Figure 1.

Macroscopic and microscopic illustration of osteoarthritis (OA). (A) Knee OA at four different stages can be evaluated by K-L scores, including “normal” at stage 1. Only cartilage degradation is shown for illustration simplicity instead of the whole-joint damages. (B) Microscopic exploration of normal and osteoarthritic chondrocytes interacting with other cell types through cell-cell communication (EV secretion) at stage 4 with possible biological outcomes. The synovial fluid-derived EVs can be extracted for OA diagnosis. Green/red curve arrows indicate cells secreting EVs with bioactive molecules that potentially are chondro-protective/chondro-destructive. Osteoarthritic chondrocytes may also secret EVs to stimulate inflammasome activation of cells in synovial space, including macrophages.