The origin and characteristics of so‐called stromal cells (stromal cell) and the osteoclast‐like giant cell series of 19 cases of giant cell tumor (G.C.T.) of bone were studied. Immunohistochemically, two interesting cases were found. The stromal cells of one case were α‐1‐antitrypsin positive and those of the other case were α‐1‐antichymotrypsin positive. The histiocytic stromal cells of the latter case seemed to be surely neoplastic since they showed mild to moderate cell atypism. There were foci consisting of fibroblastic cells or osteoid and osteoblasts within the tumor. Those cells in the foci were apparently continuous with the surrounding stromal cells, and they were, therefore, also considered to be neoplastic. These findings strongly indicate that the stromal cells originate from the undifferentiated mesenchymal cells in the bone marrow and may differentiate to osteoblastic, fibroblastic, and histiocytic cells. All cells of these three series were not stained for a high stable form of acid phosphatase (SAPhase). SAPhase activity was demonstrated only in osteoclast‐like giant cells and some mononuclear cells, which are recently believed to be non‐neoplastic. Therefore, the cell atypia of SAPhase negative stromal cells is considered to have a
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