The Vitreous Humor

Handbook of Biomaterial Properties. Edited by J. Black and G. Hastings. Published in 1998 by Chapman Hall, London. ISBN 0 412 60330 6. 

As is the case with many other structural elements of the eye, there are presently much more data on animal vitreous body than on the human counterpart. Likely, the results reviewed here cover almost everything reported so far on the human vitreous. However, in many cases it is not possible to select a most reliable single value dependable values representing a range are thus provided in several of the following tables. 

Over the past few decades the vitreous body was perceived as a typical viscoelastic material. However, its characterization by rheometry (as shown in Table C3.8) is still in its infancy. 

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PEGylated Macugen PEGylation increases the molecular weight of this aptamer from about 10 kDa to some 50 kDa, thereby increasing its half-life in the vitreous humor 14... 

Fomivirsen is injected directly into the vitreous humor of the eye. Animal studies have shown that this drug accumulates in the retina and iris over 3 to 5 days and is cleared from the vitreous humor within 7 to 10 days. Fomivirsen exhibits minimal systemic absorption and is degraded locally by cellular exonucleases. 

Because of its poor oral bioavailability, foscamet is administered intravenously. Following intravenous infusion, 14 to 17% of foscamet is bound to plasma proteins. The concentration of this compound in the vitreous humor is approximately the same as its plasma level. Foscamet accumulates in bone this property may account for its bimodal initial half-Ufe of 4 to 8 hours... 

Pharmacokinetics No information on human pharmacokinetics is available. In the rabbit model, half-life in the vitreous humor was 62 hours and 79 hours in the retina, while the retinal half-life in monkeys was 78 hours. Systemic exposure to fomivirsen following single or repeated intravitreal injection in monkeys was not quantifiable. 

The chemist John Dalton (of atomic theory fame) was color-blind. He thought it probable that the vitreous humor of his eyes (the fluid that fills the eyeball behind the lens) was tinted blue, unlike the colorless fluid of normal eyes. He proposed that after his death, his eyes should be dissected and the color of the vitreous humor determined. His wish was honored. The day after Dalton s death in July 1844, Joseph Ransome dissected his eyes and found the vitreous humor to be perfectly colorless. Ransome, like many scientists, was reluctant to throw samples away. He placed Dalton s eyes in ajar of preservative (Fig. 1), where they stayed for a century and a half. 

The vitreous humor volume is about 4 ml in an adult. Its viscosity is 2-4 times that of water and is dependent on the concentration of sodium hyaluronate. Although it has the outward appearance of a transparent, viscoelastic gel, it contains fine diameter type II collagen fibers (8-12 nm diameter) that entrap the large coiled hyaluronic acid molecules. These fibers give the gel a spherical structure with a dent in the anterior surface (the hyaloid fossa). The main constituent of the vitreous is water (98%) with a refractive index of 1.33, but the gel is... 

Measurement of amino acid levels in the vitreous humor of rats after chronic intraocular pressure elevation or optic nerve transection. Glaucoma 11, 396—405. 

HA is an unsulfated glycosaminoglycane composed of repeating disaccharide units of D-glucuronic acid and A-acetylglucosaminc linked a-( 1 —4) and p-( 1—3), respectively. HA has special importance because it is a component of the ECM [53] in the soft tissues of mammals, where it mainly ensures water retention [54], This enables the transport of nutrients to, and removal of waste from, cells that do not have a direct blood supply, such a cartilage cells. Moreover, HA is present in the synovial joint fluid, the vitreous humor of the eye, cartilage, blood vessels, and the unbilical cord. More detailed information about the biological functions and physicochemical properties of HA can be found elsewhere [55, 56],... 

Sachs, H., Bruimer, H., Gas Chromatography-Mass Spectrometry Findings of Morphine and Codeine in the Vitreous Humor and Hair, Beitr. Gerichtl. Med., 44, 281,1986. 

Transcomeal iontophoresis enhances the ocular penetration of solutes and is capable of delivering high concentrations of drug to the anterior segment of the eye. However, only small concentrations have been achieved in the vitreous humor using this method [34,35]. This is presumably because the lens-iris barrier impedes the movement of drug from the anterior to the... 

Transscleral iontophoresis has been tried as an alternative method to in-travitreal injection of antibiotics such as cephalosporins, penicillins, and aminoglycosides [38-40]. Barza et al. [39] reported that the technique of transscleral iontophoresis was a safe, effective, noninvasive way of producing high concentration of antibiotics in the vitreous humor, when gentamicin sulfate iontophoresis was applied to the eyes of monkeys. 

The vitreous humor is a viscoelastic connective tissue composed of small amounts of glycosaminoglycans, including hyaluronic acid, and of such proteins as collagen.The collagen fibrils are anchored directly to the basal lamina, which forms the boimdaries of the lens, the ciliary body epithelium, and the neuroglial cells of the retina. Although the anterior vitreous is cell free, the posterior vitreous contains a few phagocytic cells, called hyalo-cytes, and is sometimes termed the cortical tissue layer. 

Hydrophilic drugs, such as gentamicin, do not cross the blood-retinal barrier readily after systemic administration. After intravitreal administration they have a prolonged half-life of 24 hours or more in the vitreous humor.Their major route of exit is across the lens zonules and into the aqueous humor and then through the aqueous outflow pathways. For the vitreous to act as a depot for these drugs, the agents must be injected, introduced by iontophoresis, or slowly released by a surgically implanted intraocular device. 

Because the normal blood-retinal barrier resists various substances, including fluorescein, the presence of fluorescein in the vitreous humor indicates a functional breakdown of this barrier. Although physiologic factors and instrument artifacts can influence vitreous fluorescence, this technique has been used to detect retinal vascular disease, especially in diabetes.The procedure has also been used to study the integrity of the blood-retinal barrier in various other diseases, including retinitis pigmentosa, optic neuritis, and essential hypertension. 

Chitin is a heteropolysaccharides, which is made up of glycans such as Hyaluronic acid, an alternating polysaccharide of D-glucuronic acid and N-acetyl-D-glucosamine MW to 5,000,000 which serves as a lubricant in joints and is a component of the vitreous humor. 

The vitreous humor of the eye is a hydrophilic gel with water content above 99%, formed by a network of collagen and hyaluronic acid, which in some pathological situations becomes dysfunctional. For this reason, some hydrogels have been tested as substitutes for the vitreous humor of the eye, among them hydrogels of PVP, polyacrylamide (PA) and PVA, although no truly satisfactory result has been achieved. ... 

Hyaluronic acid consists of repeating disaccharides of /3-D-glucopyranosyluronic acid-(l— 3)-iV-acetyl-D-galactosamine linked p- l 4) to the next disaccharide. This proteoglycan can have between 500 and 50,000 residues per chain [92]. Hyuronic acid is found widely distributed in mammalian cells and tissues, where it is found in synovial fluid that lubricates the joints, in the vitreous humor of the eye, and in connective tissue, such as the umbilical cord, the dermis, and the arterial wall. It also occurs as a capsular polysaccharide around certain bacteria, such as pathogenic streptococci [92]. 

Hyalm onate is produced both by animals and bacteria. It was first obtained by Meyer and Palmer from the vitreous humor of cattle eyes. Since then, it has been isolated from many sources, such as Wharton s jellyconnective tissue, skin, cock s comb, synovial fluid, Rous sarcoma, myxoedemal fluids, and encapsulated strains of hemolytic streptococci. Aerobacter aerogenes may also produce hyaluronate. Group A streptococci have been principally used for the investigation of the biosynthesis of hyaluronic acid. 

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