Adipocyte-epithelial interactions regulate the in vitro development of normal mammary epithelial cells

Mammary epithelial organoids (MEO), isolated from pubescent rats, were cult ured within a reconstituted basement membrane in transwell inserts, in the presence or absence of mature mammary adipocytes in the lower well. This sy stem allowed for free medium exchange between the two compartments, without direct cell-to-cell contact. When cultured in serum-free medium supplement ed with insulin, prolactin, hydrocortisone, progesterone, and various epide rmal growth factor (EGF) concentrations, mammary adipocytes did not affect epithelial cell growth, but enhanced epithelial differentiation. Casein and lipid accumulations were monitored as indicators of functional differentia tion of MEG. Mammary adipocytes significantly enhanced casein and lipid acc umulation within the MEG, independently of EGF concentration. Furthermore, adipocytes induced MEO to preferentially undergo alveolar morphogenesis, in hibited squamous outgrowth, and increased lumen size. These findings demons trate that morphological and functional differentiation of mammary epitheli al cells is profoundly enhanced by the adipose stroma and that these effect s are mediated by diffusible paracrine factors. This new model can be explo ited in future studies to define the mechanisms whereby hormones and growth factors regulate mammary gland development and carcinogenesis. Moreover, i t could complement in vivo reconstitution/transplantation studies, which ar e currently employed to evaluate the role of specific gene deletions in the regulation of mammary development. (C) 1999 Academic Press.