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Studies on differentiation of human mammary epithelial cells in culture: distinctive specificities of conditioned media.

Epithelium cultured from human milk contains cells of two types distinguishable by morphology and keratin pattern. First, the "open" type comprises cells which are isolated from one another, and which do not form typical epithelial sheets. Second, the "joined" type grow as tightly adjoining cell sheets. Clonal analysis shows that open cells, besides reproducing the same phenotype, can give rise to mixed colonies containing joined cells, thus indicating a pathway of in vitro differentiation. When conditioned medium was used to enhance colony growth it was found that human embryo lung fibroblasts release an activity which specifically enhances the open type of mammary epithelial cell. On the contrary the Nil 8 hamster fibroblast line and certain other cells release activities which primarily affect the joined cell type. This suggests that there are naturally occurring factors with specificities for distinct stages in mammary epithelial cell differentiation.