Histologic analysis over time reveals patterns of articular degradation. Sagittal sections from sham (control), and contralateral and ipsilateral OA treatments, in (a) nonmobilized (NM) and (b) forced mobilization (FM) groups of animals were analyzed over a 20-week time course. Sections were stained with safranin-O (red stain) for articular cartilage matrix proteoglycans, fast green (green stain) for bone and fibrous tissue, and hematoxylin for nuclei (blue). In the upper row of each panel, representative images of sham and contralateral joints are shown at 2, 12, and 20 weeks after surgery. The lower row shows representative sections of ipsilateral joints at all time points assessed. Each image is presented with the femoral joint surface in the upper portion. Examples of morphologically normal articular surface (ac), surface discontinuity (arrow), vertical fissures (arrow head), delamination (del), chondrocyte clusters (cc), denudation (dn), sclerotic bone (sb), fibrocartilage-like tissue (fc), and subchondral plate failure (pf) are indicated. All images are shown at the same magnification, indicated by the scale bars.