Effects of injurious compression on matrix turnover around individual cells in calf articular cartilage explants

The effects of mechanical injury on the metabolism of cartilage matrix are of interest for understanding the pathogenesis of osteoarthrosis and the development of strategies for cartilage repair. The purpose of the present study was to examine the effects of injury on matrix turnover in a calf articular cartilage explant system for which the effects of mechanical loading on cell activity and the cell-mediated pathways of matrix metabolism are already well characterized. New methods of quantitative autoradiography were used in combination with established biochemical and biomechanical techniques for the analysis of cell and matrix responses to acute mechanical injury, with particular attention to the processes of localized matrix turnover in the cell-associated matrices of individual chondrocytes. Matrix deposition and turnover around cells in control explants was spatially dependent, with the highest rates of proteoglycan deposition and turnover and the lowest rates of collagen deposition (as indicated by [3H]proline autoradiography) occurring in the pericellular matrix. Injurious compression was associated with (a) an abrupt decrease in the tensile load-carrying capacity of the collagen matrix, apparently associated with mechanical failure of the tissue, (b) a considerable but subtotal decrease in cell viability, marked by the emergence of an apparently inactive cell population interspersed within catabolically active but abnormally large cells, and (c) sustained, elevated rates of proteoglycan turnover, particularly in the cell-associated matrices of apparently viable cells, which involved the increased release of aggregating species in addition to a spectrum of degradation fragments that were also in controls. These results may represent an in vitro model for the responses of chondrocytes and the cartilage extracellular matrix to mechanical injury.