Mesenchymal stem cells (MSC) have a therapeutic potential in patients with fractures to reduce the time of healing and treat non-unions. The use of MSC to treat fractures is attractive as it would be implementing a reparative process that should be in place but occurs to be defective or protracted and MSC effects would be needed only for the repairing time that is relatively brief. However, an integrated approach to define the multiple regenerative contributions of MSC to the fracture repair process is necessary before clinical trials are initiated. In this study, using a stabilized tibia fracture mouse model, we determined the dynamic migration of transplanted MSC to the fracture site, their contributions to the repair process initiation and their role in modulating the injury-related inflammatory responses. Using MSC expressing luciferase, we determined by bioluminescence imaging that the MSC migration at the fracture site is time- and dose-dependent and, it is exclusively CXCR4-dependent. MSC improved the fracture healing affecting the callus biomechanical properties and such improvement correlated with an increase in cartilage and bone content, and changes in callus morphology as determined by micro-computed-tomography and histological studies. Transplanting CMV-Cre-R26R-LacZ-MSC, we found that MSC engrafted within the callus endosteal niche. Using MSC from BMP-2-Lac-Z mice genetically modified using a bacterial artificial chromosome system to be β-gal reporters for BMP-2 expression, we found that MSC contributed to the callus initiation by expressing BMP-2. The knowledge of the multiple MSC regenerative abilities in fracture healing will allow to design novel MSC-based therapies to treat fractures
