ABSTRACT
The control of normal cellular proliferation involves a delicate balance between different influences in the extracellular environment, including signaling molecules and cell–cell interactions. The two major female sex-steroid hormones, estradiol and progesterone, are key elements in the regulation of growth and development of female sex organs. Estrogen, acting in concert with other hormones and growth factors, appears to be the main drive to proliferation in these tissues. In contrast with the ‘proliferative’ effects of estrogen, progesterone can be seen as the ‘differentiating’ female sex steroid. In this role it can either stimulate or inhibit proliferation. For example, progesterone causes the glandular elements of the mammary gland to grow and develop into secretory epithelium with the ultimate effect of acting in concert with other hormones, particularly prolactin, to facilitate milk production. While the biology of these processes is well documented, it is only recently that progress has been made towards understanding the molecular basis of steroidal control of cell cycle progression. Despite this progress, much remains to be learnt.
