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1. Compare and contrast gamete production in males and females.
Similarities: occurs in the gonads; process begins with
diploid germ cells and ends with haploid daughter cells; both
involve meiosis and mitosis; both are characterized by a massive
over-production of gametes in comparison with the number that are
eventually involved in fertilization.
Differences: the transition from spermatogonia to spermatozoa takes approximately 64 days, whereas oogenesis may take 50 years or longer; the process begins at puberty in males and before birth in females; the process ends at death in males and at the menopause in females; male gametes are motile whereas female gametes are non-motile; oogenesis produces one ovum from one oogonium, while spermatogenesis yields four spermatozoa from each spermatogonium; gametes are produced at a rate of approximately one per month in females and many millions each day in males.
2. Consider the physiological reasons for the production of polar bodies during oogenesis but not during spermatogenesis.
Although male and female gametes contribute equal amounts of DNA during fertilization in humans, nearly all the nutrients required to maintain the developing zygote are supplied by the female gamete. For this to be the case, gametogenesis in females is said to be ‘asymmetrical’, which means that oogenesis results in the formation of three polar bodies for every oocyte produced. The oocyte inherits all the nutrients from the parent oogonium while the nutrient-deficient polar bodies degenerate and die. On the other hand, spermatozoa can be viewed as ‘DNA delivery vessels’ and are required to survive only long enough to deliver their cargo of DNA to the oocyte. Retention of nutrient-rich cytoplasm is therefore not a priority during spermatogenesis. Each meiotic cycle during spermatogenesis therefore yields four viable spermatozoa, containing very little cytoplasm, from each spermatogonium; polar bodies are not produced.
3. Discuss the contrasting but complementary functions of Sertoli cells and Leydig cells.
Both cell types support spermatogenesis but in different ways. Sertoli cells are the epithelial cells of the seminiferous tubules and have secretory as well as structural properties. One of the secretory products of Sertoli cells is androgen-binding protein, which binds to testosterone produced by Leydig cells, thereby ensuring a high local concentration of the hormone to support spermatogenesis. The structural properties of Sertoli cells are equally important. In addition to providing the structural framework that assists with spermatogenesis, tight junctions between adjacent Sertoli cells maintain an effective blood-testes barrier. Leydig cells are non-epithelial cells lying in the stroma surrounding the seminiferous tubules. These cells secrete testosterone and other androgens under the influence of luteinizing hormone from the pituitary.
4. In the female genital system, squamous metaplasia occurs most frequently in the vicinity of the external os. Discuss the physiological reasons.
Anatomically, the external os is the point at which the endocervical canal opens into the vagina. The type of epithelium at this site varies according to age and the hormonal status of the woman. In pre-pubertal girls and in post-menopausal women, circulating oestrogen levels are low, the cervix is small and the squamocolumnar junction lies deep within the endocervical canal. Under these conditions, the os is lined with stratified squamous epithelium. During the reproductive years, hormonal fluctuations cause the cervix to vary in size and, as it does so, the position of the squamocolumnar junction changes. When oestrogen levels are high, the cervix increases in volume and the squamocolumnar junction everts. Exposure of endocervical epithelium to the low pH of the vagina initiates squamous metaplasia. Therefore, squamous metaplasia in the cervix is most likely to occur at the external os, where the delicate endocervical epithelium is transiently but frequently exposed to the acidic environment of the vagina.
Short-answer questions
- Why does meiosis occur only in the gonads?
Unlike mitosis, which occurs throughout the body as part of the natural turnover of diploid cells, meiosis produces haploid daughter cells from diploid parent cells. Meiosis is required only for the production of gametes, which takes place in the gonads.
- Describe the similarities and differences between the ‘9x3’ and the ‘9+2’ arrangement in microtubules.
Similarities: both are polymers of the protein tubulin,
both are involved in mechanical support or movement within the
cell.
Differences: The ‘9+2’ arrangement consists of nine microtubule doublets arranged in a ring plus one central doublet and is found in cilia and flagella; the ‘9x3’ arrangement consists of nine microtubule triplets and is found in centrioles and basal bodies.
- Many organelles are shed from spermatids during spermatogenesis. List the organelles that are retained and explain why they are needed.
The retained organelles are the nucleus (containing the genetic material necessary for fertilization), the acrosome (for enzymatically ‘drilling’ through the otherwise impenetrable oocyte) and mitochondria (for releasing the energy required for sperm motility).
- Unlike spermatozoa, ova are non-motile. Without intrinsic mobility, explain how a fertilized egg comes to be implanted in the lining of the uterus.
Transportation of the fertilized egg along the Fallopian tube to the uterus is facilitated by gentle tubal contractions of smooth muscle. Motile cilia in the lining of the Fallopian tube assist in the forward movement of the zygote.