together of genetic material from two parents creating a nucleus with a set of genes differing from each parent; (2) a mating type. In vertebrates, arthropods, angiosperms, and many other organisms: the characteristic of having sperm-making organs or egg-producing organs.

<Next term s/b Italic>

Sex-lethal (Sxl)@@A splicing factor involved in Drosophila sex determination. The Sxl gene product initiates a cascade of RNA processing events that eventually lead to either male-specific or female-specific transcription factors, the Doublesex proteins.

Shoot apical meristem (SAM)@@In plants, the meristem at the tip of a growing shoot that is the source of stem cells for all plant organs above ground, such as leaves and flowers.

Signal transduction pathway@@The sequential biochemical reactions within a cell that transmit an initial stimulus type though a series of molecular conformational changes that result in a specific cellular response.

Signal transduction cascades@@Pathways of response whereby paracrine factors bind to a receptor that initiates a series of enzymatic reactions within the cell that in turn have often several responses as their end point, such as the regulation of transcription factors (such that different genes are expressed in the cells reacting to these paracrine factors) and/or the regulation of the cytoskeleton (such that the cells responding to the paracrine factors alter their shape or are permitted to migrate).

Silencer@@A DNA regulatory element that binds transcription factors that actively repress the transcription of a particular gene.

Stem cell asymmetry@@Strategy in which one stem cell and one developmentally committed cell are produced at each division.

Sinistral coiling@@Left-coiling. In a snail, having its coils open to the left of its shells. See also Dextral coiling.

siRNA@@Small (or short) interfering RNA; sequences that prevent the expression of specific genes by degrading mRNA after transcription, thus preventing its translation into protein. Also known as “silencing RNA.” See also RNAi.

Sister chromatids@@Each of a pair of newly replicated chromatids. They have the same DNA sequence and are joined by a centromere.

Skeletogenic mesenchyme@@Also called primary mesenchyme, formed from the first tier of micromeres (the large micromeres) of the 60-cell sea urchin embryo. They ingress, moving into the blastocoel, and form the larval skeleton.

Slit proteins@@Proteins of the extracellular matrix that are chemorepulsive; involved in inhibiting migration of neural crest cells and in controlling growth of commissural axons.

Slow block to polyspermy@@A mechanical block to polyspermy that in sea urchins becomes complete about a minute after successful sperm-egg fusion. Its basis is the cortical granule reaction, in which enzymes from the egg’s cortical granules contribute to the formation of a fertilization envelope that blocks further sperm entry. Compare with Fast block to polyspermy.

Smad family@@Transcription factors activated by members of the TGF-β superfamily that function in the SMAD pathway.

Somatic mesoderm@@See Parietal mesoderm.

Somatic cells@@Cells that make up the body—i.e., all cells in the organism that are not germ cells. Compare with Germ cells.

Somatopleure@@Made up of somatic lateral plate mesoderm and overlying ectoderm.

Somites@@Segmental blocks of mesoderm formed from paraxial mesoderm adjacent to the notochord (the axial mesoderm). Each contain major compartments: the sclerotome, which forms the axial skeleton (vertebrae and ribs), and the dermomyotome, which goes on to form dermatome and myotome. The dermatome forms the dermis of the back; the myotome forms musculature of the back, rib cage, and ventral body. Additional muscle progenitors detach from the lateral edge of the dermomyotome and migrate into the limbs to form the muscles of the fore and hindlimbs.

Somitic mesoderm@@See Paraxial mesoderm.

Somitogenesis@@The process of segmentation of the paraxial mesoderm to form somites, beginning cranially and extending caudally. Its components are (1) periodicity, (2) fissure formation (to separate the somites), (3) epithelialization, (4) specification, and (5) differentiation.

Somitomeres@@Early pre-somites, consisting of paraxial mesoderm cells organized into whorls of cells.

Sox9@@Product of an autosomal gene involved in several developmental processes, most notably bone formation. In the genital ridge of mammals, it induces testis formation, and XX humans with an extra copy of SOX9 develop as males.

Specification@@The first stage of commitment of cell or tissue fate during which the cell or tissue is capable of differentiating autonomously (i.e., by itself) when placed in an environment that is neutral with respect to the developmental pathway. At the stage of specification, cell commitment is still capable of being reversed.

Sperm head@@Consists of the nucleus, acrosome, and minimal cytoplasm.

Sperm-activating peptides (SAPs)@@Small chemotactic peptides found in the jelly of echinoderm eggs. They diffuse away from the egg jelly in seawater and are species specific, only attracting sperm of the same species. Resact, found in the sea urchin Arbacia punctulata, is an example.

Spermatids@@Haploid sperm cells, the stage following the second meiotic division. In mammals, spermatids are still connected to one another by cytoplasmic bridges, allowing for diffusion of gene products across the cytoplasmic bridges.

Spermatogenesis@@The production of sperm.

Spermatogonia@@Sperm stem cells. When a spermatogonium stops undergoing mitosis, it becomes a primary spermatocyte and increases in size prior to meiosis.

Spermiogenesis@@The differentiation of the mature spermatozoa from the haploid round spermatid.

Spina bifida@@A congenital defect resulting from incomplete closure of the spine around the spinal cord, usually in the lower back. There are differing degrees of severity, the most severe being when the neural folds also fail to close.

Spiral holoblastic cleavage@@Characteristic of several animal groups, including annelid worms, some flatworms, and most molluscs. Cleavage is at oblique angles to the animal-vegetal axis, forming a “spiral” arrangement of daughter blastomeres. The cells touch one another at more places than do those of radially cleaving embryos, assuming the most thermodynamically stable packing orientation.

Splanchnic mesoderm@@Also called the visceral mesoderm and splanchnic lateral plate mesoderm; derived from lateral mesoderm closest to the endoderm (ventral) and separated from other component of lateral mesoderm (somatic, near ectoderm, dorsal) by the intraembryonic coelom. Together with the underlying endoderm, it forms the splanchnopleure. The splanchnic mesoderm will form the heart, capillaries, gonads, the visceral peritoneum and serous membranes that cover the organs, the mesenteries, and blood cells. Also called visceral mesoderm.

Splanchnopleure@@Made up of splanchnic lateral plate mesoderm and underlying endoderm. See Splanchnic mesoderm.

Spliceosome@@A complex made up of small nuclear RNAs (snRNAs) and splicing factors, that binds to splice sites and mediates the splicing of nRNA.

Splicing factors@@Proteins that bind to splice sites or to the areas adjacent to them.

Splicing isoforms@@Different proteins encoded by the same gene and generated by alternative splicing.

Spores@@Haploid reproductive cells.

Sporophyte@@The diploid growth stage in the alternating life cycle of plants and algae. Compare with Gametophyte.

Src family kinases (SFKs)@@Family of enzymes that phosphorylate tyrosine residues; involved in many signaling events, including the responses of growth cones to chemoattractants.

Sry@@Sex-determining region of the Y chromosome. The Sry gene encodes the mammalian testis-determining factor. It is probably active for only a few hours in the genital ridge, during which time it synthesizes the Sry transcription factor, whose primary role is to activate the Sox9 gene required for testis formation.

Stamens@@The male organs of a flower. They usually comprise a stalk (filament) and a pollen-producing anther.

STAT@@Signal transducers and activators of transcription. A family of transcription factors, part of the JAK-STAT pathway. Important in the regulation of human fetal bone growth.

Stem cell@@A relatively undifferentiated cell from the embryo, fetus, or adult that, that divides and when it does so, produces (1) one cell that retains its undifferentiated character and remains in the stem cell niche; and (2) a second cell that leaves the niche and can undergo one or more paths of differentiation. See also Adult stem cell; Embryonic stem cell.

Stem cell factor (SCF)@@Paracrine factor important for maintaining certain stem cells, including hematopoietic, sperm, and pigment stem cells. Binds to the Kit receptor protein.

Stem cell mediated regeneration@@Process by which stem cells allow an organism to regrow certain organs or tissues (e.g., hair, blood cells) that have been lost.

Stem cell niche@@An environment (regulatory microenvironment) that provides a milieu of extracellular matrices and paracrine factors that allows cells residing within it to remain relatively undifferentiated. Regulates stem cell proliferation and differentiation.

Stereoblastulae@@Blastulae that have no blastocoel, e.g., blastulae produced by spiral cleavage.

Stigma@@The surface of a carpel, usually at the peak of the style, that receives pollen.

Stomata (singular, stoma)@@In plants, the pores in the epidermis of leaves and other organs that allow for gas exchange. Each pore is bordered by two guard cells that control the size of the pore, opening and closing the stoma in response to environmental conditions.

Stomodeum@@An ectoderm-lined invagination in the oral region of the embryo that meets the endoderm of the closed gut tube to form the oral plate.

Stratum corneum@@The outermost layer of the epidermis in the skin of tetrapods (amphibians, reptiles including birds, and mammals). It is a protective layer consisting of cornified cells, which are dead cells filled with keratin protein that are shed and replaced throughout the life of the organism.

Stromal-derived factor 1 (SDF1)@@A chemoattractant. SDF1 is secreted, for example, by ectodermal placodes, thereby attracting cranial neural crest cells toward the placode.

Style@@A stalk, often elongated, between the stigma that receives pollen and the ovary, where the ovule is located.

Stylopod@@The proximal bones of a vertebrate limb, adjacent to the body wall; either the humerus (forelimb) or the femur (hindlimb).

Subgerminal cavity@@A space between the blastoderm and the yolk of avian eggs which is created when the blastoderm cells absorb water from the albumen (“egg white”) and secrete fluid between themselves and the yolk.

Subgranular zone (SGZ)@@A region of the hippocampus in the cerebrum that contains neural stem cells, allowing for adult neurogenesis in this region.

Subventricular zone@@A region in the vertebrate cerebrum that is formed as progenitor cells migrate away from the ventricular zone.

Sulcus limitans@@A longitudinal groove that divides the developing spinal cord and medulla into dorsal (receives sensory input) and ventral (initiates motor functions) halves.

Superficial cleavage@@The divisions of the cytoplasm of centrolecithal zygotes that occur only in the rim of cytoplasm around the periphery of the cell due to the presence of a large amount of centrally-located yolk, as in insects.

Suspensor@@A plant structure within the germinating seed that connects the plant embryo to the nutrients within the seed. It develops from the basal cell of the proembryo. See Proembryo.

Symbiogenesis@@A hypothesis for the origin of eukaryotic cells, wherein the first eukaryotic cells emerged from the fusion of prokaryotic organisms, one forming the nucleus, the other forming the mitochondrion.

Symbiont@@The smaller organism in a symbiotic relationship in which the other organism is much larger and serves as the host, while the smaller organism may live on the surface or inside the body of the larger.

Symbiosis@@Greek, “living together.” Refers to any close association between organisms of different species.

Sympoiesis@@The phenomena of development through the interactions of multiple species, wherein symbionts provide developmental signals needed by the host, and the host often reciprocates in facilitating symbiont reproduction.

Synapse@@Junction at which a neuron contacts its target cell (which can be another neuron or another type of cell) and information in the form of neurotransmitter molecules (e.g., acetylcholine, GABA, serotonin) is exchanged across the synaptic cleft between the two cells.

Synapsis@@The highly specific parallel alignment (pairing) of homologous chromosomes during the first meiotic division.

Syncytial blastoderm@@Describes the Drosophila embryo during cleavage when nuclei have divided, but no cell membranes have yet formed to separate the nuclei into individual cells.

Syncytial specification@@The interactions of nuclei and transcription factors, which eventually result in cell specification, that take place in a common cytoplasm, as in the early Drosophila embryo.

Syncytiotrophoblast@@A population of cells from the mammalian trophoblast that undergoes mitosis without cytokinesis resulting in multinucleate cells. The syncytiotrophoblast tissue is thought to further the progression of the embryo into the uterine wall by digesting uterine tissue.

Syncytium@@Many nuclei residing in a common cytoplasm, results either from karyokinesis without cytokinesis or from cell fusion.

Syndrome@@Greek, “happening together.” Several malformations or pathologies that occur concurrently. Genetically based syndromes are caused either by (1) a chromosomal event (such as trisomy 21, or Down syndrome) where several genes are deleted or added, or (2) by one gene having many effects.

T

Tailbud@@The caudalmost region of the vertebrate embryo, posterior to the hindlimb buds. Forms the tail of the embryo.

Telencephalon@@The anterior subdivision of the prosencephalon; will eventually form the cerebral hemispheres.

Telogen@@The resting phase of the hair follicle regeneration cycle.

Telolecithal@@Describes the eggs of birds and fish which have only one small area at the animal pole of the egg that is free of yolk.

Telson@@A tail-like structure; the posterior most segment of certain arthropods. Seen in insect larvae such as Drosophila.

Teratogens@@Exogenous agents that cause disruptions in development resulting in teratogenesis, the formation of congenital defects.

Teratology@@The study of birth defects and of how environmental agents disrupt normal development.

Terminal end buds@@The ends of the extensive branches of ducts in the mammary glands of mammals. Under the influence of estrogens at puberty, the ducts grow by the elongation of these buds.

Testosterone@@A steroid hormone that is androgenic. In mammals, it is secreted by the fetal testes and masculinizes the fetus, stimulating the formation of the penis, male duct system, scrotum, and other portions of the male anatomy, as well as inhibiting development of the breast primordia.

Tetrapods@@Latin, “four feet.” Includes the vertebrates amphibians, reptiles, birds, and mammals. Evolved from lobe-finned fish (sarcopterygian) ancestors.

<Note beta symbol In the following 2 terms>

TGF-β family@@Transforming growth factor-β. A family of growth factors within the TGF-β superfamily.

TGF-β superfamily@@More than 30 structurally related members of a group of paracrine factors. The proteins encoded by TGF-β superfamily genes are processed such that the carboxy-terminal region contains the mature peptide. These peptides are dimerized into homodimers (with themselves) or heterodimers (with other TGF-β peptides) and are secreted from the cell. The TGF-β superfamily includes the TGF-β family, activin family, bone morphogenetic proteins (BMPs), Vg1 family, and other proteins, including glial-derived neurotrophic factor (GDNF; necessary for kidney and enteric neuron differentiation) and anti-Müllerian hormone (AMH; involved in mammalian sex determination).

Thecal cells (theca)@@Steroid hormone-secreting cells of the mammalian ovary that, together with the granulosa cells, form the follicles surrounding the germ cells. They differentiate from mesenchyme cells of the ovary.

Thyroid hormone receptors (TRs)@@Nuclear receptors that bind the thyroid hormones tri-iodothyronine (T3), as well as thyroxine (T4). Once bound to the hormone, the TR becomes a transcriptional activator of gene expression. There are several different TR types, including TRα and TRβ.

Thyroxine (T4)@@Thyroid hormone containing four iodine molecules; converted to the more active tri-iodothyronine (T3) through removal of one iodine molecule. Increases basal metabolic rate in cells. Initiates metamorphosis in amphibians.

Topologically associating domains (TADs)@@References the 3-Dimensional conformation of regions of a chromosome that physically self-interact, which can influence gene regulation.

Totipotent@@Latin, “capable of all.” Describes the potency of certain stem cells to form all structures of an organism, such as the earliest mammalian blastomeres (up to the 8-cell stage), which can form both trophoblast cells and the embryo precursor cells. Compare with Pluripotent.

Trans-activating domain@@The transcription factor domain that activates or suppresses the transcription of the gene whose promoter or enhancer it has bound, usually by enabling the transcription factor to interact with the proteins involved in binding RNA polymerase or with enzymes that modify histones.

Transcript@@The mRNA for a given gene.

Transcription factor@@A protein that binds to DNA with precise sequence recognition for specific promoters, enhancers, or silencers.

Transcription initiation site@@DNA sequence of a gene that codes for the addition of a modified nucleotide “cap” at the 5′ end of the RNA soon after it is transcribed. Also called the cap sequence.

Transcription termination sequence@@DNA sequence of a gene where transcription is terminated. Transcription continues for about 1000 nucleotides beyond the AATAAA site of the 3′ untranslated region of the gene before being terminated.

Transcription@@The process of copying DNA into RNA.

Transcriptional co-regulators@@Proteins, recruited by transcription factors, that make modifications in chromatin structure, which either enhance or repress transcription of specific genes.

Transcriptome@@Total messenger RNAs (mRNAs) expressed by genes in an organism or a specific type of tissue or cell.

Transgene@@Exogenous DNA or gene introduced through experimental manipulation into a cell’s genome.

Transit-amplifying cells@@See Progenitor cells.

Transition zone@@In neural tube development in vertebrates, the zone between the region that undergoes primary neurulation and the region that undergoes secondary neurulation. The size of this zone varies among different species.

Translation initiation site@@The ATG codon (becomes AUG in mRNA), which signals the beginning of the first exon (protein-coding region) of a gene.

Translation termination codon@@Sequence in a gene, TAA, TAG, or TGA, which is transcribed as a codon in the mRNA—when a ribosome encounters this codon, the ribosome dissociates and the protein is released.

Translation@@The process in which the codons of a messenger RNA are translated into the amino acid sequence of a polypeptide chain.

Tri-iodothyronine (T3)@@The more active form of thyroid hormone, produced through the removal of an iodine molecular from thyroxine (T4).

Trigeminal placode@@In vertebrates, a pair of intermediate cranial placodes that are subdivided into the ophthalmic and maxomandibular placodes and that generate the distal neurons of the paired trigeminal ganglions, the sensory ganglions of the paired 5th cranial nerve (the trigeminal nerve).

Trimesters@@The three temporal portions (roughly 3 months each) of a human pregnancy.

Triploblasts@@See Bilaterians.

Trithorax@@Family of proteins that are recruited to retain the memory of the transcriptional state of regions of DNA as the cell goes through mitosis; keeps active genes active.

Trophectoderm cells@@In the mammalian embryo, the outer layer of cells of the blastocyst that surround the inner cell mass and blastocoel; develop into the embryonic side of the placenta.

Trophoblast@@The external cells of the early mammalian embryo (i.e., the morula and the blastocyst) that will bind to the uterus. Trophoblast cells form the chorion (the embryonic portion of the placenta). Also called trophectoderm.

Trunk neural crest cells@@Neural crest cells migrating from this region become the dorsal root ganglia containing the sensory neurons, sympathetic ganglia, adrenal medulla, the nerve clusters surrounding the aorta, and Schwann cells if they migrate along a ventral pathway, and they generate melanocytes of the dorsum and belly if they migrate along a dorsolateral pathway.

Tubal pregnancy@@See Ectopic pregnancy.

Tube cell@@Also called the vegetative cell, it is one of two cells produced by the division of the microspore nucleus in angiosperm pollen grains. It engulfs the generative cell, to give rise to the pollen tube.

 “Turing-type” model@@See Reaction-diffusion mechanism.

Type A spermatogonia@@In mammals, sperm stem cells that undergo mitosis and maintain the population of Type A spermatogonia while also generating Type B spermatogonia.

Type B cells@@A type of neural stem cell found in the rosettes of the V-SVZ of the cerebrum; fuel the generation of specific types of neurons in the olfactory bulb and striatum.

Type B spermatogonia@@In mammals, precursors of the spermatocytes and the last cells of the line that undergo mitosis. They divide once to generate the primary spermatocytes.

Type IV collagen@@A type of collagen that forms a fine meshwork; found in the basal lamina, an extracellular matrix that lies underneath epithelia.

U

<Note 4 prime symbols in the following 2 terms>

3¢ Untranslated region (3¢ UTR)@@A region of a eukaryotic gene and RNA following the translation termination codon that, although transcribed, is not translated into protein. It includes the region needed for insertion of the polyA tail on the transcript that allows the transcript to exit the nucleus.

5¢ Untranslated region (5¢ UTR)@@Also called a leader sequence or leader RNA; a region of a eukaryotic gene or RNA. In a gene, it is a sequence of base pairs between the transcription initiation and translation initiation sites; in an RNA, it is its 5′ end. These are not translated into protein, but can determine the rate at which translation is initiated.

Umbilical veins@@See Vitelline veins.

Ureteric buds@@In amniotes, paired epithelial branches induced by the metanephrogenic mesenchyme to branch from each of the paired nephric ducts. Ureteric buds will form the collecting ducts, renal pelvis, and ureters that take the urine to the bladder.

Urodeles@@Amphibian group that includes the salamanders. Compare with Anurans.

Urogenital sinus@@In mammals, the region of the cloaca that is separated from the rectum by the urogenital septum. The bladder forms from the anterior portion of the sinus, and the urethra develops from the posterior region. In females, also forms Skene’s glands; in males it also forms the prostate gland.

Uterus@@The womb; a hormone-responsive sex organ of the female reproductive tract in which the mammalian embryo develops.

V

Vagal neural crest cells@@Neural crest cells from the neck region, which overlaps the cranial/trunk crest boundary. Together with the sacral neural crest, generates the parasympathetic (enteric) ganglia of the gut, which are required for peristaltic movement of the bowels.

Vagina@@The elastic and muscular canal leading from the external genitalia to the cervix of the uterus in most female mammals. It facilitates both sexual intercourse and birth.

Vas deferens@@Derived from the Wolffian duct, the tube through which sperm pass from the epididymis to the urethra.

Vascular endothelial growth factors (VEGFs)@@A family of proteins involved in vasculogenesis that includes several VEGFs, as well as placental growth factor. Each VEGF appears to enable the differentiation of the angioblasts and their multiplication to form endothelial tubes. Also critical for angiogenesis.

Vascular tissue@@The conducting tissues in vascular plants that transport fluids (xylem) and nutrients (phloem).

Vasculogenesis@@The de novo creation of a network of blood vessels from the lateral plate mesoderm. See also Extraembryonic vasculogenesis.

Vegetal plate@@Area of thickened cells at the vegetal pole of the sea urchin blastula.

Vegetal pole@@The yolk containing end of the egg or embryo, opposite the animal pole.

Vegetal rotation@@During frog gastrulation, internal cell rearrangements place the prospective pharyngeal endoderm cells adjacent to the blastocoel and immediately above the involuting mesoderm.

Ventral pathway@@Migration pathway of trunk neural crest cells that travel ventrally through the anterior of the sclerotome and contribute to the sympathetic and parasympathetic ganglia, adrenomedullary cells, and dorsal root ganglia. Also called the ventrolateral pathway.

Ventral furrow@@Invagination of the prospective mesoderm, about 1000 cells constituting the ventral midline of the embryo, at the onset of gastrulation in Drosophila.

Ventricular cells@@Cells derived from the neuroepithelium that line the ventricles of the brain and secrete cerebrospinal fluid.

Ventricular radial glia (vRG)@@Progenitor cells that reside in the ventricular zone. They give rise to neurons, outer radial glia (oRG), and intermediate progenitor (IP) cells. See also Ventricular zone.

Ventricular zone (VZ)@@Inner layer of the developing spinal cord and brain. Forms from the germinal neuroepithelium of the original neural tube and contains neural progenitor cells that are a source of neurons and glial cells. Will form the ependyma.

Ventricular-subventricular zone (V-SVZ)@@Region of the cerebrum that contains neural stem cells and is capable of neurogenesis in the adult.

Vertical transmission@@In referring to symbiosis, the transfer of symbionts from one generation to the next through the germ cells, usually the eggs.

Visceral mesoderm@@See Splanchnic mesoderm.

Viscerocranium@@The jaws and other skeletal elements derived from the pharyngeal arches.

Vital dyes@@Stains used to label living cells without killing them. When applied to embryos, vital dyes have been used to follow cell migration during development and generate fate maps of specific regions of the embryo.

Vitelline envelope@@In invertebrates, the extracellular matrix that forms a fibrous mat around the egg outside the cell membrane and is often involved in sperm-egg recognition and is essential for the species-specific binding of sperm. The vitelline envelope contains several different glycoproteins. It is supplemented by extensions of membrane glycoproteins from the cell membrane and by proteinaceous “posts” that adhere the vitelline envelope to the membrane.

Vitelline veins@@The veins, continuous with the endocardium, that carry nutrients from the yolk sac into the sinus venosus of the developing vertebrate heart. In birds, these veins form from yolk sac blood islands, and bring nutrients to the embryo and transport gases to and from the sites of respiratory exchange. In mammals they are called omphalomesenteric veins or umbilical veins.

Viviparity@@Young are nourished in and born from the mother’s body rather than hatched from an egg, as in placental mammals. Compare with Oviparity.

Vulval precursor cells (VPCs)@@Six cells in the larval stage of C. elegans that will form the vulva via inductive signals.

W

White matter@@The axonal (as opposed to neuronal) region of the brain and spinal cord. Name derives from the fact that myelin sheaths give the axons a whitish appearance. Compare with Gray matter.

Whorls@@In plants, a set of leaves, sepals, petals, or branches that emerges simultaneously in a ring around the stem or apex.

Wolffian duct@@In vertebrates, the duct of the developing excretory system that grows down alongside the mesonephric mesoderm and induces it to form kidney tubules. In amniotes, it later degenerates in females, but in males, becomes the epididymis and vas deferens. Also called nephric duct.

Wound epidermis@@In salamander limb regeneration, the epidermal cells that migrate over the stump amputation to cover the wound surface immediately following amputation; later thickens to form the apical ectodermal cap.

X

X-chromosome inactivation@@In mammals, the irreversible conversion of the chromatin of one X chromosome in each female (XX) cell into highly condensed heterochromatin—a Barr body—thus preventing excess transcription of genes on the X chromosome. See also Dosage compensation.

Xylem@@In vascular plants, the conduits for bringing water and nutrients upward through the plant.

Y

Yellow crescent@@Region of the tunicate zygote cytoplasm extending from the vegetal pole to the equator that forms after fertilization by the migration of cytoplasm containing yellow lipid inclusions; will become mesoderm. Contains the mRNA for transcription factors that will specify the muscles.

Yolk cell@@The cell containing the yolk in a fish embryo, once the yolk-free cytoplasm at the animal pole of the egg divides to form individual cells above the yolky cytoplasm. Initially, all the cells maintain a connection with the underlying yolk cell.

Yolk plug@@The large endodermal cells that remain exposed on the vegetal surface surrounded by the blastopore of the amphibian gastrulating embryo.

Yolk sac@@The first extraembryonic membrane to form, derived from splanchnopleure that grows over the yolk to enclose it. The yolk sac mediates nutrition in developing birds and reptiles. It is connected to the midgut by the yolk duct (vitelline duct), so that the walls of the yolk sac and the walls of the gut are continuous.

Yolk syncytial layer (YSL)@@A cell population in the zebrafish cleavage stage embryo formed at the ninth or tenth cell cycle, when the cells at the vegetal edge of the blastoderm fuse with the underlying yolk cell, producing a ring of nuclei in the part of the yolk cell cytoplasm that sits just beneath the blastoderm. Important for directing some of the cell movements of gastrulation.

Z

Zeugopod@@The middle bones of the vertebrate limb; the radius and ulna (forelimb) or tibia and fibula (hindlimb).

Zona pellucida@@Glycoprotein coat (extracellular matrix) around the mammalian egg, synthesized and secreted by the growing oocyte.

Zone of polarizing activity (ZPA)@@A small block of mesodermal tissue in the very posterior of the limb bud progress zone. Specifies the anterior-posterior (thumb-pinkie) axis of the developing limb through the actions of the paracrine factor Sonic hedgehog.

Zygote@@A fertilized egg with a diploid chromosomal complement (zygotic genome). The single-celled stage of embryogenesis.

Zygotic genome@@The chromosomal constitution of the single-cell embryo (zygote), resulting from fusion of the haploid maternal and paternal pronuclei.