Progression of histopathological changes in knee joint sections stained with Fast Green and Safranin O. A: Control animals show unaltered articular cartilage (AC) and subchondral bone (SB). Animals injected with 250 U collagenase (B, C, D, E) show increased thinning of the articular cartilage throughout time, along with focal chondrocyte disorganization. Animals injected with 500 U (F, G, H, I) show signs of OA onset at week 4 ( H ) that further develop until week 6 ( I ) when extensive damage of the articular cartilage is observed. Bar: 50 μm.