GROWTH-FACTORS AND ANTRAL FOLLICULAR DEVELOPMENT IN DOMESTIC RUMINANTS

Growth factors of endocrine and/or paracrine origin play determinant r oles in antral follicular development. They modulate survival, prolife ration and differentiation of follicular cells, acting in interaction with gonadotropins. It is thought that factors belonging to the famili es of the insulin-like growth factor (IGF-I and -II), epidermal growth factor (EGF and TGF-alpha) and fibroblast growth factor (FGF) can sup port growth of small antral follicles by enhancing granulosa cell prol iferation. During terminal follicular development, i.e. development of large antral follicles strictly dependent on gonadotropin supply, fac tors such as IGFs and inhibin potentiate the differentiation-promoting actions of gonadotropins on granulosa and thecal cells, respectively. In addition, binding proteins modulate bioavailability of these growt h factors in follicles. In particular, terminal follicular development in domestic ruminants is accompanied by important decreases in intraf ollicular concentrations of the IGF-binding proteins IGFBP-2, -4 and - 5, due to both a decrease in their synthesis by follicular cells and a n increase in their degradation by specific intrafollicular proteinase s. As a result, IGF bioavailability increases in large antral follicle s, enhancing the amplification of gonadotropin action on follicular ce lls. In contrast, atresia is characterized by an increase in the expre ssion of IGFBP-2, -4 and -5 mRNAs and a decrease in proteolytic degrad ation of the corresponding proteins, resulting in an increase in their intrafollicular concentrations and a decrease in IGF bioavailability. All these intrafollicular mechanisms, in addition to the privileged e ndocrine dialogue that establishes between the dominant follicle(s) an d the hypothalamo-pituitary complex, contribute to ensuring its (their ) final development and to triggering off atresia of the gonadotropin dependent follicles of the cohort. (C) 1996 by Elsevier Science Inc.