Gametogenesis: oogenesis

Oogenesis starts before birth: during embryonic development, a couple of million oogonia (gamete mother cells) form in each foetal ovary, and no new oogonia are added after birth.

Oogonia begin meiosis and halt in prophase I; these arrested cells are primary oocytes. A primary oocyte surrounded by a layer of granulosa cells is a primary follicle.

Many follicles degenerate between birth and puberty, so each ovary has only about 60,000–80,000 primary follicles left at puberty.

A secondary follicle forms when a primary follicle is surrounded by additional layers of granulosa cells and acquires a new theca. Once a fluid-filled cavity called the antrum appears, the follicle is tertiary, and its theca has become organised into theca interna and theca externa.

Inside the tertiary follicle the primary oocyte grows and finishes meiosis I. The division is unequal: it gives a large haploid secondary oocyte that keeps most of the cytoplasm and a tiny first polar body.

The tertiary follicle matures into the Graafian follicle, in which the secondary oocyte lies enclosed by a covering called the zona pellucida.

At ovulation the Graafian follicle ruptures and releases the secondary oocyte (the ovum). Meiosis II of this cell is completed only if a sperm enters it.

Net result: one primary spermatocyte gives four sperms, but one primary oocyte gives only one functional ovum, with the rest of the chromosomes discarded in polar bodies.