Circulating CD34-Positive Cells Promote Early Osteogenic differentiation of Human Fracture Hematoma-Derived Progenitor Cells in

INTRODUCTION Human circulating CD34+ cells, an endothelial progenitor cell (EPC)enriched population [1], have been also reported to differentiate into osteoblasts [2]. Previously, we demonstrated that that human circulating CD34+ cells, systemically transplanted into immunodeficient rats with nonhealing fracture, were recruited into fracture sites, contributed to a favorable environment for fracture healing by enhancing vasculogenesis and osteogenesis, and finally led to functional recovery from fracture [3]. Hematoma occurring at a fracture site is known to play an important role in fracture healing. Recently, we demonstrated that mesenchymal progenitor cells (MPCs) with multilineage differentiation potential exist in human fracture hematoma, indicating its critical role in the process of fracture healing [4]. However, the influence of circulating CD34+ cells on the differentiation of fracture hematoma-derived MPCs (HMPCs) into osteogenic cells in the process of fracture healing is not yet well understood. We therefore studied the effect of peripheral blood-derived CD34+ cells on human HMPCs differentiation towards an osteoblastic phenotype by indirect contact co-cultures.