Timing for Intravenous Injection of Mesenchymal Stem Cells to Enhance Fracture Healing

When delivered intravenously, mesenchymal stem cells (MSCs) enhance fracture healing and can be injected at various time points, but the optimal timing remains unclear. This study aimed to clarify the optimal timing of MSC delivery to enhance the healing. For-ty-nine Wistar rats with femoral shaft fractures were randomized according to injection timing: immediately after fracture (group A, n = 14), 24 h after fracture (group B, n = 14), 7 days after fracture (group C, n = 14), and control group (group D, n = 7). Allogeneic bone-marrow-derived MSCs (5.0 × 10⁶ cells) were administered intravenously. Rats were euthanized at 6 weeks post-fracture for analysis. In each group, seven rats were evaluated for new bone formation and histological examination. Another seven rats in group A, B, and C were evaluated for VEGF, TGF-β1, and BMP-2 expression. Group C showed significantly higher new bone formation than groups A, B and D (p = 0.017, p = 0.003, and p = 0.005). Histological grades were higher in group C than in groups D (p = 0.018). In Western blot analysis, VEGF and BMP-2 were higher in group C (p < 0.05). Real-time polymerase chain reaction revealed higher VEGF and TGF-β1 RNA expression in group C (p < 0.05). MSCs administration at 7 days post-fracture is suggested to be the optimal timing for intravenous injection to enhance fracture healing in a rat long bone fracture model.