The follicle has a fundamental reproductive role in the ovary. Follicle growth takes 85 days in humans and most follicles become artistic at some stage with the oocyte continuing to play a critical role in follicular control and the regulation of oogenesis, ovulation rate and fecundity. The first primordial follicle in human foetuses is formed at 15-22 week gestation, at which point oocytes are enclosed by a single layer of pre-granulosa cells. Upon activation of primordial follicles, progression from the primary to secondary stage of follicular development requires further oocyte expansion, granulosa cell (GC) proliferation and investment of a luteinizing hormone (LH)-responsive theca cell layer. The pre-antral follicle continues its development and becomes increasingly follicle stimulating hormone (FSH) responsive. Follicular antrum formation and antral expansion are absolutely dependent on FSH. Luteinizing hormone appears to play a major role in mediating the final phase of follicular development.