The Biomechanical Properties of the Chondrocyte Pericellular Matrix: Micropipette Aspiration of Mechanically Isolated Chondrons

Introduction: The mechanical environment of the chondrocyte is one of several important environmental factors which influence the normal balance between synthesis and degradation of the articular cartilage extracellular matrix (ECM). Previous studies have shown that compression of cartilage explants alters cellular metabolism in a timeand spatially-varying manner which is correlated with the local mechanical environment of the chondrocytes [1]. These findings suggest that local mechanical factors in the microenvironment of each cell are responsible for governing chondrocyte response to matrix compression. Therefore, detailed information on the stresses and strains that the individual cell is exposed to during cartilage compression would improve our understanding of the sequence of events involved in mechanotransduction. Within the ECM, chondrocytes are surrounded by a pericellular matrix (PCM), which together with the chondrocyte and a surrounding capsule are termed the chondron [2]. This region is characterized by the presence of type VI collagen and increased proteoglycan concentration relative to the ECM. The functional role of this complex structural unit is not known, although the fact that the PCM completely surrounds the cell suggests that it influences the biochemical and biomechanical interactions between the ECM and the chondrocyte. In this regard, previous studies have utilized finite element modeling to predict the mechanical environment of the chondrocytes within the extracellular matrix [3]. With this technique, it was shown that the cellular environment is highly dependent on the relative material properties of the ECM, the PCM, and the chondrocytes. While the mechanical properties of the cartilage ECM and of isolated chondrocytes have been extensively characterized, little data is available on the mechanical properties of the PCM. In this study, the micropipette aspiration technique was combined with a detailed theoretical analysis of a layered half-space of the chondron to test the hypothesis that the mechanical properties of the PCM differ between the surface and deep zones of articular cartilage.