Regulation of Ovarian Follicular Development in Primates: Facts and Hypotheses

IN THE adult ovary, folliculogenesis starts when follicles leave the pool of resting follicles (RF) to enter the growth phase. From there, the early growing follicle undergoes a developmental process including a dramatic course of cellular proliferation and differentiation. In primates, only one follicle commonly reaches the preovulatory stage every cycle; most follicles fail to complete this maturation scheme, dying in the process termed atresia. In recent years, a picture has emerged depicting the classic endocrine control of ovarian function by LH and FSH, entangled in a maze of regulatory systems hinging on cell-cell interactions between follicular cells, via action of a variety of molecules (1–3). Different types of cell-cell interactions have been described. In paracrine regulations, a molecule synthesized by one cellular type is released into the interstitial milieu to act directly on another cellular type. In autocrine regulations, molecules synthesized by one cellular type are released to act either on the same tissue (autocrine action), on immediate neighbor cells (juxtacrine action), or are not released and act inside the producing cell itself (intracrine action). These paracrine and autocrine regulations may be involved at every stage of folliculogenesis by acting either alone or by modifying the multiple functions of FSH and LH. Finally, it appears from these findings that during folliculogenesis a finely tuned process of progressive differentiation occurs in all constituents of the follicle.