Importance of the IGF system in early folliculogenesis

In the primate ovary, androgens promote oocyte insulin-like growth factor (IGF) I expression as well as initiation of follicle development. In most other mammals, IGFs do not appear to be required for the entry of follicles into the pool of growing follicles and for their gonadotropin-independent development. In contrast, IGFs are involved in the increase in FSH-responsiveness of granulosa cells when follicles enter into the gonadotropin-dependent stages of follicular development (200 microm in mouse, 2 mm in sheep, 5 mm diameter in cattle). In the late stages of folliculogenesis, the decrease in IGFBPs participates in the increase in IGF bioavailability, leading to a further amplification of FSH action.