Phagocytic recognition of bone by macrophages

There is now persuasive evidence that osteoclasts are formed by the fusion of mononuclear phagocytes. The presence of osteoclasts on bone thus infers the phagocytic recognition bone by mononuclear phagocytes. I compared the phagocytic recognition by macrophages of hydroxyapatite, and native, demineralised and deproteinated bone powder. Removal of mineral resulted in loss of phagocytic recognition by macrophages, while removal of the organic component resulted in avid binding of bone fragments to a similar degree to that observed for hydroxyapatite crystals. The organic component of bone therefore protects the hydroxyapatite crystals of the mineral component from phagocytic recognition by macrophages. A simple model for bone resorption would be that resident bone-lining cells (active and resting osteoblasts) remove the organic protection under appropriate stimuli and thus allow phagocytic recognition of the underlying mineral by macrophages, which accumulate, fuse and commence digestion.