Biophysics of the Hyaluronan-Rich Pericellular Coat

The articular cartilage consists of a complex extracellular matrix (ECM), which is subject to a high mechanical loading. To counteract the ongoing abrasion, a specialized cell type is embedded within the ECM. These so called chondrocytes constantly renew the ECM. To live and even divide in such a mechanically challenging environment, chondrocytes are protected by a several micron thick pericellular coat (PCC). This coat is of vital biological importance for example in cell proliferation and migration, but also in diseases like osteoarthritis or with age.The PCC consists mainly of water. Thus it remains invisible in all light microscopy techniques. Where the individual components and even their molecular interactions are well understood, much less is know about the mesoscopic structure of the PCC, which in turn is vital to understand force transductions.In order to analyze the molecular architecture of the PCC, we established a new set of tools to measure and manipulate the PCC on living cells. The micromechanical profile was measured with position-sensitive passive microrheology. In contrast to other mechanical techniques applied to the PCC, it is independent of the adjacent cell body and enables the measurement in the z direction normal to the cell surface. The observed profiles show a decreasing viscoelasticity within the PCC correlating to a decreasing concentration of the polymer backbone (hyaluronan).Further, we demonstrate that the PCC expression depends on the cell's interactions with the ECM. In order to precisely control adhesion, we employed nanostructured surfaces. Chondrocytes not only change their adhesion areas in respect to the ligand density, but also the size of their PCC. The modified PCC size could also be related to changes in its mechanical properties analyzed in ongoing experiments.