Journal of Molecular Endocrinology

Outline of the main steps of folliculogenesis. Folliculogenesis starts with primordial follicles that are subsequently recruited to become primary follicles. Proliferating GCs progressively form several layers around the oocyte (secondary to late preantral follicles). From this stage, a layer of theca cells surrounds the follicle, and the follicles start to produce oestrogens. Theca cells produce androgens, which are converted into oestrogens in granulosa cells. Whereas follicular growth to the secondary/preantral stage is independent of gonadotrophins, progression beyond this stage strictly depends on FSH stimulation. As GC continue to proliferate, the antrum is formed and separates de facto granulosa cells into two general populations: cumulus and mural GCs. At this stage, selection occurs between growing follicles, and only one or a limited number of follicles continue growing to the preovulatory stage while others undergo atresia. Two layers of theca (theca interna and externa) can be distinguished around selected follicles. After ovulation, which is triggered by a peak of FSH and LH, theca cells and mural granulosa cells luteinise to produce progesterone.