The Interplay of Insulin-Like Growth Factors, Gonadotropins, and Endocrine Disruptors in Ovarian Follicular Development and Function

Over the past 20 years, the expression, signaling mechanisms, and roles of members of the insulin-like growth factor (IGF) family (ligands, receptors, binding proteins, and binding protein proteases and their inhibitors) have been elucidated in ovarian follicle function in humans and other species. In vitro studies with human, nonhuman primate, and farm animal granulosa and thecal cells and genetic approaches using mouse knockout models for IGF family members have revealed that IGFs are key intraovarian regulators of follicle growth, selection, atresia, cellular differentiation, and steroidogenesis, oocyte maturation, and cumulus expansion. Some of these actions are synergistic with gonadotropins, although most are not sustainable with IGFs alone and require gonadotropin actions, thereby designating IGFs as co-gonadotropins. In the human disorder of polycystic ovarian syndrome, characterized by small antral follicle arrest, the IGF system appears to contribute to the observed resistance to foillicle-stimulating hormone action at the level of the granulosa compartment and the persistence of an androgen-dominant milieu in the arrested follicles. Interestingly, recent studies demonstrate that endocrine-disrupting chemicals can compromise IGF activity and signaling in the ovarian follicle, affecting follicle development, steroidogenesis, and oocyte quality. The successful development of a healthy oocyte and appropriate granulosa and theca cell steroidogenesis on a cyclic basis are contingent on multiple factors, including a properly functioning intraovarian IGF system. Disruption of even one component of this system can lead to abnormal follicular development and function and compromised reproductive capacity.