Malignant fibrous histiocytoma of soft parts: an ultrastructural quantitative study.

Thirty-two cases of soft tissue malignant fibrous histiocytoma (MFH), including 24 of common storiform-pleomorphic type, 7 of myxoid type, and 1 of inflammatory type, were analyzed in an ultrastructural quantitative study, the objective being to clarify the cellular composition and distribution of the tumor. Approximately 100 unselected cells in each tumor were classified into seven types as follows: undifferentiated (10%-62%, m: 29%), histiocytelike (8%-69%, m: 37%), fibroblastlike (0.9%-26%, m: 11%), myofibroblasts (0%-22%, m: 3%), intermediate between histiocytelike and fibroblastlike cells (1%-41%, m: 15%), multinucleated giant (0%-1%), and xanthomatous cells (0%-1%). The undifferentiated, histiocytelike, fibroblastlike, and intermediate cells constituted the principal types, invariably present, although in varying proportions in each tumor. No statistical difference in ultrastructural cellular composition was evident in different portions of the same tumor, and such was also the case between the two main subtypes of MFH, common and myxoid. Naphthyl thiol acetate (NTA) esterase was demonstrated in fibroblastlike cells, using the ultrastructural cytochemical technique, in both cases examined. The possibility that MFH may be of alternative undifferentiated mesenchymal cell origin has to be considered.