An in vivo mouse model for human cartilage regeneration

Cartilage regeneration methods have been examined in various animal models. The major limitation of those studies is the biological difference between human and animal cartilage. We propose an in vivo model for human chondrocytes in a human cartilage defect environment. Human full-thickness (2-4 mm) articular cartilage discs (diameter 10 mm) attached to 3-6 mm subchondral bone, were obtained from human femur heads. Chondral defects (diameter 4 mm) were set within the cartilage disc without violating the subchondral bone. Human chondrocytes were isolated, cultivated for three passages and then suspended at a concentration of 10(7) cells/ml. The defect was completely filled with the cell suspension (approximately 30 microl) and then covered with a thin sheet of human periosteum, fixed with fibrin sealant. Discs were implanted subcutaneously in the backs of nude mice for 5 and 8 weeks. Controls were uncovered discs filled with cell suspension and covered discs without cells. Histological evaluation revealed a gradient of differentiation from the cartilage lateral side to the centre of the defect. A proteoglycan-rich matrix was formed with some chondron-like structures at the border of native cartilage, whereas fibrous tissue was built in the centre of the defect. After 8 weeks the areas of differentiating cells enlarged compared to 5 weeks, indicating the progress of cartilage repair. The control discs without cells or cover showed no chondrogenesis. Interestingly, uncovered discs filled with cells showed comparable areas of differentiating cells at the defect surface but lack of fibrous tissue in the middle. The histological results were supported by MRI measurement.