NEW CONCEPTS ON OSTEOCLAST ORIGIN - RELAT IONSHIP WITH NORMAL AND INFLAMMATORY MACROPHAGES

Osteoclasts are bone resorbing multinucleated giant cells. They derive from the fusion of hemopoietic mononucleated precursor cells, althoug h their precise origin along the hemopoietic differentiation pathway i s still a matter of discussion. Recent data obtained both in vivo and in vitro sustain the idea that osteoclasts derive from fusion of cells at the late stages of the monocytic pathway, or even directly from ma crophages. Osteoclasts, as well as macrophages, are phagocytic cells s haring in common many surface antigens. In vit ro models have been dev eloped enabling formation of osteoclasts directly from resident tissue macrophages. Furthermore, analysis of osteopetrotic mice, obtained fr om either spontaneous mutations or after homologous recombinations, ha ve shown that macrophages and osteoclasts are closely related. Finally , osteoclasts appear also to be highly related to polycaryonic macroph ages found in inflammatory tissues. Here, we propose that only one way of differentiation leads from monocytes/macrophages to either inflamm atory polycaryonic macrophages or bone resorbing osteoclasts, the fina l commitment depending upon microenvironment conditions.