1606 in Vitro Origin of Osteoclasts

During the last two decades, ample evidence has been obtained that osteoclasts, the multinucleated calcified-matrix resorbing giant cells of bone, which form by fusion of mononuclear precursor cells, are of hematogenous origin. The evidence stems from experiments done in parabionts of labeled animals (1), studies on osteopetrotic animals and humans (2-4), and quail-chick and mouse-quail transplantation experiments (5-7). Essentially, these in vivo studies have shown that osteoclasts derive from bone marrow or other hematopoietic tissues and have indicated mononuclear phagocytes as the most likely candidates for the precursor cells which fuse to form an osteoclast. However, in vitro evidence for the direct transformation of monocytes and/or tissue macrophages into bone-resorbing osteoclasts is still lacking, although it has long been known that cultured macrophages can form foreign-body giant cells in vitro by fusion (8-10). Several in vitro studies have dealt with the destruction of calcified bone matrix by mononuclear phagocytes; for this work, use was made of human peripheral blood monocytes (11, 12) or rodent macrophages (13) in combination with devitalized bone particles. Monocytes and macrophages were able to resorb mineral in a contactmediated fashion, but did not form cells with the morphological characteristics of osteoclasts (13). Recent investigations, however, point to the importance of interactions between bone-forming and -resorbing cells during osteoclastic bone resorption (14). This means that studies done on devitalized bone without viable bone-forming cells might be of limited value with respect to the formation of osteoclasts and osteoclastmediated bone resorption. We recently found (7) that early removal of the perichondrium-periosteum from embryonic mouse long-bone primordia prevents the formation of osteoclasts during organ culture of such bones. In mouse-quail transplantation studies, such stripped bone rudiments are invaded by quail osteoclasts, but mouse osteoclasts are not formed because the stripping procedure has removed the osteoclast precursor cells. In the present study, stripped live bone rudiments were used to assess the capacity of various populations of mononuclear phagocytes to form osteoclasts in vitro. Stripped bone rudiments were co-cultured with embryonic liver as well as with