Cell Surface Bound Pericellular Matrix has a Spatially Varying Mesh Size

An invisible but thick polymer matrix of polysaccharides extends outward from the surface of many cell types including fibroblasts, smooth muscle cells, and chondrocytes. Grafted to the cell surface, the key backbone polymer hyaluronan is dramatically stretched outward from the surface by large bottlebrush molecules (e.g. aggrecan, versican). This extended, cell bound structure is called the pericellular matrix or cell coat. The shape and thickness of the cell coat is strongly correlated with adhesion-dependent and mechanical activities of the cell in particular proliferation and migration, for example during wound healing, embryogenesis, and cancer metastasis. Yet, much remains to be understood about the cell coat, which is invisible to both phase contrast and DIC microscopy, and which until recently was difficult to fluorescently-label without collapsing its hydrated structure. Our measurements based on optical force probe microscopy and quantitative particle exclusion assays provide some of the first quantitative mechanical and ultra-structural measurements of the cell coat. The assays reveal that cell coat structure on RCJ-P chondrocytes is not crosslinked and further that the pericellular matrix maintains a varying osmotic pressure throughout its structure which influences how objects move through the matrix. We show for the first time that the cell coat has a spatially varying mesh size perpendicular to the cell surface, ranging from approximately ∼100nm to 500nm at eight microns. The accessibility of the cell surface through the pericellular matrix both for molecules and nanoparticles as well as the cell's interaction with its surroundings will depend strongly on the cell coat configuration and its varying mesh size (e.g. adhesion to other cells and the extracellular matrix; endocytosis). These measurements provide first experimental clues for how cell coat ultrastructure and mechanics might physically regulate cell interactions with the extracellular environment.