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Hyaluronic acid (HA)

Hyaluronic acid, also called sodium hyaluronate, or hyaluronan or HA, is a bio-polymeric material where repeat unit comprising N-acetyl-D-glucosamine and D-glucuronic acid is linearly repeated in connection, which polymer plentifully exists in vitreous humor, synovial fluid, connective tissues etc. Hyaluronic acid belongs to the group of glycosaminoglycans which also includes compounds containing sulfate groups (chondroitins, keratans and the heparans). As a consequence of variable distribution of sulfate residues, this group of glycosaminoglycans is an heterogeneous assemblage of different molecules. By contrast, hyaluronic acid only contains the disaccharide unit N-acetyl glucosamine and glucuronate. Hyaluronic acid (HA) is unique among glycosaminoglycans in that it is not covalently bound to a polypeptide. HA is also unique in having a relatively simple structure of repeating nonsulfated disaccharide units composed of D-glucuronic acid (GIcUA) and N-acetyl-D-glucosamine (GIcNAc). Natural hyaluronic acid is polydisperse in respect of molecular weight and is known to show excellent biocompatibility even when implanted or injected into the body by virtue of the absence of species and organ specificity. Hyaluronic acid is ubiquitous in the organism, with the highest concentration found in soft connective tissue and joint fluid. It is a constituent of the intercellular matrix of connective tissue that exists in almost all vertebrates. Hyaluronic acid plays an important role in the biological organism, firstly as a mechanical support of the cells of many tissues, such as the skin, the tendons, the muscles and cartilage and it is therefore the main component of the extracellular matrix. HA also performs other functions in the biological processes, such as the hydration of tissues, lubrication, cellular migration, cell function and differentiation. Hyaluronic acid plays a fundamental role in the tissue repair process, especially in the first stages of granulation, stabilising the coagulation matrix and controlling its degradation, favouring the recruitment of inflammatory cells such as fibroblasts and endothelial cells and, lastly, orienting the subsequent migration of the epithelial cells. Hyaluronic acid is characterized by having a high moisture retention property, a high viscoelasticity and a high lubricity, and making use of such characteristics, it is blended as a humectant in cosmetics, and its availability is widely recognized in the field of medicine as e.g. an injection drug for treatment of various arthropathy or adjuncts for ophthalmic operation. Hyaluronic acid is a natural component of cartilage and joint fluid. Hyaluronic acid is an important component of the intercellular matrix. Specifically, the highest levels are found in the eye and synovial fluid of joints. In joints, its primary role is that of lubrication, reducing pain, and inflammation. HA lubricates and absorbs shock in the joint. Hyaluronic acid may also play an important role in the control of intercellular and interstitial permeability. This theory is supported by the fact that the depolymerization of hyaluronic acid, which occurs in certain pathological conditions of connective tissues, results in an increased permeability of the connective tissue barrier.

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