Electron microscopy of bone tumors--osteosarcoma, chondrosarcoma, giant cell tumor of bone.

Six cases of osteosarcoma, 5 cases of chondrosarcoma, and 6 cases of giant cell tumor of bone were studied by electron microscopy. The cells comprising osteosarcoma were basically osteoblasts and in addition to them, malignant chondrocytes, myofibroblasts and undifferentiated cells. The ratio of number of these cells were variable according to the portions designated microscopically as osteoblastic, chondroblastic and fibroblastic. Previous ultrastructural studies on osteosarcoma have scarcely revealed the presence of myofibroblasts. It is likely that osteosarcoma arises from undifferentiated mesenchymal cells capable of differentiating into various cells such as osteoblasts, chondroblasts, and fibroblasts; especially capable of differentiating into osteoblasts. On the other hand, chondrosarcoma was composed of the cells in the series of maturation from undifferentiated cells to chondrocytes. In high-grade malignant cases, glycogen particles and rough endoplasmic reticulum became less conspicuous, and occasionally malignant multinucleated cells were seen. In one case, endoplasmic reticular inclusions which were thought to be viral nucleocapsid by some authors were found. They, however, seemed to be formed by condensation of intracisternal granular material secreted by the rough endoplasmic reticulum. The cells comprising giant cell tumor of bone were multinucleated giant cells and stromal cells classified into four types. In a malignant case, giant cells showed no remarkable morphological change while stromal cells underwent an atypical change. This finding suggests that the mononuclear stromal cells are the principal tumor cells and the establishment of histological grading should mainly depend on the atypism of stromal cells. The tumor is likely to originate from undifferentiated mesenchymal cells which have the tendency of histiocytic and fibroblastic differentiation. The morphogenesis of multinucleated giant cells is not clarified in this study. Although the possibility that stromal cells are the origin of giant cells is still remained, it could also be speculated that giant cells are somehow related to immune response to the tumor having macrophage-related origin.