Synovial fluid-derived mesenchymal stem cells increase after intra-articular ligament injury in humans

Objective. The existence of mesenchymal stem cells (MSCs) in SF was previously reported. However, the behaviour and properties of MSCs derived from SF have not been fully elucidated. Methods. Human SFs were obtained from 19 knee joints with anterior cruciate ligament injury around the time of reconstruction surgery, and from three healthy volunteers. SF was plated, cultured and examined for colony-forming number, in vitro differentiation, surface epitopes and gene profiles. Also, rabbit synovium-MSCs were injected into the knee joint in a rabbit partial anterior cruciate ligament defect model, and the injected cells were traced. Results. SF-MSCs from IA ligament injury patients were 100 times more in number than those from healthy volunteers. Total colony number was positively correlated with post-injury period. No significant differences were observed among the cells derived from SF around the time of the surgery in relation to surface epitopes and differentiation potentials. Cluster analysis of gene profiles demonstrated that SF-MSCs were more similar to synovium MSCs than bone marrow MSCs. In rabbit experiments, the MSCs injected into the knee in which IA ligament was partially defective were observed more on the defected area than on the intact area of the ligament at 24 h. Conclusion. We demonstrated that SF-MSCs, similar to synovium MSCs, increased in number after IA ligament injury and surgery without marked alteration of the properties.