The avian ovarian hierarchy: A balance between follicle differentiation and atresia

In reproductively active females from several vertebrate groups, including many birds and reptiles, the paired or single ovary(ies) consist of an orderly arrangement of follicles representing, at a single point in time, all stages of development ranging from slow-growing, relatively undifferentiated follicles to fully differentiated, preovulatory follicles. Under normal environmental and physiological conditions, this ovarian follicle hierarchy is established by a combination of follicle growth, follicle attrition (by the process of atresia), and most importantly the shedding of mature, preovulatory follicles at ovulation. At the termination of the breeding season, all partially developed follicles are lost via atresia and the ovary returns to a quiescent state until the next breeding season. At present, hormonal and cellular factors which maintain the follicle hierarchy during the breeding season are incompletely understood. The present review focuses largely on recent information describing the actions of endocrine and paracrine factors which regulate ovarian follicle differentiation and determine follicle viability, as well as the intracellular mechanisms of their actions.