Malignant plasmacytoid myoepithelioma of the palate: histological observations compared to benign predominant plasmacytoid myoepithelial cells in pleomorphic adenoma of the palate.

Predominant benign plasmacytoid myoepithelial cells in pleomorphic adenoma and malignant plasmacytoid myoepithelioma cells were investigated morphologically. The cells of both tumors were plasmacytoid in appearance and sheet-like. Immunohistochemically, they were positive for keratin, vimentin, and S-100 protein, and negative for alpha-smooth muscle actin. In the malignant cells, large nuclei with irregular nuclear membranes and distinct nucleoi and occasional intranuclear inclusions and nuclear grooves were seen. Ultrastructural findings showed that the benign cells were richer in intermediate filaments and had fewer mitochondria. The intranuclear inclusions and nuclear grooves of the malignant cells were caused by invagination of the irregular nuclear membranes. Taken in their entirety, the above light microscopical nuclear findings may be useful as an adjunct for distinguishing malignant from benign plasmacytoid neoplastic myoepithelial cells of the salivary gland.