Ovarian Follicular Atresia

Abstract

Throughout ovarian development and function in mammals, a highly orchestrated, periodic process known as follicular atresia occurs that destroys and eliminates follicles and oocytes from the ovary. Follicular atresia is pervasive. In humans, it is estimated to account for 99.9% of the loss of oocytes from development of the fetal ovary until reproductive senescence (Baker, 1963; Faddy et al., 1992). Overall, the process of follicular atresia eliminates all but 300-400 oocytes, some of which become available for selection, ovulation and potential fertilization. Similar phenomena of loss are observed in other mammals. The rationale for such extensive elimination of oocytes during fetal and adult life is unknown. However, in the case of larger, more mature follicles (i.e., antral follicles), the importance of follicular atresia is attributed to a finite lifespan of the oocyte. Hence, in the adult female, atresia ensures that only the healthiest follicles, containing oocytes of optimal quality for fertilization, remain available throughout the reproductive period. In this chapter, we provide a broad overview of the physiological process of follicular atresia, giving emphasis to the cellular and molecular mechanisms that influence the process in two monovulatory species, the cow and human female.