Primary fibrosarcoma and malignant fibrous histiocytoma of bone - A comparative ultrastructural study: Evidence of a spectrum of fibroblastic differentiation

As primary bone fibrosarcoma (FS) and malignant fibrous histiocytoma (MFH) have similar clinical, radiographic, or survival manifestations, ultrastruc tural and immunohistochemical studies were undertaken to determine the diff erentiation pathways of constituent malignant cells. Twelve cases of primar y intraosseous FS and MFH were selected for this ultrastructural comparativ e study and were analyzed for fibroblastic or modified fibroblastic differe ntiation. There were 4 FS cases and 8 MFH cases, of which 5 were storiform- pleomorphic, 2 were giant cell, and 1 was myxoid type. All FS consisted of spindle fibroblasts with a prominent rough endoplasmic reticulum and Golgi apparatus, variable amounts of vimentin intermediate filaments, and extrace llular collagen fibrils. The MFH were composed of a mixture of spindle and pleomorphic fibroblasts (8/8), histiofibroblasts (4/8), and myofibroblasts (3/8). Variable numbers of undifferentiated cells were found in both tumors . In conclusion, fibroblastic differentiation and collagen production was n oted in all cases. The often pleomorphic histiofibroblasts present in some MFH cases most likely represent "modified fibroblasts," similar to myofibro blasts. These findings support the hypothesis that the fibroblast and its v ariants are the predominant cell types found in these tumors, suggesting th at the diagnostic entity MFH should be classified as a pleomorphic fibrosar coma.