Follicular growth and atresia in the mouse.

Follicles were classified on the basis of the number of layers of follicle cells, the presence and degree of development of the zone pellucide, and the presence of an antrum. Formation of an antrum in follicles with less than 7 to 8 cell layers and/or presence of necrotic cells was considered indicative of degeneration. When classified in this manner, the data suggest that follicles and their contained oocytes are committed to either normal development or atresia by the time a third layer of granulosa cells is formed.