High magnification images of sagittal sections of articular cartilage, stained with safranin-O and fast-green, reveal detailed cartilage histology. (a) Healthy-appearing sham cartilage has intact superficial, mid, and deep zones (from top to bottom of image) that stain deeply with safranin-O (red) for glycosaminoglycans. The chondrocytes are arranged in columns. (b) Two week FM ipsilateral cartilage demonstrates delamination (del) of the superficial zone. (c) Four week FM ipsilateral cartilage shows the development of vertical fissures (vf) into the mid-zone, and loss of glycosaminoglycans (pale green stain in mid-zone is red in panel a). (d) Matrix erosion of the superficial and mid-zones is evident by 8 weeks in FM ipsilateral cartilage, as well as the formation of chondrocyte clusters (cc). (e) By 16 weeks, NM ipsilateral cartilage shows almost complete denudation (dn) of the articular cartilage, and evidence of bone repair appears beneath the subchondral plate (br). (f) Fibrocartilage-like tissue (fc) is evident in the articular cartilage of 20-week FM ipsilateral joints, which is indicative of abnormal repair processes. All images are shown at the same magnification, indicated by the scale bar. FM, forced mobilization; NM, nonmobilized.