Oogenesis

Oogenesis is the process that produces the female gamete, the ovum, inside the ovary, and unlike spermatogenesis it starts well before birth. During foetal development, cells in the germinal epithelium of the foetal ovary divide mitotically to produce millions of oogonia (egg mother cells); critically, no further oogonia are ever formed after birth. These oogonial cells then enter prophase I of meiosis I to become primary oocytes, which are temporarily arrested at that stage. Each primary oocyte is soon surrounded by a single layer of granulosa cells, forming a primordial (or primary) follicle. A great many of these follicles simply degenerate between birth and puberty, so that by puberty only about 60,000-80,000 follicles remain in each ovary. From puberty onward, a recruited primary follicle is surrounded by additional granulosa cell layers and a new theca layer to become a secondary follicle; a fluid-filled cavity, the antrum, then develops, turning it into a tertiary follicle, with the theca layer itself organising into an inner theca interna and outer theca externa. Within this tertiary follicle, the primary oocyte grows and finally completes meiosis I — an unequal division that produces one large haploid secondary oocyte and a much smaller first polar body, which simply degenerates. The tertiary follicle matures into the Graafian follicle. Meiosis II then begins in the secondary oocyte but is completed only if fertilisation actually takes place, producing the ovum and …