Benign myoepithelioma of the skin.

A case of cutaneous myoepithelioma is reported. The tumor was composed of spindle-shaped, epithelioid, and plasmacytoid (hyaline) cells. It exhibited a widespread immunoreactivity for low molecular weight keratins and protein S-100, being irregularly positive for smooth muscle actin. Ultrastructural studies of tumor cells showed a variable content of intermediate filaments, with focal densities resembling smooth muscle dense bodies. A well-developed basal lamina, pinocytotic vesicles, and some desmosomes were also observed. In spite of being accepted as an individual entity, myoepitheliomas probably belong to a family of lesions that include mixed tumors. Therefore, this case can be considered as a salivary-gland-type tumor, probably originating from myoepithelial cells of sweat glands. The existence of this unique neoplasm provides further support to the debated role of myoepithelial cells in the development of mixed tumors.