Squamous differentiation in human breast epithelial cells in culture, with comparative observations on intermediate filaments from intact epithelium.

Intact, grossly normal human mammary tissue and cultured breast epithelial cells were examined by transmission electron microscopy for indicators of squamous differentiation. Intact epithelium contained 7-10 nm filaments which we interpreted as cytokeratin intermediate type filaments; many formed bundles which lacked conspicuous electron density. Two types of cell process were present: one was luminal and covered in a glycocalyx, whereas the other lacked a coat and was confined to lateral membranes. In culture, mammary epithelial cells contained intermediate filament bundles which were conspicuous by their enhanced electron density compared with those of intact epithelium. These were acceptable as dense tonofibrils i.e. indicators of squamous differentiation. Cultured cells also possessed cell processes broadly comparable to those of intact epithelium. This study confirms the phenomenon of squamous differentiation consequent upon culture, in a system which differed methodologically from previous studies, and makes hitherto neglected comparisons between intact and cultured breast epithelium.