Hyaluronic acid (HA) is the most abundant glycosaminoglycan (GAG) in mammalian tissue. It is present in high concentrations in connective tissue, such as skin, vitreous humour, cartilage, and umbilical cord, but the largest single reservoir is the synovial fluid (SF) of the diarthrodial joints, where concentrations of 0.5-4 mg/mL are achieved. Hyaluronic acid is the major hydrodynamic nonprotein component of joint SF. Its unique viscoelastic properties confer remarkable shock absorbing and lubricating abilities to SF, while its enormous macromolecular size and hydrophilicity serve to retain fluid in the joint cavity during articulation. HA restricts the entry of large plasma proteins and cells into SF but facilitates solute exchange between the synovial capillaries and cartilage and other joint tissues. In addition, HA can form a pericellular coat around cells, interact with proinflammatory mediators, and bind to cell receptors, such as cluster determinant (CD)44 and receptor for hyaluronate-mediated motility (RHAMM), where it modulates cell proliferation, migration, and gene expression. All these physicochemical and biologic properties of HA have been shown to be molecular weight (MW) dependent. The diverse physicochemical properties of HA arise from its unique macromolecular structure. The HA is an exceptionally long (3-30 μm) and unbranched nonsulfated GAG composed of repeating disaccharide units of N-acetylglucosamine and glucuronic acid glycosidically linked through their respective 1-4 ring positions. Hydroxyl group oxygens at the glucuronyl-1 and glucosamine 3-positions are used for further polymerization of the HA disaccharide units to form chains that, when released from the cell plasma membrane, are of variable length and thus polydispersity. Despite the simplicity of the HA primary structure, this linear polyelectrolyte adopts complex conformations in solution, which engender it with diverse biologic properties. Within the joint cavity, HA molecules are predominately synthesized by the type B synovial cells (PMID: 12219318).