Figure 1.

Fibrosis in synovial and cartilage tissue. (A) The normal joint consists of a smooth layer of articular cartilage and a smooth layer of synovial membrane on the side that maintains the synovial fluid in the synovial cavity. In the joint of a patient with osteoarthritis (OA), the increased proliferation of synovial cells induces synovial fibrosis that results in joint swelling, stiffness, and pain. The increasing levels of synovium eventually affect the cartilage and bone tissue, inducing further degradation of the cartilage tissue and bone erosion. (B) The normal articular cartilage has a smooth extracellular matrix (ECM) that is mostly composed of aggrecan and collagen type II. Normally, chondrocytes remain in small spaces called lacunae. However, chondrocytes in a defected cartilage lesion undergo abnormal proliferation that leads to their de-differentiation into fibroblast-like fibrotic chondrocytes. Then, these chondrocytes secrete ECM proteins such as collagen type I instead of aggrecan or collagen type II. These changes lead to a stiffer type of cartilage and eventually completely change the characteristics of the articular cartilage.