Myoepithelial-like cell lines from normal mammary glands of neonatal Ludwig Wistar rats, rat mammary (Rama) 401 and Rama 704E, were injected into fat pads of syngeneic animals or were single-cell cloned in vitro. Rama 401 produced
tumors that were predominantly composed of elongated cells, while the subclones of both cell lines yielded multilayered structures of elongated cells when grown on floating 0.3%
collagen gels in vitro. Immunocytochemical analysis of histologic sections for markers of myoepithelial cells revealed that anti-actin-
myosin and human
keratin sera failed to
stain the Rama 401
tumor cells or subclones of both cell lines on
collagen gels, but both were stained with antilaminin serum. Immunofluorescent analysis of cultures of Rama 401
tumors showed that the resulting elongated cells failed to
stain with antikeratin serum, but abundant staining was observed with antilaminin and antivimentin sera, as in the
tumors. Ultrastructural analysis of the Rama 401
tumor cells identified intermediate junctions and extracellular basement membrane-like material in the vicinity of plasma membrane-associated pinocytotic vesicles, but neither true desmosomes nor myofilamental bundles were observed. Thus growth of rat mammary myoepithelial-like cells as
tumors in syngeneic animals or as subclones in vitro can lead to selective loss of myofilaments and
prekeratin-containing intermediate filaments. Similar relatively undifferentiated elongated cells may be responsible for some of the cellular heterogeneity observed in certain
carcinogen-induced rat mammary
tumors.