Fig. 21. The two major classes of graafian follicles: healthy and atretic. Each undergoes a regulated course of progressive change that results in ovulation or apoptosis. (From Erickson GF: The graafian follicle: A functional definition. In Adashi EY (ed): Ovulation: Evolving Scientific and Clinical Concepts. New York: Springer-Verlag, 2000.)