Fig. 1.

The essential steps of development and degeneration of follicles and the CL in a mammalian ovary. PGC cysts form in the embryonic period (blue area). After the PGC depletion before birth, the primordial follicles start to gradually develop into antral follicles. During the juvenile period (green area) all antral follicles undergo atresia. During the reproductive period (yellow area) a surge of FSH rescues the dominant follicle, while smaller antral follicles degenerate via atresia. Dominant follicles next undergo ovulation stimulated by FSH and LH and transform into CL , which subsequently involutes into corpus albicans in a PGF2α-dependent manner. Primordial follicles are repeatedly recruited until the whole ovarian reserve is depleted (pink area) and the reproductive period ends. The black arrows indicate survival and development, the red arrows indicate degradation events, and the lightning symbols indicate the crucial involvement of endocrine regulators