Ovarian steroid hormone-regulated cell proliferation is essential for the growth and development of the breast. The fact that the normal epithelium contains receptors for both estrogen and progesterone lends support to a receptor-mediated mechanism as the likely candidate for hormonal regulation of mammary gland development.
Previous studies by the authors of proliferative activity in the mammary epithelium of both rats and humans have demonstrated that cell division varies with the degree of structural differentiation of the mammary parenchyma. In humans, the highest level of cell division is observed in the undifferentiated lobules type 1 (lob1) present in the breast of young nulliparous females. The progressive differentiation of lob1 into lob2 and lob3, occurring under the hormonal influence of the menstrual cycle, and the full differentiation into lob4 during pregnancy, results in a concomitant reduction in the proliferative activity of the mammary epithelium. Interestingly, the authors have also shown that the content of estrogen receptor alpha (ER-alpha) and progesterone receptor (PR) in the lobular structures of the breast is directly proportional to the rate of cell proliferation.