The IGF family and folliculogenesis

The process of ovarian follicular development is a long and arduous one, marked by dramatic proliferation and differentiation of both the somatic and germ cell elements. To a large extent, this explosive agenda is under the control of conventional endocrine principles, involving hormones such as pituitary gonadotropins and ovarian steroids. Importantly, it has become increasingly apparent that several phenomena central to ovarian physiology are not fully accounted for by conventional endocrine principles. As an example, consideration must be given to the process of follicular selection whereby a predetermined number of follicle(s) is recruited, selected, allowed to assert dominance, and ultimately ovulate despite the fact that all follicles are afforded comparable gonadotropic stimulation. These observations have given rise to the suggestion that the regulation of ovarian function may be under the control of yet another group of modulatory principles known under the general term 'putative intra-ovarian regulators'. It is generally envisioned that the exquisitely-timed and highly-regionalized expression of these locally-derived, often peptidergic, principles may finally account for those aspects of the ovarian life cycle which at this time defy conventional explanation. It has been hypothesized that the role of IGF-1 in the context of ovarian physiology is to serve as an amplifier of gonadotropin hormonal action. At this time, significant support for this hypothesis can be documented. More speculative is the notion that IGF-1 may partake in intrafollicular intercompartmental coordination, a concept presupposing enhanced more coordinated follicular development, due to granulosa-theca-interstitial cell cross-talk and improved coupling. Entirely in the speculative realm is the proposition that IGF-1 may in fact partake in the process of follicular selection, a notion for which relatively limited support can be derived at this time.