Alveolar soft part sarcoma. Immunohistochemical evidence for muscle cell differentiation.

Seven cases of alveolar soft part sarcomas (ASPSs) were studied immunohistochemically for the presence of a number of differentiation markers in an attempt to define the cellular nature of this tumor. Desmin-positive tumor cells were found in three and muscle actin-positive cells in four cases when studied in formaldehyde-fixed and paraffin-embedded material. In one case studied in frozen sections, focal desmin positivity but no other intermediate filaments were found. Immunostaining for synaptophysin, a general neuroendocrine marker, was negative in all cases. All tumors were positive with a monoclonal antibody NK1C3, which consistently stains melanomas, and three cases showed significant numbers of S-100 protein-positive tumor cells, but immunostaining with HMB-45, a melanoma-specific monoclonal antibody, was negative in all cases. However, several rhabdomyosarcomas studied for NK1C3 and S-100 protein for comparison were also at least focally positive. Electron microscopic examination, performed in three cases, showed uniform paucity of all kinds of filaments in the tumor cells of ASPS, and it specifically failed to reveal any signs of smooth or striated muscle cell differentiation. Thus, the results of the present study do not unequivocally define the nature of ASPS but speak against its paraganglionic character and present evidence for muscle cell differentiation.