Ultrastructural evidence in vitro of osteoclast-induced degradation of calcium phosphate ceramic by simultaneous resorption and phagocytosis mechanisms

Osteoclasts are physiological polykaryons specialized in the resorption of calcified tissue. In the context of the clinical use of calcium-phosphate ( CaP) ceramics as bone substitutes, this study used transmission electron mi croscopy to investigate the in vitro mechanisms of CaP ceramic degradation by osteoclastic cell types. Osteoclasts cultured on CaP ceramic developed t ypical ultrastructural features of bone osteoclasts, such as a polarized do me shape, a clear zone and a ruffled border. Modification of the shape and density of CaP crystals under the ruffled border indicated an acidic microe nvironment. Moreover, osteoclasts were able to degrade ceramic by simultane ous resorption and phagocytosis mechanisms. Phagocytosis did not alter the ability of osteoclasts to resorb CaP ceramic. The phagocytosis mechanism co nsisted of three steps: crystal phagocytosis, disappearance of the endophag osome envelope membrane and fragmentation of phagocytosed crystals within t he cytoplasm. The common mechanism of phagocytosis described here is simila r to that observed with the monocyte/macrophage lineage, confirming that os teoclasts are part of the mononuclear phagocyte system. Osteoclasts are thu s clearly involved in CaP degradation by means of resorption and phagocytos is.