AbstractColony‐forming epithelial cells exfoliated in human milk have been examined by immunofluorescence using antibodies to cytokeratins (tonofilaments), and to high molecular weight desmosomal core proteins. The cells may be classified by their ability to form junctional complexes with their neighbours. Those deficient in desmosomal junctions, called D − cells, grow into colonies of noncontiguous cells without desmosomes, and with a perinuclear network arrangement of cytokeratins. Junction forming, or D + cells, grow as contiguous cell sheets with abundant desmosomes and well developed bundles of tonofilaments. D − cells may also segregate D + cells among their progeny yielding mixed clones, and a gradual increase in the overall number of D + cells during culture. Established D + cells have surface markers characteristic of mammary epithelium and are presumably derived by exfoliation of luminal cells of the alveoli or ducts which contain desmosomal junctions. D − cells also possess mammary epithelial cell markers, but their origin is unknown. Medium conditioned by the Nil 8 line of hamster cells contains a junction‐promoting activity that accelerates the rate, or frequency, of segregation of D + cells from D − cells, so that milk cells grown in this medium predominently give closed colonies of D + cells. Medium conditioned by the MRC5 strain of human embryo lung cells, however, contains a junction‐inhibiting activity, which prevents new junction formation and probably destroys existing junctions, so that cells in this medium mostly grow as open colonies of cells with D − phenotype. It is hoped that studies with this experimental system will assist in the better understanding of normal and abnormal regulation of desmosomal junctions and their role in t
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