Myofilament localization and immunoelectron microscopic detection of muscle-specific actin in neoplastic myoepithelial cells in pleomorphic adenomas and myoepitheliomas.

Elucidating the cellular characteristics of the nonluminal or myoepithelial cells of pleomorphic adenomas is one approach to establishing the diagnostic criteria for myoepitheliomas. Ultrastructural features of nonluminal tumor cells in 22 pleomorphic adenomas and of tumor cells in 9 myoepitheliomas were assessed from micrographs of routinely fixed and epoxy resin-embedded samples. Recognizable myofilaments were only moderately prominent in 1 myoepithelioma. In the rest of the cases, irrespective of whether nonluminal cells of pleomorphic adenomas or tumor cells of myoepitheliomas were spindle, angular, round, or plasmacytoid in form, myofilaments were noted only in one third of the cases and were present even in these in a small proportion of the tumor cells. Intermediate filament accumulations and basal lamina were more frequent findings associated with nonluminal tumor cells. Six pleomorphic adenomas and 2 myoepitheliomas had been fixed in half-strength glutaraldehyde and embedded in LR White resin for immunoelectron microscopic detection of muscle-specific actin. In 3 (2 pleomorphic adenomas and myoepitheliomas) of these 8 cases, readily visualized bands of filaments in many tumor cells were strongly labeled by the colloidal gold probe detecting muscle-specific actin even when myofilaments were minimal and infrequent in 2 cases and undetectable in the third by routine transmission electron microscopy. Lack of myofilament detection by immunocytochemistry or routine electron microscopy does not exclude a diagnosis of pleomorphic adenoma or myoepithelioma when growth patterns and cytology indicate such diagnoses. Immunoelectron microscopy, in fact, shows that muscle-specific actin can be detected even when myofilaments or muscle actin are apparently absent or minimal by routine electron microscopy or immunohistochemistry, respectively. Because examples of pleomorphic adenoma and myoepithelioma each with similar histologic and cytologic features of the myoepitheliomatous cells can have variable degrees or complete absence of expression of myofilaments or muscle-specific actin, the time-honored term myoepithelial for the nonluminal cells of pleomorphic adenomas and the term myoepithelioma are legitimate even in the absence of those markers that are specific for normal myoepithelial cells.