Matrix remodeling and cytological changes during spontaneous cartilage repair

During the repair of articular cartilage, type I collagen (COL1)-based fibrous tissues change into a mixture of COL1 and type II collagen (COL2) and finally form hyaline cartilaginous tissues consisting of COL2. In order to elucidate the changes that occur in the matrix during cartilage repair and the roles of fibroblasts and chondrocytes in this process, we generated a minimal cartilage defect model that could be spontaneously repaired. Defects of 0.3 mm were created on the patellofemoral articular cartilage of rats using an Er:YAG laser and were observed histologically, ultrastructurally and histochemically. At week 2 after this operation, fibroblastic cells were found to be surrounded by COL1 throughout the area of the defect. These cells became acid phosphatase positive by week 4, both taking in and degrading collagen fibrils. Thereafter, the cells became rounded, with both COL1 and 2 evident in the matrix, and showed immunolocalized matrix metalloproteinase-1 or -9. In the region of the bone marrow, the cells became hypertrophic and were surrounded mainly by COL2 and proteoglycans. By the eighth week, the cartilaginous matrix was found to contain abundant COL2, in which collagen fibrils of various diameters were arranged irregularly. These morphological changes suggested that the fibroblastic cells both produce and resolve the matrix and undertake remodeling to become chondrocytes by converting from a COL1- into a COL2-dominant matrix. This process eventually forms new articular cartilage, but this is not completely identical to normal articular cartilage at the ultrastructural level.