Circulating Osteogenic Cells

This chapter summarizes biological characteristics and possible roles of circulating osteogenic precursor (COP) cells that have been studied using a variety of experimental approaches. The experimental approaches include parabiosis experiments, bone marrow transplantation experiments, and ectopic bone formation experiments. Several key concepts appear to be generally true regarding the physiological functions of COP cells. The first is that the bone-forming function of COP cells may not be their primary role but is an adaptive response in conditions of injury, repair, or abnormal cytokine signaling. Second, COP cell homing may be mediated by the CXCR4/SDF-1 axis that is shared by multiple processes requiring the migration of stem cells. Finally, the burgeoning area of COP cell biology holds promise for development of gene and cell therapy protocols to enhance bone formation as well as diagnostic tests based on COP cell levels as a biomarker for disease states.