[Cellular differentiation in storiform-pleomorphic malignant fibrous histiocytomas. An electron microscopic study of histogenesis].

The most common form of malignant fibrous histiocytoma is the storiform-pleomorphic subtype composed of spindle-shaped fibroblast-like cells, mononucleated histiocytic elements and a changing amount of pleomorphic giant cells. In relation to the changing cellular structures 14 pleomorphic-storiform malignant fibrous histiocytomas were investigated electronmicroscopically. In all tumors several types of cells varying in shape, and size as well as in organelle composition could be demonstrated: 1. Undifferentiated cells, which are relatively small and have a scanty cytoplasm with few organelles. 2. Fibroblast-like cells with well developed rough endoplasmic reticulum, mostly arranged in a storiform pattern. 3. Myofibroblasts corresponding to fibroblasts and showing bundles of thin filaments (4 to 6 nm) with focal dense bodies in the peripheral area of the cytoplasm. 4. Histiocyte-like cells characterized by filopodia-like projections and abundant cytoplasm containing lysosomes and phagolysosomes and also lipid droplets. 5. Chimeric cells, which are intermediate forms with features of fibroblast-like and histiocyte-like tumor cells. 6. Multinucleated tumor giant cells which can be subdivided into fibroblast-like and histiocyte-like types and intermediate forms. On the basis of our ultrastructural studies the storiform pleomorphic malignant fibrous histiocytoma is interpreted as a tumor of an undifferentiated mesenchymal cell with the potency of fibroblastic or histiocytic differentiation. The origin of this cell is uncertain. Dedifferentiation of a differentiated connective tissue cell (fibroblast, pericyte) into a proliferating undifferentiated precursor cell is discussed.