Mesenchymal stem cell sheets revitalize nonviable dense grafts: Implications for repair of large-bone and tendon defects

Background. Large musculoskeletal defects are commonly reconstructed with allogeneic grafts. As cryopreserved allogeneic grafts lack viable cells, this often results in poorer clinical outcome. Current technology can not incorporate large number of cells to the dense grafts. This study aimed to investigate the feasibility of fabricating sheets of mesenchymal stem cells (MSCs) to revitalize cryopreserved grafts. Methods. Human MSCs were isolated, characterized, and cultured to form a cell sheet in the presence of ascorbic acid. Once a sheet of MSCs was obtained, it was assembled onto the demineralized bone grafts or frozen tendon grafts by a wrapping technique. Then the assembled structure was cultured for 3 weeks. The macro morphology, histology, and immunohistochemistry of the grafts were evaluated. Results. It was found that MSCs were able to form coherent cellular sheets within 3 weeks. When assembled with demineralized bone matrix, MSC sheets were similar to in situ periosteum and were able to differentiate into the osteochondral lineage. When assembled with frozen tendon graft, MSCs sheets were well-incorporated within the tissue sheath (peritenon) around the tendon, and adopted the characteristic spindle-shaped morphology of tenocyte-like cells. Conclusions. The results therefore demonstrated that MSCs sheets are easily fabricated and can maintain their differentiation potential within particular scaffolds, which would suggest a novel and convenient strategy for revitalizing large tissue grafts to improve clinical outcome.