Multinucleated giant cells in primary cultures derived from canine bone marrow--evidence for formation of putative osteoclasts.

Mononucleated cells derived from canine bone marrow were maintained in vitro for up to 6 weeks. The culture characteristics and development of these cells were evaluated by histological, ultrastructural and histochemical methods. Within 1 week the cells had fused together to form flattened, multinucleated cells. Further fusing with one another and other mononucleated cells produced large (diameters more than 300 microns), multinucleated cells which frequently contained more than 50 nuclei per cell and exhibited ultrastructural and histochemical features that were strikingly similar to those displayed by osteoclasts. The confluent monolayer of mono- and multinucleated cells present at 4 weeks had, by the sixth week, become altered such that fibroblast overgrowth obliterated all other cells. During the development of the culture adipocytes became differentiated from mononuclear cells and frequently were located within spherical multicellular aggregates (spheroids). Functional assessments were employed to investigate whether the multinucleated cells generated in this way, represented osteoclast-like cells, or alternatively, were related to macrophage polykarya as found in foreign body granulomata in vivo. Neither resorption pits on sperm whale dentine slivers (diagnostic of osteoclasts), nor formation of granulomata in vitro, were observed. We believe that the present results indicate that the multinucleated cells generated from canine bone marrow mononuclear precursors in vitro, merit designation as osteoclast-like cells. Definitive characterisation however, must await further functional assessments of hormone responsiveness.