The essentials
Gametogenesis is the production of haploid gametes from diploid germ cells. Spermatogenesis begins at puberty in the seminiferous tubules. Spermatogonia (2n, 46 chromosomes) multiply by mitosis; some become primary spermatocytes, which complete meiosis I to form two secondary spermatocytes (n = 23). Meiosis II gives four haploid spermatids. Spermatids are transformed into spermatozoa by spermiogenesis, and sperm are released from Sertoli cells by spermiation. The process is started by GnRH from the hypothalamus, which stimulates the anterior pituitary to secrete LH and FSH. LH acts on Leydig cells to stimulate androgen secretion; FSH acts on Sertoli cells to aid spermiogenesis.
A sperm has a head, neck, middle piece and tail. The head contains a haploid nucleus capped by the acrosome, which holds enzymes for fertilisation. The middle piece has mitochondria that supply energy for movement.
Oogenesis starts during foetal life: millions of oogonia form in each foetal ovary and no new ones are added after birth. Primary oocytes begin meiosis I and stay arrested at prophase I. At puberty only about 60,000–80,000 primary follicles remain in each ovary. A tertiary follicle has a fluid-filled antrum. The primary oocyte completes meiosis I inside the growing follicle, forming a large secondary oocyte and a tiny first polar body. The Graafian follicle ruptures to release the secondary oocyte at ovulation; meiosis II is completed only when a sperm enters.
Key points
- One primary spermatocyte → 4 sperm; one primary oocyte → 1 ovum + polar bodies.
- Unequal cytoplasmic division in oogenesis keeps most food in the ovum.
- A normal ejaculate has about 200–300 million sperm; at least 60% should have normal shape and size, and at least 40% vigorous motility for normal fertility.
Common mistakes
- Ovulation releases a secondary oocyte, not a mature ovum.