Repair Mechanisms in Articular Cartilage—A Porcine in Vitro Study

Explants are excellent systems for studying homeostasis in cartilage. The systems are very useful in pharmacological studies involving OA-treatment and in studies of repair mechanisms during injury to hyaline cartilage. The purpose of this study was to evaluate the reparative processes occurring in a young age porcine cartilage explant model examining tissue by Light (LM) and Transmission Electron Microscopy (TEM). Explants of articular cartilage were dissected from the femoral condyles of immature one-year-old pigs and cultured in DMEM/F12 medium with FCS (stimulated explant) or in medium without FCS (control explant) for up to 4 weeks. After 1 4 weeks of culture with FCS, LM showed migration and proliferation of chondrocytes in cartilage close to the injured surface differentiating two areas: proliferative zone and necrotic zone. The chondrocytes present in the necrotic zone showed a polarization towards the injured surface. After budding through the injured surface, the chondrocytes formed repair tissue in an interface repair zone and in outer repair tissue. TEM showed chondrocytes in expanded lacunae involving the proliferative zone. The pericellular matrix of the expanded lacunae was partly dissolved, indicating release of matrix-degrading enzymes during proliferation and remodeling. Migratory chondrocytes were identified in oval lacunae close to the injured surface. The pericellular matrix of these oval lacunae was significantly dissolved and immunohistochemistry demonstrated strong staining with a polyclonale collagenase antibody around these units, suggesting release of matrix-degrading collagenase contributing to chondrocyte mobility. We describe an explant model comprising two different repair systems in immature articular cartilage. This model provides us with new reference points that are important in understanding the repair mechanisms.