Repair of chronic osteochondral defects using predifferentiated mesenchymal stem cells in sheep model

Introduction: The aim of this project was to investigate the repair of a chronic osteochondral defect of the knee joint in sheep by using autologous, bone marrow-derived ovine mesenchymal stem cells (oMSCs). The issue of whether chondrogenic predifferentiation of the oMSCs in vitro bears an influence on the repair in vivo is also to be addressed. Methods: In an initial operation the osteochondral lesions (diameter 7 mm, depth 2 mm) were created at the medial femoral condyles of the hind legs in 10 Merino sheep and bone marrow aspirates were harvested. For each implant 1.8x10e6 expanded oMSCs were mixed with a collagen-I-hydrogel (Arthro Kinetics Biotechnology). One part of the oMSC gels was predifferentiated with chondrogenic medium+ 10ng/ml TGF-beta3, whilst the other part was not predifferentiated. To achieve chronification, implantation of the oMSCs gels was not performed until 6 weeks after defect creation. A histological evaluation followed after 6 months. Results: With regard to proteoglycan content, cell number, gel contraction, apoptosis, and progress of chondrogenic differentiation, a differentiation period of 14 days in vitro was considered optimal. After 6 months in vivo, the defects treated with predifferentiated oMSC gels showed significantly better histological scores with morphologic characteristics of hyaline cartilage such as zoning, presence of collagen type II and marginal expression of collagen type I compared to the un-differentiated oMSC gel group. Conclusion: The approach based on autologous predifferentiated MSCs in vitro is promising for facilitating repair of focal osteochondral defects that become chronic.