This study indicated that SDF-1 played an important role in endochondral bone repair in our TBI model. Firstly, determining the expressing pattern of SDF-1 was crucial for understanding its role during accelerated bone healing. So we performed gene expression and immunohistochemical analysis and observed that the surrounding tissue of the long bones indeed expressed the SDF-1 mRNA. The expression of the SDF-1 and CXCR4 mRNAs was significantly up-regulated in the TBI/fracture group during the acute phase of bone repair, whereas a lower increase was detected in the fracture only group. It was previously reported that SDF-1 is expressed at the endosteum and the growth plate of normal long bones in adults 
, but a recent study showed that SDF-1 is expressed at the periosteum during embryonic endochondral bone development and that expression is substantially reduced after birth 
. Our observations indicated that there was no apparent SDF-1 expression at those locations. Another important issue for understanding the role of SDF-1 during accelerated bone repair is its regulated expression during the repair process. Increased expression of the SDF-1 mRNA was observed on days 1 and 2 in our model, although other studies have demonstrated an increase in SDF-1 expression within 24 hours after injury 
. It was previously reported that SDF-1 is regulated by a hypoxia-specific transcriptional factor, hypoxia-inducible factor 1 (HIF-1), and that the expression of SDF-1 may increase rapidly after loss of blood supply 
. However, the function of trophic vasculature would be affected after TBI and subsequent hypoxia may have resulted in a relative increase in SDF-1 expression in the TBI/fracture group. Furthermore, the up-regulation of SDF-1 was lower during the acute phase in the fracture-only group, in which the oxygen concentration was well preserved before surgery. This differential increase in SDF-1 expression between the two groups suggested that this molecule might be a key regulator involved in accelerating bone repair.
We observed that SDF-1 promoted the migration of MSCs in vitro in a dose-dependent manner, as based on in vitro and in vivo chemotactic assays. We also found that the BrdU-labeled mouse BMSCs that were injected intravenously could be recruited to the damaged bone. In addition, AMD3100, an antagonist of CXCR4, could inhibit this migration. These results strongly supported that SDF-1 was an essential molecule for the migration of MSCs to sites of bone repair in vivo. Recently, studies have shown that the recruitment of autologous stem cells may be substantially enhanced with localized release of stem cell chemokines 
. It has been previously mentioned that the SDF-1α/CXCR4 axis might be involved in recruitment of expanded MSCs to damaged tissues 
. Enhanced recruitment of autologous stem cells could improve the tissue responses and jumpstart stem cell participation in healing 
. And our research has proven that SDF-1α is a promising candidate for in situ recruitment in bone regeneration 
. However, the effects of SDF-1 in accelerated bone repair have not been shown. In our research, we demonstrated that the intravenously transplanted MSCs migrated to the site of bone repair in the TBI/fracture model. The mobilization of BrdU-positive cells was observed around the damaged bone, and AMD3100 could decrease the number of migrated cells. These results supported the idea that the SDF-1/CXCR4 axis was involved in the migration of cells to the sites of bone repair. Moreover, the percentage of BrdU-positive chondrocytes in the endochondral callus was 27.6%, demonstrating that the migrated cells had differentiated into chondrocytes. These results indicated that the migrated cells were mesenchymal cells and actually participated in endochondral bone formation. To our knowledge, this study is the first to show the effect of SDF-1 on accelerated bone repair in vivo. Another findings was that there was a significant correlation between the decreased volume of newly formed bone and the blockade of SDF-1 or CXCR4. The loss-of-function studies revealed that treatment with an anti–SDF-1 neutralizing antibody or AMD3100 remarkably decreased the area of new bone formation.
Taken together, these data indicated that the SDF-1/CXCR4 axis may play a key role in accelerated bone healing and contribute to endochondral bone repair. Further studies are needed to clarify the regulatory mechanism of SDF-1 expression in bone repair. Furthermore, we hope that the therapeutic use of SDF-1 to achieve successful bone repair will be feasible in the near future.
The SDF-1/CXCR4 axis plays a crucial role in accelerated bone healing and contributes to endochondral bone repair in a TBI/fracture model. This study lays the foundation for the use SDF-1 to promote bone fracture healing in clinic setting.