[Keratin 5-positive cells in the breast are progenitor cells of glandular and myoepithelial differentiation. A cell biological model as a basis of breast pathology].

In the present computer-assisted labelling study we demonstrate for the first time that in normal breast tissue keratin (K) 5+ progenitor cells give rise to either glandular (K 8/18+) or to myoepithelial cells (sm-actin = SMA+) via intermediary cells (either K 5/6+; K 8/18+ or K 5/6+; SMA+). Furthermore we have conclusive evidence that benign proliferative breast disease lesions are progenitor cell-derived lesions. In contrast, most breast cancers appear to evolve from glandularly differentiated cells. Western blotting experiments which show large amounts of K 5 in benign proliferative breast lesions, but not in ductal carcinoma in situ confirm the immunofluorescence data. Our results pave the way towards a new cell biological model of breast regeneration, benign proliferative breast disease and breast cancer.