Transplanted blood-derived endothelial progenitor cells (EPC) enhance bridging of sheep tibia critical size defects

The angiogenic events that accompany bone regeneration function as a "limiting factor" and are the primary regulatory mechanisms that direct the healing process. The general aim of this study was to test whether blood-derived progenitor cells that have endothelial characteristics (EPC), when applied to a large segmental defect, would promote bone regeneration. We established a critical-sized gap platform in sheep tibiae. Our model system takes advantage of the physiological wound healing process that occurs during the first two weeks following injury, and results in the gap being filled with scar tissue. EPC were expanded ex-vivo and 2 × 10⁷ cells/0.2 ml were implanted into a wedged-shaped canal excavated in the fibrotic scar tissue. Sham treated sheep served as controls. Bone regeneration was followed every two weeks for three months by X-ray radiography. At the end of the experimental period, the regenerating segments were subjected to micro-computed tomographic (μCT) analysis. While minimal bone formation was detected in sham-treated sheep, six out of seven autologous EPC-transplanted sheep showed initial mineralization already by 2 weeks and complete bridging by 8-12 weeks post EPC transplantation. Histology of gaps 12 weeks post sham treatment showed mostly fibrotic scar tissue. On the contrary, EPC transplantation led to formation of dense and massive woven bone all throughout the defect. The results of this preclinical study open new therapeutic opportunities for the treatment of large scale bone injuries.