Gametogenesis

Same starting cell, same two divisions — and yet spermatogenesis produces four working gametes while oogenesis produces one. That asymmetry is deliberate, and understanding why it exists is the fastest way to keep the two processes apart in your head.

The shared pattern

Before separating the two, notice what they have in common. Both start in the germinal epithelium, the outer layer of cells of the gonad. Both use mitosis first to build up numbers, then growth and differentiation, then meiosis to halve the chromosome number. The differences are in the timing and in what happens to the cytoplasm.

Spermatogenesis

Sperm production takes place in the testes from puberty onwards. The testes contain many small tubules known as seminiferous tubules. The gaps between them, the interstices, are filled with interstitial cells, sometimes called Leydig cells, which produce testosterone.

Spermatogenesis begins in the germinal epithelium, the layer of cells making up the outer layer of the seminiferous tubules. Cells here divide by mitosis to produce diploid cells called spermatogonia. Of the two daughter cells, one goes on to eventually become a sperm cell while the other stays in the germinal epithelium, where it can continue dividing — which is exactly why a male can keep producing sperm for decades.

Spermatogonia then migrate from the germinal epithelium towards the lumen of the tubule, moving through the gaps between Sertoli cells, which form the inner lining of the tubules. They differentiate into immature sperm cells called primary spermatocytes. These mature and divide by meiosis: meiosis I forms secondary spermatocytes, and meiosis II forms spermatids.

Spermatids stay associated with the Sertoli cells while they mature into sperm cells, or spermatozoa. Once fully mature, they detach and move along the tubule lumen towards the sperm duct, passing through the coiled epididymis on the way.

Oogenesis

The timeline here is completely different, and it is the timeline that examiners test.

Ova production begins in the ovaries of the female foetus before birth. The ovaries are surrounded by a germinal epithelium, and cells in this layer divide by mitosis throughout the first 7 months of foetal development to form diploid cells called oogonia, which migrate throughout the tissues of the ovaries. This process stops after 7 months, by which time several million oogonia have been produced — and those are all the oogonia the ovaries will ever produce.

During the few months leading up to birth, the oogonia grow in size and enter meiosis I, and a layer of follicle cells develops around them. A partially divided oogonium plus its follicle cells is a primary follicle. At this point oogenesis pauses until puberty. Many of the several million primary follicles present at birth degrade over a woman’s lifetime and never reach maturity.

When puberty begins, FSH stimulates the continued development of several primary follicles, but only one reaches maturity. Meiosis I continues and the primary follicle divides to form two new cells — and here is the key detail: the division of cytoplasm is not equal. The result is a large secondary oocyte plus a very small cell called a polar body, which has very little cytoplasm and does not mature further.

The secondary oocyte enters meiosis II and, at this point, leaves the ovary with its layer of follicle cells — that is ovulation. The remains of the follicle left behind develop into the corpus luteum.

Meiosis II is not finished at ovulation. The secondary oocyte only completes it after a sperm cell enters, and even then it finishes just before the nuclei fuse. The cell is an ovum very briefly, between the end of meiosis II and the fusion of the two nuclei, and a second polar body is produced.

Worked examples