Initial and cyclic recruitment of ovarian follicles

Mammalian ovaries consist of follicles as basic functional units. The total number of ovarian follicles is determined early in life. and the depletion of this pool leads to reproductive senescence. Each follicle develops to e ither ovulate or, more likely, to undergo degeneration. The dynamics of ova rian follicle development have interested endocrinologists and developmenta l biologists for many years. with the advent of assisted reproductive techn iques in humans, the possibility of regulating follicle development in vivo and in vitro has gained clinical relevance. In this review, we focus upon key branching points during the development of ovarian follicles as well as factors involved in determining the eventual destiny of individual follicl es. We discuss inconsistencies in the literature regarding the definitions of follicle recruitment and selection and propose to name the two major ste ps of follicle development as initial and cyclic recruitment, respectively. Because some of these disparities have risen due to differences in the ani mal systems studied, we also compare the development of the ovarian follicl es of both humans and rats. We also review the status of knowledge of sever al puzzling clinical issues that may provide important clues toward unlocki ng the mechanisms of follicle development.