The Bcl-2 and Bax proteins are well characterized, and studies have shown that they are important regulators in the ovary. Yang and Rajamahendran demonstrated that progesterone-induced antral follicle atresia was associated with an increased ratio of Bax levels to Bcl-2 levels in bovine follicles. Another study showed that overexpression of Bcl-2 during embryonic life resulted in a greater number of primordial follicles at birth. Similarly, a Bcl-2 knockout mouse model was found to have fewer than the normal complement of primordial follicles at birth. Mice deficient in Bax, a proapoptotic protein, were found to have greater numbers of ovarian primordial follicles that were maintained into advanced chronological age.
In addition to being important apoptotic regulators in the ovary, Bcl-2 and Bax have been shown to be involved in chemically induced apoptosis. For example, dimethyl-benz[a]anthracene, a polyaromatic hydrocarbon that has been shown to induce ovarian atresia in juvenile catfish, also has been shown to induce Bax expression and apoptosis in mouse oocytes.
Atresia can occur at any stage of follicular development, but antral follicles are most vulnerable to atresia.