Myxoid variant of malignant fibrous histiocytoma: ultrastructural observations.

The ultrastructural findings in 4 cases of the myxoid variant of malignant fibrous histiocytoma (MFH) are described. Although this neoplasm is characterized by a high rate of local recurrence, the overall prognosis is considerably better than the usual nonmyxoid MFH. The neoplasm is composed of an admixture of round, stellate and multinucleated giant cells within a myxoid and well-vascularized stroma. Electron microscopy demonstrated four principal cell types: a primitive mesenchymal cell, spindle cells of fibroblastic and histiocytic nature, and multinucleated giant cells. These observations complement the light microscopic features but probably are insufficient to differentiate critically this entity from other myxomatous lesions of mesenchymal soft tissue such as myxoma, pseudosarcomatous fasciitis, myxoid liposarcoma. The polymorphic cellular composition lends support to the concept that the neoplasm is probably derived from a primitive multipotent mesenchymal cell capable of structural and functional modulation toward more cytodifferentiated forms.