[An electron microscopic study of malignant giant cell tumor of bone]

Three cases of malignant giant cell tumors were studied by light microscope and electron microscope. Light microscopic observation revealed three different types of tumor cells, namely, malignant stromal cells, fibrosarcomatous cell and osteosarcomatous cells. Electron microscopic observation revealed that fibrosarcomatous cells had ultrastructure similar to that of malignant stromal cells. Both types of cells showed nuclear pseudo-inclusions (NPI), and well developed organellae related to matrix synthesis in the cytoplasm. Polysaccharide staining of the same cells revealed positive products in the cytoplasm. Osteosarcomatous cells showed irregular nuclei which contained NPI and well developed r-ER. The cells were located in the non-calcified matrix which consisted of collagen fibers. No alkaline phosphatase activity was observed in this area. These facts suggest that: (1) malignant stromal cells and fibrosarcomatous cells synthesize collagenous stroma as normal fibroblasts do, and (2) osteosarcomatous cells are devoid of the ability to promote matrix calcification.