Apoptosis in the estrous and menstrual cycles.

In the absence of pregnancy, the adult mammary gland is subjected to cyclic fluctuations of hormonal stimulation that constitute the estrous and menstrual cycles. The mammary epithelium responds to these systemic hormonal changes by regional proliferation, differentiation and cell death by apoptosis. The fact that the mammary epithelial response involves only a minor subset of the epithelial cells implies a delicate local control of epithelial cell fate resulting in either cell death or survival. Evidence gleaned from descriptive data suggests that the apoptosis-related genes of the Bcl-2 gene family, tissue remodeling genes, protein tyrosine kinases and master genes of the homeotic gene cluster may be involved in determining epithelial cell fate during the estrous cycle.