Endothelial Progenitor Cells and Mesenchymal Stem Cells Seeded onto beta-TCP Granules Enhance Early Vascularization and Bone Healing in a Critical-Sized Bone Defect in Rats

Question/aim: Lack of vessels indicates an insufficient nutritional supply of a bone graft and may limit the recruitment of bone-forming cells. Our aim was to evaluate the influence of endothelial progenitor cells (EPCs) alone or in combination with mesenchymal stem cells (MSCs) on early vascularization and bone healing in critical-sized defect (CSD) in vivo. Methods: MSCs from human bone marrow and EPCs from buffy coat were used. A femoral CSD in adult athymic rats was created and stabilized by an external fixateur. The remaining defects were filled with fibronectin-coated beta-tricalcium phosphate (beta-TCP) granules, EPCs seeded on beta-TCP, MSCs seeded on beta-TCP, coculture of EPCs/MSCs seeded on beta-TCP, or autologous bone. Vascularization and bone formation were determined by immunohistology, mu CT analysis, and biomechanical testing after 1, 4, and 8 weeks. Results: Early vascularization was significantly improved in EPC/MSC group or EPC group, respectively. At 4 weeks bone formation increased significantly when the CSD was treated with coculture of MSCs/EPCs. Eight weeks after transplantation CSD showed significantly more bony bridgings and significantly increased ultimate load in the EPC/MSC group compared to the other groups. Discussion: This cell approach suggests that there is a synergistic effect and that the initial stage of neovascularization by EPCs is considered to be crucial for complete bone regeneration in the late phase.