The various structural components of pilomatrixoma (calcifying epithelioma of Malherbe) were studied for the expression of hair-specific (trichocytic) cytokeratins as well as epithelial cytokeratins, using immunoperoxidase and immunofluorescence microscopy of frozen sections as well as two-dimensional gel electrophoresis and immunoblotting. Trichocyte-type cytokeratins were detected in only a minor subpopulation of basophilic cells but more prominently in most "transitional" cells as well as in "shadow" cells. In contrast, antibodies against certain epithelial cytokeratins (including antibody KA1 against cytokeratins of stratified squamous epithelia and antibodies against cytokeratin 19) revealed an extensive but heterogeneous staining of basophilic cells. In regions of squamoid cells, epithelial cytokeratins in an unusual pattern were found. An antibody against vimentin was negative on pilomatrixoma cells. An antibody against desmoplakins decorated true desmosomes in basophilic and transitional cells. Biochemically, trichocytic cytokeratin polypeptides as well as epithelial cytokeratins 5, 6, 14, 16, 17, and 19 were positively detected. These results provide evidence in support of the notion of a trichocytic differentiation and probably derivation of pilomatrixoma. According to the cytokeratin expression patterns the majority of pilomatrixoma cells resemble the normal hair cortex lineage but some enter a pathway of squamous cell differentiation.
