Gametogenesis

Gametogenesis is the making of gametes: spermatogenesis in the testes and oogenesis in the ovaries.

Spermatogenesis starts at puberty and goes on through life. In the seminiferous tubules:

• Spermatogonia (diploid, 46 chromosomes) divide by mitosis; some grow into primary spermatocytes.
• Each primary spermatocyte undergoes meiosis I, giving two haploid secondary spermatocytes.
• Each of these undergoes meiosis II, giving two spermatids. So one primary spermatocyte gives four spermatids, each with 23 chromosomes.
• The spermatids change shape into sperms: this is spermiogenesis.
• The sperm heads stay attached to Sertoli cells; finally the sperms are released into the tubule: spermiation.

Hormones start it all at puberty. The hypothalamus releases more GnRH, which makes the pituitary release two hormones:

• LH acts on the Leydig cells, which make androgens; androgens drive spermatogenesis.
• FSH acts on the Sertoli cells, which help in spermiogenesis.

A sperm has a head, with the nucleus and, on top, the acrosome, a cap of enzymes that help it enter the ovum; a neck; a middle piece full of mitochondria, which supply energy for swimming; and a tail. One ejaculation releases 200-300 million sperms, and for normal fertility at least 60% should be normal in shape and 40% should swim vigorously.

Oogenesis begins before birth. In the fetal ovary, millions of oogonia form; no new oogonia are made after birth. They start meiosis and then stop in prophase I: these are primary oocytes, each wrapped in cells as a primary follicle. Many die before puberty.

After puberty, a few follicles develop each month: primary follicle → secondary follicle (more layers of cells) → tertiary follicle, which has a fluid-filled cavity, the antrum → mature Graafian follicle.

Inside the tertiary follicle, the primary oocyte completes meiosis I, but the division is unequal: one large secondary oocyte keeps nearly all the cytoplasm (needed to feed the future embryo), and a tiny first polar body is cast off. The Graafian follicle bursts and releases the secondary oocyte: ovulation.

So the difference is striking: one primary spermatocyte makes four sperms, but one primary oocyte makes only one ovum.