Understanding ovarian life cycle

Abstract

Ovarian reserve is determined by the number of primordial follicles in the ovary. Quiescent primordial follicles are recruited as primary follicles, which continue to grow until they reach gonadotropin responsive antral stage where after another wave of cyclic recruitment occurs to select a cohort of antral follicles for further growth, dominance and ovulation. What triggers the initiation of growth in primordial follicles remained a mystery for decades. But now a growing body of evidence suggests that rather than a single hormone or signaling pathway an orchestrate of many signals arising from different compartments in the ovary such oocyte, granulosa and theca cells and stroma coordinate the activation of primordial follicles and the early stages of follicle growth. Furthermore these locally produced hormones can modify the response of the growing follicles to gonadotropins; may act as luteinization inhibitors at later stages of follicle growth; and thence may influence the success of assisted reproduction techniques in human. Therefore the aim of this article is to provide an update on folliculogenesis that highlights important features in ovarian life cycle such as formation of germ cells, follicle assembly, growth and ovulation along with the concept of postnatal oogenesis in the ovary from current and historical perspectives.