Fig. 1

Timing of ovary development in mouse and human. The gonadal ridge forms during fetal development (i), after which it is invaded by proliferating primordial germ cells (ii). Meiosis is then initiated, prior to ovarian follicle formation (iii). After entering Prophase I of meiosis, germ cells, now termed oocytes, become arrested at the dictyate stage of prophase I (iv), at which point they interact with surrounding somatic pregranulosa cells to form primordial follicles (v). From this point on, follicles gradually leave the resting primordial phase and initiate growth (vi)