Programmed cell death in the reproductive system.

The reproductive system presents some of the best examples of programmed cell death, which is to be expected considering the dramatic cycles of tissue growth and regression in females. Hormones from the pituitary gland, gonads and uterus are responsible for coordinating cycles in which the preservation of cell survival and inhibition of apoptosis are important. In the ovary, atresia regulates the size of the follicle cohort for ovulation and is an archetype of apoptosis as induced by hormone withdrawal. The fate of an antral follicle--growth or atresia--is determined by circulating levels of gonadotrophins, and follicle-stimulating hormone (FSH) in particular. At the end of a menstrual cycle or pregnancy or lactation, hormone withdrawal triggers cell death and tissue remodelling and initiates a fresh cycle. In the endometrium, breast and prostate gland, steroid hormones are the principal survival factors and castration triggers regression of responsive tissues, which is sometimes decisive in the fight against disease. But while the primary trigger of cell death varies between tissues, underlying cellular mechanisms are more conservative and cell death/survival genes, such as bcl-2, bax and others that are expressed in other tissues, play important roles in the reproductive system too.