Expression and distribution of vimentin and keratin filaments in heterokaryons of human fibroblasts and amnion epithelial cells.

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Abstract
The expression of intermediate filaments of the keratin and vimentin type was studied in heterokaryons of human fibroblasts and amnion epithelial cells by immunofluorescence microscopy. Fibroblasts and their homokaryons showed a fibrillar, vimentin-specific fluorescence throughout the cytoplasm but were negative when stained for keratin. Amnion epithelial cells and their homokaryons showed a keratin-specific fibrillar staining, and only some of them also contained detectable vimentin. When suspended epithelial cells were fused with adherent fibroblasts, keratin fibrils spread within 3 h into fibroblasts, intermixing with the vimentin fibrils. At 1-3 d after fusion, both vimentin and keratin filaments were expressed as typical fibrillar cytoplasmic arrays, and the distribution of keratin in heterokaryons closely resembled that of vimentin. A typical cell-to-cell arrangement of keratin fibrils, seen in cultures of amnion epithelial cells, could also be found between heterokaryons. Treatment of the cultures with vinblastine sulfate induced coiling of the vimentin filaments in both homo- and heterokaryons, whereas the keratin organization was only slightly affected. Results show that both vimentin and keratin filaments are incorporated into the cytoskeleton of heterokaryons formed between fibroblasts and epithelial cells, and that they behave in the same way as in their parental cells. Both epithelial and fibroblastic characteristics appear coexpressed in such heterokaryons.