Immunohistochemical analysis of cytokeratin expression in various trichogenic tumors.

The immunophenotypes, especially expression of cytokeratins, in 13 cases of trichogenic tumors were examined to investigate their histogenesis. Four cases of multiple trichoepithelioma, five cases of classical solitary trichoepithelioma, one case of desmoplastic trichoepithelioma, one case of trichogenic trichoblastoma, one case of trichoblastic fibroma, and one case of giant solitary trichoepithelioma were retrieved. The immunoreactivities of the epithelial nests and the keratinous cysts in these tumors were similar to those of the outer root sheath and the infundibulum of normal hair follicles, respectively. From the comparative studies of the immunophenotypes with those of normal hair follicles, we speculated that all trichogenic tumors differentiate mainly toward the outermost layer of the outer root sheath between the lower part of the permanent portion and the upper part of the transient portion and some parts of them differentiate toward various other parts of the follicles. Although differentiation toward the other follicular structures can vary from case to case, there is no particular staining pattern specific for each kind of trichogenic tumor and no significant differences in immunoreactivity among them. Our observations support a recent notion that all neoplasms of follicular germinative cells should be grouped as a single entity.