[Ultrastructural studies of healing process of injured articular cartilage]

This study was undertaken to investigate the healing process and the nature of the repaired tissue of injured articular cartilage. Full-thickness defects were made in the articular cartilage of the femoral condyles of 48 immature rats, and were examined by using light and transmission electron microscopes from one to 48 weeks after the injury. Under the light microscope, toluidine blue metachromasia was demonstrated in the deep matrix at 4 weeks after the injury. The repaired tissue was similar to the articular cartilage in controls at 12 weeks after the injury, but decreased number of cells and irregular surface were recognized in the specimen taken at 48 weeks after the injury. By electron microscopic examinations, at 12 weeks after the injury, cells in the repaired tissue were indistinguishable from chondrocytes, and the tidemark showed a fine structure similar to that in the articular cartilage in controls where many electron dense particles were observed. Collagen fibrils in the superficial layer were arranged in parallel to the articular surface. Three dimensional meshwork of collagen fibrils was also recognized in the intermediate and deep layers, but was inferior to that in the articular cartilage in controls. At 48 weeks after the injury, the surviving cells in the repaired tissue contained deposits of glycogen, large lipid droplets and vacuoles. Amianthoid change and fibrous long-spaced collagen were occasionally observed in the matrix. Collagen fibrils in the areas with poor cell population were smaller in diameter and loosely arranged. The above findings indicate that the anomalous arrangement of collagen fibrils is a main cause for the weakness of the repaired tissue to the mechanical load, resulting the degeneration.