EFFECTS OF INJURIOUS COMPRESSION ON MATRIX TURNOVER AROUND INDIVIDUALCELLS 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 turn over in a calf articular cartilage explant system for which the effect s of mechanical loading on cell activity and the cell-mediated pathway s 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 ar ound cells in control explants was spatially dependent, with the highe st rates of proteoglycan deposition and turnover and the lowest rates of collagen deposition (as indicated by [H-3]proline autoradiography) occurring in the pericellular matrix. Injurious compression was associ ated 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 viabi lity, marked by the emergence of an apparently inactive cell populatio n interspersed within catabolically active but abnormally large cells, and (c) sustained, elevated rates of proteoglycan turnover, particula rly 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 r esults may represent an in vitro model for the responses of chondrocyt es and the cartilage extracellular matrix to mechanical injury.