Gametogenesis in both sexes

The testis: two functions in one organ

A testis is a mass of seminiferous tubules. Inside their walls lies the germinal epithelium, where the gametes are made; between the tubules sit the interstitial cells of Leydig, which secrete testosterone.

One organ, two functions — exocrine (spermatozoa) and endocrine (testosterone) — and they are anatomically separate, which is why they can be affected independently.

Inside the tubule the Sertoli cells run from the basement membrane to the lumen. They nurse the developing germ cells, form the blood–testis barrier that protects them from the immune system, and secrete inhibin, the hormone that feeds back on FSH.

Spermatogenesis

It runs continuously from puberty to the end of life, takes about 74 days, and moves from the basement membrane towards the lumen.

A diploid spermatogonium multiplies by mitosis, grows into a primary spermatocyte, and undergoes the first meiotic division to give two haploid secondary spermatocytes; the second division gives four spermatids.

Spermiogenesis then transforms each round spermatid into a spermatozoon without any further division: the nucleus condenses, the Golgi builds the acrosome over it, a flagellum grows from the centriole, mitochondria coil into the midpiece, and most of the cytoplasm is discarded.

One spermatogonium therefore yields four functional gametes.

The ovary and its follicles

The ovary is likewise both exocrine and endocrine, but the two functions are carried by the same structure: the follicle produces the oocyte and secretes the hormones.

A follicle grows from primordial to primary, secondary, tertiary and finally to the mature (de Graaf) follicle, whose antrum is filled with fluid and whose theca and granulosa cells secrete oestradiol.

Only about 400 follicles will ever ovulate. All the others undergo atresia at some stage, which is why the ovarian reserve falls steadily and why it cannot be replenished.

Oogenesis

Oogenesis begins before birth: all the oogonia have entered meiosis by the fifth month of fetal life and then arrest in prophase I. A girl is born with her entire stock — one to two million, falling to about 400 000 at puberty — and makes no new ones.

The first meiotic division only completes at ovulation, releasing a secondary oocyte and a first polar body. Meiosis II then arrests again in metaphase and is completed only if a spermatozoon fertilises the oocyte, at which point the second polar body is expelled.

The divisions are also deliberately unequal: nearly all the cytoplasm stays in the oocyte, so one primary oocyte gives one gamete and two or three polar bodies, not four gametes.