DIAGNOSTIC-CRITERIA FOR NEOPLASTIC MYOEPITHELIAL CELLS IN PLEOMORPHICADENOMAS AND MYOEPITHELIOMAS - IMMUNOCYTOCHEMICAL DETECTION OF MUSCLE-SPECIFIC ACTIN, CYTOKERATIN-14, VIMENTIN, AND GLIAL FIBRILLARY ACIDICPROTEIN

Objective: Markers for normal salivary gland myoepithelium were used t o determine the extent of their expression in the neoplastic myoepithe lial (nonluminal) cells of pleomorphic adenomas and then in the tumor cells in myoepitheliomas and to gather information necessary to establ ish diagnostic criteria, especially muscle actin expression, for myoep itheliomas. Study design. Methanol/acetic acid-fixed and paraffin-embe dded tissue was used to immunohistochemically study expression of inte rmediate and smooth-muscle actin filaments in nonluminal cells in 14 p leomorphic adenomas and to compare this to their expression in five my oepitheliomas. Results. In routine histologic sections, the morphologi c variants of nonluminal tumor cells-spindle, stellate, polygonal, ang ular, and plasmacytoid-in pleomorphic adenoma mirror the spectrum of t umor cells in myoepitheliomas. Immunocytochemical similarities are als o apparent. Two specific markers for myoepithelial cells in the normal salivary gland, muscle-specific actin and cytokeratin 14, were both v ariably, independently, and never uniformly expressed in nonluminal ce lls of pleomorphic adenoma and tumor cells in myoepitheliomas regardle ss of their morphology. Cytokeratin 14 in addition labels basal cells of excretory ducts. Both muscle-specific actin and cytokeratin 14 pref erentially localized to single layers of periductal cells in pleomorph ic adenoma. Although collections of nonluminal spindle cells were most often labeled for muscle-specific actin in pleomorphic adenomas, angu lar, polygonal, and plasmacytoid cells preferentially expressed cytoke ratin 14. Similar patterns were noted in the three myoepitheliomas wit h reasonable expression of the two markers. Only isolated single cells or small groups of plasmacytoid cells in four pleomorphic adenomas wi th a significant component of these cells and the two plasmacytoid myo epitheliomas immunostained for muscle-specific actin and cytokeratin 1 4. In both tumor types, vimentin was nearly uniformly expressed in non luminal tumor cells of all morphologic types, including plasmacytoid c ells. Conclusions. The range and transition of morphology of nonlumina l cells in pleomorphic adenomas is reflected in myoepitheliomas. Incom plete or absent expression of the myoepithelial/basal cell markers, mu scle-specific actin, and cytokeratin 14, and the general expression of vimentin is common to both tumors. Because these findings apply to th e majority of plasmacytoid cells in pleomorphic adenomas, tumor cells with a similar morphology and immunoprofile are to be expected in myoe pitheliomas; the term plasmacytoid myoepitheliomas is thus appropriate regardless of the presence or absence of muscle-specific actin.