MYOFILAMENT LOCALIZATION AND IMMUNOELECTRON MICROSCOPIC DETECTION OF MUSCLE-SPECIFIC ACTIN IN NEOPLASTIC MYOEPITHELIAL CELLS IN PLEOMORPHICADENOMAS AND MYOEPITHELIOMAS

Elucidating the cellular characteristics of the nonluminal or myoepith elial cells of pleomorphic adenomas is one approach to establishing th e 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, i rrespective of whether nonluminal cells of pleomorphic adenomas or tum or cells of myoepitheliomas were spindle, angular, round, or plasmacyt oid in form, myofilaments were noted only in one third of the cases an d were present even in these in a small proportion of the tumor cells. Intermediate filament accumulations and basal lamina were more freque nt findings associated with nonluminal tumor cells. Six pleomorphic ad enomas and 2 myoepitheliomas had been fixed in half-strength glutarald ehyde and embedded in LR White resin for immunoelectron microscopic de tection of muscle-specific actin. In 3 (2 pleomorphic adenomas and myo epitheliomas) 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 an d infrequent in 2 cases and undetectable in the third by routine trans mission electron microscopy. Lack of myofilament detection by immunocy tochemistry or routine electron microscopy does not exclude a diagnosi s of pleomorphic adenoma or myoepithelioma when growth patterns and cy tology indicate such diagnoses. Immunoelectron microscopy, in fact, sh ows that muscle-specific actin can be detected even when myofilaments or muscle actin are apparently absent or minimal by routine electron m icroscopy or immunohistochemistry, respectively. Because examples of p leomorphic 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-sp ecific actin, the time-honored term myoepithelial for the nonluminal c ells of pleomorphic adenomas and the term myoepithelioma are legitimat e even in the absence of those markers that are specific for normal my oepithelial cells.