Possible mechanisms operative in cartilage regeneration by mesenchymal stem cells. The anti-inflammatory and immunosuppressive properties of bone marrow stromal cells (BMSCs) ensure that these cells can reduce inflammation in the knee. Concurrently, BMSCs may initiate the repair process by differentiating into chondrocytes or by inducing proliferation and differentiation of the remaining healthy chondroprogenitos into mature chondrocytes or both. A whole host of transcription factors, biological modulators, and extracellular matrix proteins expressed or produced by BMSCs may play a pivotal role in enhancing neocartilage formation. The various factors implicated for cartilage tissue synthesis are depicted in this figure. BMP, bone morphogenic protein; FGF, fibroblast growth factor; Gli3, gliobastoma transcription factor 1; HoxA, homeobox protein A; IGF-1, insulin-like growth factor 1; IL, interleukin; PTHrP, parathyroid hormone-related protein; Runx2, Runt related transcription factor 2; SOX9, SRY (sex determining region Y)-box 9 gene; STAT 1, signal tranducers and activators of transcription factor 1; TGF-β, transforming growth factor-beta.