Ultrastructural and immunohistochemical characteristics of mucoepidermoid carcinoma of the breast.

Mucoepidermoid carcinoma is a rare subtype of breast carcinoma. In an attempt to elucidate the histogenesis of these tumors, two mucoepidermoid carcinomas were studied electron microscopically and immunohistochemically with antisera to S-100 protein and epidermal cytokeratin. Light microscopic examination showed these tumors to be composed of mixtures of neoplastic mucus-secreting, squamous, and intermediate cells. Ultrastructural examination revealed squamous cells, mucus-secreting luminal cells, and classic myoepithelial cells. The intermediate cells, which contained focal peripheral myofilaments with dense bodies and pinocytotic vesicles associated with centrally aggregated tonofilaments, were interpreted as modified myoepithelial cells. Classic dendritic myoepithelial and intermediate (modified myoepithelial) cells stained with antisera to S-100 protein and epidermal cytokeratin. Both the ultrastructural and the immunohistochemical features indicate that myoepithelial and duct luminal cells play an important role in the histogenesis of mucoepidermoid tumors of the breast.