Plasmacytoid monomorphic adenoma of salivary glands. Absence of myogenous differentiation and comparison to spindle cell myoepithelioma.

The immunohistochemical and ultrastructural features of two salivary gland monomorphic adenomas composed of plasmacytoid cells (so-called plasmacytoid myoepitheliomas) were studied to determine if the plasmacytoid cells contained detectable evidence of myogenous differentiation. The results were compared with the immunohistochemical profile of three salivary gland myoepitheliomas of spindle-cell type. The plasmacytoid tumors were each immunoreactive for vimentin, cytokeratin, S100 protein, and glial fibrillary acidic protein (GFAP). They were negative for muscle-specific actin (MSA), smooth-muscle actin (SMA), and desmin. Conversely, two of three spindle-cell myoepitheliomas were immunoreactive for MSA and SMA, in addition to vimentin, cytokeratin, and S100 protein. One tumor also contained focal positivity for desmin and GFAP, and a single spindle-cell tumor was vimentin-positive only. Ultrastructurally, plasmacytoid cells were characterized by focal desmosomes, basal lamina, and abundant intermediate cytoplasmic filaments. Dense bodies typical of smooth-muscle cells and actin-sized filaments were absent. Immunohistochemically and ultra-structurally, the plasmacytoid cells lack any evidence of myogenous differentiation and should not be considered a subtype of myoepithelioma.