Differential effects of cyclic and static pressure on biochemical and morphological properties of chondrocytes from articular cartilage

Background Mechanical stresses are known to play important role on articular cartilage functions in vivo and also on cartilage explants and chondrocytes monolayer culture. This study examined the differential effect of cyclic and static pressures on chondrocytes cultured in alginate matrix, which is physiologically closer to the in vivo environment of cells in cartilage. Methods Goat knee joint articular cartilage chondrocytes cultured in alginate beads were exposed to 1.2 and 2.4 MPa cyclic and static loadings via a custom-made cam/follower based machine. Biochemical contents (glycosaminoglycan, collagen, DNA) and protease activity of cells were analyzed separately in cellular matrix, further removed matrix and in culture medium. Morphology of chondrocytes was studied under transmission electron microscopy. Findings Compared with controls (unloaded cells), cyclic loading increased the glycosaminoglycan content of cells at 1.2 and 2.4 MPa in cellular matrix and further removed matrix (P < 0.001) whereas it decreased at similar static loads (P < 0.001). In alginate matrix, chondrocytes released a metalloprotease, which required Mn2+ for activity. Both cyclic load levels inhibited its specific activity in cellular matrix but increased it at static loading (P < 0.001). The protease specific activity in further removed matrix increased at both cyclic and static loadings (P < 0.001). Transmission electron microscopy data showed improved cells ultrastructure and cell–matrix interactions under cyclic load whereas these deteriorated under static loadings. Interpretation The study suggests that cyclic load has a positive effect on chondrocytes metabolism and morphology whereas static load has a degenerative effect.