Intraepidermal pilar epithelioma: a new dermatopathologic interpretation of a skin tumor.

An intraepidermally developed epithelial cell tumor, forming multiple nests, was examined to identify its cytologic characteristics. Histochemically, the tumor cells contained neither glycogen nor lipid substance. By N-(7-dimethylamino-3-methyl-4-coumarinyl)maleimide staining, the cytoplasm of the tumor cells in the periphery of each nest was rich in SH groups but not in SS linkages, whereas centrally located homogeneous tumor cells contained SS diffusely but no SH. The tumor cells showed no activity of phosphorylase and a weak activity of succinic dehydrogenase. Immunohistochemically, antihair keratin monoclonal antibodies specific for hair cells decorated the tumor cells, but carcinoembryonic antigen staining showed no positivity. Ultrastructurally, the tumor cells underwent a keratinization forming a fingerprint pattern of keratin filaments; however, membrane-coating granules and marginal bands were not formed. These intraepidermal tumor cells may have cytologic natures similar to those of hair cortical cells. The term intraepidermal pilar epithelioma is proposed as a diagnosis for this tumor.