Circulating cells with osteogenic potential are physiologically mobilized into the fracture healing site in the parabiotic mice model

Based on the accumulating evidence of osteogenic cells present in the systemic circulation, we hypothesized that circulating osteogenic connective tissue progenitors (CTPs) home to fracture site and contribute to skeletal repair. Parabiotic animals were formed by surgically conjoining transgenic mice constitutively expressing green fluorescent protein (GFP) in no erythroid tissue and syngeneic wild-type mice. After 3 weeks parabionts, equilibrium in blood chimerism between partners was established. A transverse fibular fracture was made in the contralateral hind limb of the conjoined wild-type partner. The contribution of circulating cells to the fracture callus was assessed based on analysis of GFP+ cells and co-localization of alkaline phosphatase (AP) staining nonfracture and at 1, 2, 3, and 4 weeks after fracture. Histomorphometric analysis at the fracture site showed significant increase of GFP+ cells after 2 (5.4%) and 3 (5.6%) weeks compared to nonfractured controls (1.7%). Of the GFP+ cells, percentage of the cells expressing AP activity at 1 (37.4%) and 2 (85.3%) weeks postfracture time was statistically higher than that in nonfractured controls (10.8%). The rate of mobilization of circulating osteogenic CTPs to fracture callus was also examined using 1 week parabionts at week 0-1 and week 1-2 postfracture. There was significant increase of GFP+/AP+ cells from week 0-1 (0.1%) and week 1-2 (1.8%). These data indicate that circulating osteogenic CTPs are mobilized to fracture site and contribute to osteogenesis in the early stage of fracture healing.