Investigation for neoplastic myoepithelial cells in pleomorphic adenomas and myoepitheliomas-Histopathlogical and immunohistochemical valuations-

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Determining criteria for identifying neoplastic myoepithlium is criticalfor proper classification of salivary gland tumors. Muscle-specific actin (MSA) and intermediate filament expression were determined in methanol-fixed pleomorphic adenomas (PA ; n=14) and myoepithliomas (MYO ; N=5), as well as myofilament formation in electron micrographs of 22 PA and 10 MYO.Immunoelectron microscopy (IEM) for MSA was performed on 6 PA and 2 MYO.Considerably less MSA and cytokeratin 14 (CK14), two markers for normal myoepithlial cells, were detected in nonluminal cells of PA and tumor cells MY than expected ; only 5/14 PA (36%) and 2/5 MYO (40%) had significant numbers (ranging from 5-80%) of positive tumor cells. Expression of CK14 was also limited (50% of PA and 40% of MYO with 10-70% and 10-40% cells positive, respectively). Vimentin was strongly and diffusely (90-100% of cells) expressed in all PA and MYO,while GFAP was noted in 93% of PA (25-100% cells positive) and 100% of MYO (10-60% cells positive).By routine EM,only one-third of PA and MYO had small numbers of tumor cells with myofilaments (in only one MYO were these moderately prominent), while IEM (n=8) revealed MSA-colloidal gold positive myofilaments in 2 PA and 1 MYO,all of which appeared to lack myofilaments by routine EM.Clearly, the lack of myofilament detection by immunohistochemistry or EM does not exclude a diagnosis of PA or MYO when growth patterns and cell type indicate such a diagnosis ; this even applies to plasmacytoid MYO.IEM shows muscle-actin can be detected even when myofilaments are apparently absent or minimal by routine EM.Vimentin and GFAP are more consistent markers for neoplastic myoepithelial cells than actin and CK14.