Impaired recruitment and differentiation of osteoclast progenitors by osteocalcin-deplete bone implants

This is a report of an experimental system to study differentiation of bone-resorbing osteoclasts and demonstrates that osteocalcin, an extracellular bone-specific component, is necessary for the recruitment of osteoclast progenitor cells. The subcutaneous implantation of devitalized bone particles (BPs) elicits the recruitment and differentiation of osteoclasts that resorb the BPs. In a previous study, we showed by histomorphometric analysis that BPs that were deficient in osteocalcin were resorbed only 60% as well as normal BPs. In this study, the mechanism of this difference was investigated by measurements of recruitment, differentiation and activity of bone resorbing cells by normal and osteocalcin-deficient BP. Mononuclear cells were attracted to control BPs soon after implantation. In dramatic constrast, cellularity was depressed around osteocalcin-deficient BPs with very few mononuclear cells within the implant on day 5 (35% of control cellularity). In implants of normal BPs, tartrate-resistant acid phosphatase-positive multi-nucleated cells were evident by day 5; very few appeared in implants of osteocalcin-deplete BPs even by day 12. The amount of tartrate-resistant acid phosphatase activity in homogenates of the osteocalcin-deficient bone particle specimens not only lagged behind controls but never reached the maximum activity of control BP specimens. These data support the hypothesis that osteocalcin may function as a matrix signal in the recruitment and/or activation of cells for bone resorption.