Anterior pituitary cell renewal during the estrous cycle.

The anterior pituitary gland undergoes a process of cell renewal during the estrous cycle. Although the occurrence of proliferation and death of anterior pituitary cells at specific stages of the estrous cycle is well known, the underlying mechanisms that regulate these processes are still being uncovered. In spite of the recognized proliferative effects of estrogens on lactotropes, recent evidence shows that estrogens can also trigger antiproliferative and apoptotic responses in anterior pituitary cells. In the present review we analyze the actions of gonadal steroids on proliferation and death of anterior pituitary cells during the estrous cycle and the mediators involved in these actions. Estradiol sensitizes anterior pituitary cells not only to mitogenic stimuli but also to apoptotic signals and upregulates local synthesis of tropic growth factors as well as proapoptotic cytokines. Several growth factors and cytokines have been shown to induce estrogen-dependent lactotrope proliferation and death, whereas progesterone antagonizes estrogen-induced effects. These locally synthesized factors may mediate the effects of gonadal steroids in the process of anterior pituitary cell renewal during the estrous cycle.