What is Hyaluronic Acid (HA)
Basic functions of Hyaluronan
Hyaluronan is present in every tissue of the body. It is most concentrated in the synovial fluid which occupies the spaces between the joints, in the vitreous fluid in the eye, and in the skin. Retention of water is one of the most important biological functions of Hyaluronan1, second only to providing nutrients and removing waste from cells that do not have a direct blood supply, such as cartilage cells. With a lower than adequate amount of Hyaluronan, nutrients cannot be moved into these cells and waste cannot be eliminated from cells. Hyaluronan is found in the synovial joint fluid, the vitreous humor of the eye, the cartilage, blood vessels, extracellular matrix, skin and the umbilical cord.2 Hyaluronan is sometimes abbreviated as HA and can also be known as Hyaluronic acid and Hyaluron.
Hyaluronan plays a crucial role in maintaining tissue homeostasis and consequently the intactness of the tissue; its function is to maintain the degree of hydration, turgidity, plasticity and viscosity. Hyaluronan also acts as a cementing substance and shockproof molecule, and as an efficient lubricant. All these properties prevent tissue cell damage caused by physical stress.
In addition to the mechanical and protective activities deriving from the chemical/structural properties of the molecule, Hyaluronan possesses other important biological activities such as aiding tissue repair processes due to its regenerating and anti-inflammatory properties.
Hyaluronan exists in synovial joint fluid and in cartilage
Our joints (like the elbows and knees) are surrounded by a membrane called the synovial membrane, which forms a capsule around the ends of the bones. This membrane secretes a liquid called the synovial fluid, which is found in joint cavities. It has many functions, including serving as a lubricant, shock absorber and a nutrient carrier. The fluid protects the joints and bones. Cartilage is immersed in the synovial fluid and is a fibrous connective tissue. Cartilage is avascular, meaning it contains no blood vessels, which is why the synovial fluid is so important. Synovial fluid is the only way in which nutrients can be carried into the cartilage and waste can be removed.2 Cartilage is a specialized form of connective tissue. Hyaline cartilage is the most predominant form of cartilage in the body and lends strength and flexibility to the body. A key component of cartilage is Hyaluronan. Cartilage is also avascular – with no blood vessels. Nutrients are brought by the synovial fluid, which is rich in Hyaluronan, to the cartilage.2
Hyaluronan is the main constituent of a family of polysaccharides which are similar in terms of structure and behaviour, and contain amino sugars better known as glycosaminoglycans. In chemical terms, it is a non-branched linear polymer consisting of a disaccharide unit formed from Glucosamine as N-acetylglucosamine into glucuronic acid and thence into Hyaluronan. This is repeated in the molecule numerous times, reaching molecular weights of up to several million Daltons. Interest in this molecule has increased considerably in recent years and it has been used with great success in the field of cosmetic surgery, ophthalmic surgery and oral care, and as a therapeutic aid in some degenerative joint diseases. Hyaluronan is needed to cushion and lubricate joints, eyes, skin and heart valves; and also helps increase supplies of joint lubricating synovial fluid.
REFERENCES
1. Block, A., and Bettelheim, F.: Water Vapor Sorption of Hyaluronan, Biochim Biophys Acta 201, 69, 1970
2. Goa K. L. and Benfield P.: Drugs 1994, 47: 536-566.
3. Oral Delivery of Hyaluronan Absorbs Effectively in Joints, Apr 18, 2004.