Editorial Commentary: Stem Cells. They Are in the Fat Tissue, Bone Marrow, and Even in the Synovial Fluid of the Knee Joint.

Adult stem cells have been isolated in bone marrow and adipose tissue. These mesenchymal stromal cells (MSCs) have the ability to differentiate into osteogenic, chondrogenic, and adipogenic cell lines. The study by Branch et al. has identified MSCs in the synovial fluid of the knee in patients after anterior cruciate ligament injury and in patients with osteoarthritis of the knee. When mixing synovial fluid with whole blood and using a commercially available platelet-rich plasma-processing system, the total number of MSCs doubled in both groups when compared with the cell count in synovial fluid only. However, it is not clear whether the MSCs in the processed synovium-whole blood mix include synovial MSCs versus MSCs from only the blood. In addition, cell counts were substantially lower when compared with the typical concentrations of MSCs in bone marrow aspirate. The clinical application is yet to be defined.