Strong hyaluronan expression in the full-thickness rat articular cartilage repair tissue.

Articular cartilage lesions have a poor capacity to regenerate. In full-depth articular cartilage defects, the repair process involves an ingrowth of mesenchymal cells from the bone marrow to the injured area, and these cells attempt to restore the lesion with cartilage-like repair tissue. In this study, we investigated histologically the distribution of hyaluronan in the rat repair tissue in relation to other glycosaminoglycans. Full-depth lesions were drilled to the weight-bearing region of rat medical femoral condyle. The rats were divided into two groups: intermittent active motion (IAM) and running training (RT) groups. In the RT group, programmed exercise was started 1 week after surgery, while the rats in the IAM group could move freely in their cages. The lesions were investigated 4 and 8 weeks after the surgery. Semiquantitative histological grading showed no significant differences in the repair between the groups. In normal articular cartilage, hyaluronan was stained mainly around chondrocytes. During repair, strong hyaluronan staining was observed in loose mesenchymal tissue, while in the repair area undergoing endochondral ossification, hyaluronan was intensively stained mainly around the hypertrophic chondrocytes. Remarkably strong staining for hyaluronan was noticed in areas of apparent mesenchymal progenitor cell invasion, the areas being simultaneously devoid of staining for keratan sulphate. In conclusion, hyaluronan is strongly expressed in the early cartilage repair tissue, and its staining intensity and distribution shows very sensitively abnormal articular cartilage structure.