Question

Men and women differ greatly in the way they make gametes. Compare and contrast male and female gametogenesis on the bases of: overall timing of the process (when it starts, stops, etc.), timing of the meiotic divisions, number of functional cells produced, size and content of the gametes, numbers of gametes produced (per day and in the long term), contribution to the zygote, and reproductive lifespan. Explain the hormonal control of spermatogenesis. Why is it important for inhibin to act on only FSH and not LH?

Answer 1

Male Gametogenesis	Female Gametogenesis
Male gametogenesis is known as spermatogenesis and takes place in testes	Female gametogenesis if known as oogenesis and occurs in the ovary
Timing: Spermatogenesis is initiated only after puberty when there is testosterone secretion is initiated. It occurs in lumen of seminiferous tubules. Mitotic replication embryonic development occurs which leads to differentiation of gonocytes to fetal spermatogonia. This process is regulated by FSH. Germ cells are present in a sequence in basement membrane of seminiferous tubules. Then, they form the primary spermatocytes, which form secondary spermatocyte. These will form the spermatids which now move towards the lumen. Spermatogonia and early spermatocytes are supported in the basal compartment by tight junctions. Each primary spermatocyte form two secondary spermatocyte. Each secondary spermatocyte form four haploid spermatids.	Timing: Three week after fertilization of the oocyte, oogenesis begins in the embryo. Hence, oogenesis starts before birth. The yolk sac has primordial germ cells that migrate to genital ridge, where they undergo mitosis to form oogonial stem cells. These germ stem cells will expand to nearly ten time the number by 20th week of gestation. At the 7th week of gestation, the cells form the primitive medullary cords and sex cord. At week 16-18, primordial follicles are formed, which stop mitosis and undergo meiosis. This results in formation of primary oocyte. The mesenchymal somatic cells will form the basement membrane to form the granulosa layer. Ovary has maximum oocytes by 4-5 weeks of gestation which decreases to 1-2 million at birth and less than 50000 at puberty. These immature oocytes will remain dormant till puberty. At puberty, hormonal secretion by hypothalamus- pituitary axis leads to development of primary follicles to secondary and then antral follicles. Every month, one dominant follicle is selected, which mature rapidly in response to estrogen and will ovulate. This releases the ovum. The thecal cells remining behind will secrete progesterone that is required for endometrial development. Only one oocyte is release per menstrual cycle of a month.
Both maturation divisions occur in the testes.	First maturation occurs in ovary, while second maturation (second polar body removal) occurs in the fallopian tube during fertilization (outside the ovary)
Nearly 20 million to 300 million sperm cells are produced by testis per day. Around 525 billion sperm are produced in a lifetime. Males are fertile from puberty to 54-78 years of life. In humans, spermatocyte maturation is 25.3 days, spermiogenesis is around 21.6 days. Spermatogenesis takes around 74 days	Only one ovum/egg/gamete is produced per cycle per month. 300-400 eggs are produced in a lifetime. Female reproductive lifespan is around 28 years.
Primary spermatocytes undergo first meiotic division to form secondary spermatocytes. Short lived second meiotic division occurs in secondary spermatocytes to form spermatids. Primary spermatocytes undergo first meiotic division to form secondary spermatocytes. Short lived second meiotic division occurs in secondary spermatocytes to form spermatids.	First meiotic division occurs in primary oocyte and the first polar body is lost after anaphase. Oocyte is arrested in prophase I, till meiotic competence is reached. Second meiotic division occurs secondary oocyte and there is an arrest in metaphase II. It continues after fertilization when the second polar body is lost.
Sperm cells are smallest cell which has an acrosome, head, midpiece and tail. It is motile. This is because sperm has to swim through the female reproductive tract.	Egg cells are largest covered with a basement membrane and zona pellucida. The sperm nuclei are injected into the egg and the fertilized egg develops into the zygote.

Comparison: Both processes occur in gonads: testis and ovary. The process is initiated in the germinal epithelium, the outer layer of the gonads. Multiplication, growth and maturation are three steps in gametogenesis. Meiosis occurs in both processes.

Hormonal control of spermatogenesis:

When puberty starts, hypothalamus secretes gonadotrophin releasing hormone (GnRH) in a pulsatile manner. The pituitary secreted the follicle stimulating hormone (FSH) and Leydig cells (LH) in response to GnRH. FSH stimulates the Sertoli cells for nourishment sperm produced by spermatogenesis. The LH also stimulates the interstitial Leydig cells of testes to release testosterone, which is released in bloodstream. Testosterone is responsible for secondary sexual characteristics. Its main function is to stimulate spermatogenesis. When the testosterone secretion reaches peak levels, it will negatively regulate GnRH to stop LH and FSH secretion. The Sertoli cells produce the hormone inhibin, when sperm count is high. Inhibin will then inhibit release of GnRH and FSH to reduce sperm production.

Inhibin will not negatively regulate LH, because LH is required. If inhibin inhibits LH secretion, the spermatogenesis will be inhibited. This will lead to no sperm production or azoospermia. Inhibin action of FSH will only inhibit sperm nourishment, so would affect sperm maturation. LH is required for testosterone secretion, which also is required for development of secondary male characteristics. Inhibition of LH but not FSH inhibits cell cycle, especially DNA synthesis. Hence, LH should not be inhibited by inhibin.