Hyaluronan and Chondroitin Sulfate
Hyaluronan is present in nearly all vertebrate connective tissues and is nontoxic and nonimmunogenic. It is a polysaccharide (glycosaminoglycan, or GAG) that has a repeating unit of glucuronic acid and N-acetylglucosamine. At physiologic pH, hyaluronan is a weak polyanion because of the ionization of the carboxyl groups present in each glucuronic acid residue. This ionization, together with the GAG residues, confers a negative charge on hyaluronan. The negative charge attracts sodium and thereby water, resulting in hydration of the vitreous. Production of hyaluronan begins around the time of birth, when the corresponding hydration has been proposed to contribute to vitreous transparency and growth of the eye.
In free solution, hyaluronan occupies an extremely high volume relative to its weight and may fill all the space in the vitreous except for that occupied by the collagen fibers (see Fig 11-2B). Hyaluronan molecules of the vitreous may undergo lateral interactions with one another, and such interactions may be stabilized by noncollagenous proteins. Both the concentration and the molecular weight of hyaluronan in the vitreous vary, depending on the species and on the location in the vitreous body, with higher concentrations typically found in the posterior pole.
Chondroitin sulfate is also a GAG, but unlike hyaluronan, it is sulfated. Chondroitin sulfate plays an independent role in maintaining the ultrastructure of the vitreous. Versican is the predominant form of chondroitin sulfate in the vitreous, where it interacts with hyaluronan. Versican has been reported to participate in the formation of the vitreous gel.
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Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.