Dr. Meganck served as a consultant for GE Healthcare. All other authors state that they have no conflicts of interest.
Article first published online: 5 JAN 2009
Copyright © 2009 ASBMR
Journal of Bone and Mineral Research
Volume 24, Issue 6, pages 1043–1054, June 2009
How to Cite
Taylor, D. K., Meganck, J. A., Terkhorn, S., Rajani, R., Naik, A., O'Keefe, R. J., Goldstein, S. A. and Hankenson, K. D. (2009), Thrombospondin-2 Influences the Proportion of Cartilage and Bone During Fracture Healing. J Bone Miner Res, 24: 1043–1054. doi: 10.1359/jbmr.090101
Published online on January 5, 2009
- Issue published online: 4 DEC 2009
- Article first published online: 5 JAN 2009
- Manuscript Accepted: 29 DEC 2008
- Manuscript Revised: 19 AUG 2008
- Manuscript Received: 27 FEB 2008
Thrombospondin-2 (TSP2) is a matricellular protein with increased expression during growth and regeneration. TSP2-null mice show accelerated dermal wound healing and enhanced bone formation. We hypothesized that bone regeneration would be enhanced in the absence of TSP2. Closed, semistabilized transverse fractures were created in the tibias of wildtype (WT) and TSP2-null mice. The fractures were examined 5, 10, and 20 days after fracture using μCT, histology, immunohistochemistry, quantitative RT-PCR, and torsional mechanical testing. Ten days after fracture, TSP2-null mice showed 30% more bone by μCT and 40% less cartilage by histology. Twenty days after fracture, TSP2-null mice showed reduced bone volume fraction and BMD. Mice were examined 5 days after fracture during the stage of neovascularization and mesenchymal cell influx to determine a cellular explanation for the phenotype. TSP2-null mice showed increased cell proliferation with no difference in apoptosis in the highly cellular fracture callus. Although mature bone and cartilage is minimal 5 days after fracture, TSP2-null mice had reduced expression of collagen IIa and Sox9 (chondrocyte differentiation markers) but increased expression of osteocalcin and osterix (osteoblast differentiation markers). Importantly, TSP2-null mice had a 2-fold increase in vessel density that corresponded with a reduction in vascular endothelial growth factor (VEGF) and Glut-1 (markers of hypoxia inducible factor [HIF]-regulated transcription). Finally, by expressing TSP2 using adenovirus starting 3 days after fracture, chondrogenesis was restored in TSP2-null mice. We hypothesize that TSP2 expressed by cells in the fracture mesenchyme regulates callus vascularization. The increase in vascularity increases tissue oxemia and decreases HIF; thus, undifferentiated cells in the callus develop into osteoblasts rather than chondrocytes. This leads to an alternative strategy for achieving fracture healing with reduced endochondral ossification and enhanced appositional bone formation. Controlling the ratio of cartilage to bone during fracture healing has important implications for expediting healing or promoting regeneration in nonunions.