From the chemical point of view, carbohydrates can be classified based on the Polymerization degree (PD) in:
Monosaccharides: formed by a single molecule; do not release more simple molecules when hydrolyzed, e.g. glucose, galactose, fructose.
Oligosaccarides: Formed by 2-9 monomers linked by glycosidic bonds (release 2-9 monosaccharides when hydrolyzed). The most important subgroup of oligosaccharides is the sub group of Disaccharides, formed by just two molecules of monosaccharides. Disaccharides that appear naturally are lactose or milk sugar, (formed by galactose and glucose), and sucrose or table sugar, formed by fructose and glucose. Other important disaccharides, products of the digestion of starch and glycogen, are maltose and isomaltose, formed both of them by two molecules of glucose, but linked in a different way. Cellobiose is a disaccharide formed in animals that can breakdown cellulose.
Polysaccharides are carbohydrates formed by more than 9 monosaccharides linked by glycosidic bonds. When they are formed by the same kind of monosaccharides, they are called homopolysaccharides, like starch, glycogen and cellulose, formed each of them by hundreds of molecules of glucose linked by glycosidic linkages. If the polysaccharides molecules are formed by different kinds of monosaccharides, they are considered heteropolysaccharides. Since Hyaluronic acid is formed by thousands of alternative units of N-acetyl glucosamine and glucuronic acid, it is a heteropolysaccharide.
Heteropolysaccharides provide extracellular support to very different organisms, from bacteria to humans; together with fibrous proteins, like collagen, elastin and others, heteropolysaccharides are the most important components of the extracellular matrix. Other important heteropolysaccharides in the extracellular matrix are condroitin sulfates and dermatan sulfates. These heteropolysaccharides are formed by the repetition of a disaccharide unit of an aminosugar and an acid sugar.
Besides the rol of hyaluronic acid as an important component of extracellular matrix of skin and connective tissue, it also provides viscosity to the synovial fluid, lubricating joints, gives consistency to the vitreous humor and contributes to tensile strength and elasticity in cartilages and tendons.
Hyaluronic acid has received a big attention from the media as an important tool in the anti-aging process.