oogenesis

Oogenesis is the production of female gametes, or ova, in animals. It begins with oogonia that proliferate by mitosis during fetal development in many species, then enter meiosis to become primary oocytes, which arrest in prophase I.

In humans, oogenesis proceeds within ovarian follicles. By birth, a female's ovaries contain a finite supply of primary oocytes arrested in prophase I. Most primordial follicles degenerate through atresia; only a small subset reach maturity.

At puberty, hormonal cycles recruit a group of follicles each cycle. The selected primary oocyte resumes meiosis I, forming a secondary oocyte and the first polar body; meiosis I is asymmetric, yielding less cytoplasm in the polar body. The secondary oocyte begins meiosis II but arrests at metaphase II and is released during ovulation.

Meiosis II completes only after fertilization, producing a mature ovum and a second polar body; if fertilization does not occur, the oocyte does not become a zygote. The corpus luteum forms from the remnant follicle and secretes hormones in early pregnancy.

Compared with spermatogenesis, oogenesis yields typically one functional gamete per meiotic cycle, with long periods of arrest, unequal cytoplasmic distribution, and a finite number of oocytes in life.