Summary

The Timing of Reproductive Cycles

• In iteroparous animals that live in environments with regular seasonal cycles, the reproductive cycle is nearly always timed to coordinate with the seasonal cycle of environmental conditions, in at least certain ways. Photoperiod and environmental temperature are the most commonly used environmental cues employed to achieve this coordination. The most dramatic aspect of the coordination is that individuals or populations often become physiologically incapable of reproducing during the most unfavorable time of year (winter).
• Sperm storage and embryonic diapause are commonly employed mechanisms that permit certain steps in the reproductive process to be coordinated relatively independently of other steps. Sperm storage uncouples the times of mating and fertilization. Embryonic diapause uncouples the times of fertilization and completion of embryonic development.
• Embryonic diapause in placental mammals is called delayed implantation. It may be obligate or facultative. Obligate delayed implantation is typically employed to create down time so as to adjust the total length of the reproductive cycle to be about 365 days. Facultative delayed implantation in small mammals is typically used to create offsets in developmental timing between litters that a mother is simultaneously nursing and nurturing in her uterus.
• Among small- and medium-sized iteroparous species, a minority reproduce just once per year at highly circumscribed times (e.g., palolo worms, red foxes, some birds). Large-bodied mammals with gestation periods that are lengthy, yet shorter than 12 months, often employ delayed implantation or short-day breeding to ensure that their young are born at a favorable time of year. The mammals of largest size, such as elephants and large whales, require more than a single year to complete a reproductive cycle.

Reproductive Endocrinology of Placental Mammals

• The ovaries produce oocytes and secrete hormones. Oocytes are shed from the ovaries during each estrous or menstrual cycle during the reproductive season. Each cycle has three main ovarian phases: development of follicles (follicular phase), ovulation, and function of the corpus luteum (luteal phase). The uterine endometrium grows thicker prior to ovulation and becomes secretory after ovulation.
• The gonadotropins known as luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—released in response to gonadotropin-releasing hormone (GnRH)—stimulate granulosa cells in ovarian follicles to secrete estrogen (particularly estradiol). Estrogen acts as a paracrine/autocrine agent that stimulates proliferation of granulosa cells. It also acts as a blood-borne hormone that stimulates growth of the uterine endometrium, affects behavior in species with estrus, and feeds back on the anterior pituitary gland and hypothalamus.
• Ovulation is induced by copulation in some species, but is spontaneous in most. A surge in secretion of LH triggers ovulation in either case. In spontaneous ovulators, the surge is a consequence of endogenous interactions of endocrine and neuroendocrine tissues, notably the hypothalamus, anterior pituitary gland, and ovarian follicles. A key part of the process is stimulation of kisspeptin neurons by estrogen, followed by kisspeptin-stimulated secretion of GnRH.
• After ovulation, the cells of each ruptured ovarian follicle reorganize into a corpus luteum, which secretes progesterone, estrogen, and inhibin. These hormones inhibit or decrease folliculogenesis in the ovaries by reducing secretion of LH and FSH from the anterior pituitary. Progesterone supports the secretory state of the uterine endometrium and inhibits contraction of the smooth muscles of the myometrium and oviducts.
• If fertilization does not occur, the corpus luteum degenerates, and uterine endometrial tissue is resorbed or discharged as menstrual flow. If fertilization does occur, the corpus luteum is rescued and continues to function. Its function is essential for maintenance of the uterus and placenta in early pregnancy.
• Fertilization occurs in the oviduct when a secondary oocyte is present and sperm have undergone capacitation. Upon fusion of the two gametes’ cell membranes, the oocyte completes meiosis and undergoes the cortical reaction, which blocks fertilization by any additional sperm. The zygote moves down the oviduct to the uterus, developing into a blastocyst, which ultimately implants (possibly following delayed implantation in some species).
• After implantation, the embryonic trophoblast and maternal endometrium form the placenta, a structure in which embryonic and maternal blood vessels closely intermingle, permitting exchange of O₂, nutrients, and wastes between the two bloodstreams (although not mixing of blood). The placenta secretes hormones, such as progesterone, which are crucial for maintaining the placenta and sustaining pregnancy.
• Birth is accomplished by coordinated contractions of the uterine myometrium (smooth muscle). Birth is facilitated by a positive feedback loop—mediated partly by hypothalamic oxytocin secretion—that causes the contractions to become more and more powerful.
• Milk is produced by alveolar epithelial cells in the mammary glands. Prolactin stimulates milk secretion by the epithelial cells. Oxytocin causes milk ejection (milk let-down) by stimulating the contraction of myoepithelial cells surrounding the alveoli. Suckling increases secretion of both prolactin and oxytocin.
• Males produce sperm continuously during the breeding season. Sperm are produced with the aid of Sertoli cells in the seminiferous tubules in the testes. Leydig cells embedded in connective tissue between the seminiferous tubules secrete testosterone. The functions of the seminiferous tubules and Leydig cells are controlled by continuous secretion of LH and FSH, released in response to GnRH.
• In males, LH stimulates the Leydig cells. Testosterone from the Leydig cells promotes sperm production via effects on the Sertoli cells, sometimes with additive effects exerted by FSH on the Sertoli cells.
• Erection of the penis results from blood flow into spongy tissue, controlled by nitric oxide (NO). The parasympathetic nervous system initiates NO production, but then further NO is released from blood vessel endothelial walls, forming a positive feedback loop. During ejaculation, sperm are mixed with secretions of male accessory glands (principally the prostate gland and paired seminal vesicles in humans) to produce the semen that is emitted.