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An electron microscopic examination of tissue from a giant‐cell tumor of bone revealed that the fine structure of the giant cells was indistinguishable from that of osteoclasts or of giant cells from other lesions. The giant cells appeared to form by fusion of mononucleated stromal cells and to increase in size and number of nuclei by merger with additional stromal cells. Giant cells did not show evidence of division and, therefore, could not be properly considered the principal tumor cell. Mononuclear stromal cells did divide, fulfilling a criterion of neoplasia, and are probably the main tumor cells. The resemblance of many stromal cells to macrophages, sinusoidal lining cells, and cells of lymphoid series suggested that the tumor may originate from these cells. An additional observation was the presence, in mitotically dividing cells, of paired and triple cisternae.

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