ovarian follicle

An ovarian follicle is the functional unit of the ovary — a tiny capsule that contains a single developing egg (oocyte) wrapped in layers of supporting, hormone-producing cells. If the egg is a seed, the follicle is the entire seed pod that protects it, feeds it, and decides when to release it.

Follicles progress through stages of maturation. They begin as dormant primordial follicles laid down before birth; under hormonal stimulation a few advance to primary, secondary, and then antral follicles, the last named for the fluid-filled cavity (antrum) that forms inside. Two cell layers do the endocrine work: the inner granulosa cells and the outer theca cells, which together build estradiol. Each month one follicle usually outcompetes the rest to become the dominant follicle that will ovulate.

The follicle is therefore both the source of the egg and the cycle's main hormone gland during the first half of the menstrual cycle. A practical caveat is that the great majority of follicles never ovulate; they die by a programmed process called atresia. This relentless attrition, not ovulation, accounts for most of the lifelong decline in egg number.