Mammary fibroblasts stimulate growth, alveolar morphogenesis, and functional differentiation of normal rat mammary epithelial cells

Stromal-epithelial interactions play a profound role in regulating normal a nd tumor development in the mammary gland. The molecular details of these e vents, however, are incompletely understood. A novel serum-free transwell c oculture system was developed to study the natural paracrine interactions b etween mammary epithelial cells (MEC) and mammary fibroblasts (MFC) isolate d from normal rats during puberty. The MEC were cultured within a reconstit uted basement membrane (RBM) in transwell inserts with or without MFC in th e lower well. The presence of MFC stimulated epithelial cell growth, induce d alveolar morphogenesis, and enhanced casein accumulation, a marker of the functional differentiation of MEG, but did not induce ductal morphogenesis . Potent mitogenic, morphogenic, and lactogenic effects were observed when the MFC were cultured either on plastic or within a layer of RBM. Although most MFC maintained on plastic died after 1 wk in serum-free medium, fibrob last survival was enhanced significantly when the MFC were cultured within the RBM. Taken together, this in vitro model effectively reconstitutes a ph ysiologically relevant three-dimensional microenvironment for MEC and MFC, and seems ideal for studying the locally derived factors that regulate the developmental fate of the epithelial and fibroblast compartments of the mam mary gland.