Experimental mammary epithelial morphogenesis in an in vivo model: evidence for distinct cellular progenitors of the ductal and lobular phenotype.

An in vivo transplantation system has been used to evaluate the developmental capacities of specific mouse mammary epithelial cell populations. Specifically, mouse mammary epithelial cells with distinctly limited developmental potentials have been identified using this procedure. Two distinct epithelial cell progenitors have been identified by experiments designed to determine whether basal lobular and ductal phenotypes could develop independently under conditions imposed by a limiting dilution. The prediction that these separate epithelial progenitors must exist was based upon the results from transplantation experiments carried out in epithelium-divested mammary fat pads of syngeneic mice with mammary epithelium from two different transgenic mouse models. The results presented here demonstrate the following points: 1) lobular, i.e. secretory, progenitor cells are present as distinct entities among the mammary epithelial cells found in immature virgin female mice; 2) similarly, ductal epithelial progenitors are present within the same population; 3) lobular progenitors are present in greater numbers, although both cell populations are extremely small; 4) as expected, some inocula produce outgrowths with simultaneous development of both lobular and ductal phenotypes--it is not known whether this indicates cooperative interaction between the two epithelial progenitors or signals the presence of a third progenitor type capable of producing both ductular and lobular committed daughters; 5) these findings have important consequences in the design of experiments aimed at testing the effects of known and putative mammary oncogenes and tumor suppressor genes, using techniques which include cellular transformation in vitro followed by in vivo cultivation and evaluation.