Fig. 2

The OA microenvironment. Risk factors enter into the joint microenvironment and induce abnormities of cartilage. Firstly, the molecular composition and organization of ECM are changed to activate the proliferation and secretion of chondrocytes. The proliferating cells may produce high amounts of catabolic enzymes/ pro-inflammatory mediators which lead to chondrocyte apoptosis and ECM degradation. Innate immune cells such as the macrophages are motivated and the inflammatory responses then appear under the impact of various molecules. ECM: extracellular matrix, GDF5: growth differentiation factor 5, HIF: hypoxia-inducible factor