ROLES OF ESTROGEN AND PROGESTERONE IN NORMAL MAMMARY-GLAND DEVELOPMENT - INSIGHTS FROM PROGESTERONE-RECEPTOR NULL MUTANT MICE AND IN-SITU LOCALIZATION OF RECEPTOR

In contrast to most other organs, the development of normal mammary gl ands occurs mostly in the postnatal state and in a discontinuous fashi on. In all species, the glands are composed of various cell types, and it is the epithelium, embedded in the fatty stroma (commonly known as the ''fat pad''), that is targeted for proliferation and differentiat ion. Hormones, in particular the female sex steroids estrogen and prog esterone, are absolutely essential for the proliferation of mammary ep ithelial cell. However, despite intensive efforts by many laboratories spanning nearly 4 decades, at present, the precise role of these ster oids and their relative importance in mammary development remains uncl ear. This article reviews the general features and the effects of estr ogen and progesterone on normal mammary gland development, followed by a discussion of our recent studies, which emphasize the primary impor tance of progesterone and progesterone receptor for the normal mammary epithelial cell proliferation and differentiation. (C) 1997, Elsevier Science Inc.