Articular cartilage: Matrix assembly, mediation of chondrocyte metabolism, and response to compression

Chondrocytes mediate the synthesis, assembly, degradation, and turnover of the macromolecules which comprise the cartilage extracellular matrix (ECM). It is this matrix which performs the mechanical function of cartilage in vivo, and which in turn affects the physical and biochemical microenvironments of individual chondrocytes during tissue deformations. Such stimuli are well known to affect cartilage matrix metabolism; chondrocyte activities are therefore influenced by a confluence of biochemical and biomechanical factors. Understanding of the mechanisms by which tissue-length scale mechanical forces are interpreted and responded to by individual chondrocytes would be valuable for the understanding of the chondrocyte phenotype in both health and disease.

We have developed novel methods of quantitative autoradiography for the measurement of spatiotemporal distributions of matrix macromolecules around individual chondrocytes in mechanically compressed cartilage explants. These methods were applied to the quantification of proteoglycan (PG) synthesis, assembly, and deformation around chondrocytes within cartilage under geometrically well-defined static and dynamic compression protocols. Patterns of PG matrix assembly were compared with measured cell and matrix deformations, and calculated fluid flow distributions, for the identification of important micromechanical stimuli which appear to play prominent roles in the regulation of chondrocyte metabolism. Results suggest the importance of matrix mechanical deformations, fluid flows, and macromolecular transport limitations as dominant microphysical stimuli which influence PG synthesis within mechanically compressed cartilage explants.

Cell-length scale quantitative autoradiography methods were also used for the visualization of PG and collagen matrix assembly and turnover in cartilage explants, under conditions of cytokine/biochemical factor stimulation and mechanical injury. This allowed for the visualization of distinct pools of matrix macromolecules which appear to become incorporated into the ECM and turn over at different rates as a function of position relative to the chondrocyte cell membrane. These spatially-localized macromolecular pools appear to respond differentially in the context of mechanical injury, suggesting the importance of cell-mediated processes in the short-term response of cartilage to mechanical injury. Comparison with tissue-average biochemical data, including immunochemical analysis of PG degradation fragments, suggest a high degree of structure and cell-length scale spatial organization to chondrocyte-mediated ECM assembly, degradation, and turnover.

These results implied a significant degree of structural anisotropy in the cartilage ECM at cell-length scales, which would have important consequences for attempts at the analysis of cellular biomechanics. In anticipation of these efforts, and in response to existing data and speculating regarding the effects of mechanical deformations on cartilage material properties, we have developed an analytical constitutive model for hydraulic permeability of anisotropically-deformed tissue matrices. The proposed model suggests that hydraulic permeability of an initially isotropic cartilage-like matrix may become significantly anisotropic in the context of applied mechanical deformations. Predictions of the model compare well with existing data, and may have important implications for theories of joint lubrication and other biomechanical analyses at the tissue- and cell-length scales.