Ovarian follicle atresia is the periodic process in which immature ovarian follicles degenerate and are subsequently re-absorbed during the follicular phase of the menstrual cycle. Typically around 20 follicles mature each month and only a single follicle is ovulated. The rest undergo atresia. That single dominant follicle becomes a corpus luteum following ovulation. Atresia is a hormonally controlled apoptotic process that depends dominantly on granulosa cell apoptosis.