Thy-1: a differentiation marker of potential mammary myoepithelial cells in vitro.

The rat mammary cell line Rama 25 [Bennett, D.C., Peachey, L.A., Durbin, H. & Rudland, P.S. (1978) Cell 15, 283--298] differentiates morphologically in vitro from a cuboidal form to a fusiform cell resembling myoepithelial cells. This differentiation occurs in all clonal isolates of the line. By using three different rabbit antisera specific for Thy-1, we have found that antigenic differentiation accompanies morphologic change to the fusiform state. Very few cuboidal cells had Thy-1 detectable on their surfaces in the living state; but after acetone fixation cytoplasmic Thy-1 was detected by immunofluorescence in all cells of the cuboidal type. Thy-1 specificity was established by the fact that immunofluorescence induced by rabbit anti-rat thymocyte serum was abolished by absorption with rat brain but not with erythrocytes, kidney, or liver; immunofluorescence induced by rabbit antiserum to purified Thy-1 glycoproteins from mouse lymphomas was absorbed by Thy-1 positive mouse and rat lymphomas. Surface Thy-1 provides a potentially valuable antigenic marker in the Rama 25 line for studying the differentiation of mammary myoepithelial cells.