Bone matrix stimulates osteoclastic differentiation in cultures of rabbit bone marrow cells.

Cells showing osteoclastic characteristics have not been identified outside bone. Because osteoclasts originate from an extraosseous source, this suggests that identifiable osteoclastic features do not develop until the precursors enter bone, where the local microenvironment may signal osteoclastic differentiation or maturation. We assessed the influence of bone matrix on osteoclastic differentiation by incubating bone marrow cells, after removal of pre-existing osteoclasts, on plastic coverslips or slices of devitalized cortical bone. We found that there was a threefold increase in the number of osteoclast-specific MAb-positive cells on the bone matrix compared with plastic coverslips. The number of MAb-positive cells correlated with the extent of excavation of the surface of the bone slices. Multinuclearity correlated with MAb-positive cell density, and for any given density the proportion of MAb-positive cells that were multinucleate was similar on plastic and bone. We conclude that, in the presence of 1,25-(OH)2 vitamin D3, bone matrix stimulates the generation of osteoclasts but has no demonstrable influence on the fusion of mononuclear osteoclastic precursors.