Participation of two different mesenchymes in the developing mouse mammary gland: synthesis of basement membrane components by fat pad precursor cells

Two different types of mesenchyme, fat pad precursor cells (FP) and fibroblastic cells (MM) are involved in the morphogenesis of mammary gland epithelium of mouse embryo. Especially, an interaction between FP and the epithelium is necessary for its characteristic shaping of ductal branching structure. To assess the relative participations of the mesenchymes, we have analysed the extracellular matrix products by immunofluorescent staining method using antibodies to laminin, proteoheparan sulphate, and fibronectin. The staining patterns suggested that, after the 16th day of gestation when fatty substances first appeared in FP and the epithelial rudiments started to elongate and branch rapidly, FP initiated synthesis of laminin and proteoheparan sulphate, while MM synthesized fibronectin at all times. Attention was also paid to differences in the epithelial basement membranes (BM) concomitant with ones in the mesenchyme. BM were always stained with antibodies to laminin and proteoheparan sulphate. However, topographical differences in thickness were observed: the one facing FP, often seen at the tip region of the end bud, was thin, while the other surrounded by MM, often at the flank region of the duct, was thick. Specific elaboration of BM-like extracellular matrix products by FP may attribute to observed differences in BM thickness which are related to the characteristic shaping of the mammary gland.