Vitreous humour

  The vitreous humour (British spelling) or vitreous humor (U.S. spelling) is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrates. It is often referred to as the vitreous body or simply "the vitreous".

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Composition, properties and function

The vitreous is the transparent, colorless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye. It contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field), no blood vessels, and 99% of its volume is water with salts, sugars, and a network of collagen fibers with hyaluronic acid accounting for the rest. However, the vitreous has a viscosity two to four times that of pure water, giving it a gelatinous consistency. It also has a refractive index of 1.336^[1].

Although the vitreous is in contact with the retina and helps to keep it in place, it does not adhere to the retina, except in three places: all around the anterior border of the retina; in the macula, the tiny spot in the retina which gives us our "detail" and central vision; and at the optic nerve disc (where the retina sends one million nerve fibers to the brain).

Unlike the fluid in the frontal parts of the eye (aqueous humour) which is continuously replenished, the gel in the vitreous chamber is stagnant. Therefore, if blood, cells or other byproducts of inflammation get into the vitreous, they will remain there unless removed surgically (see floaters). If the vitreous pulls away from the retina, it is known as a vitreous detachment. As we age, the vitreous often liquefies and may collapse. This is more likely to occur, and occurs much earlier, in eyes that are nearsighted (myopia). It can also occurs after injuries to the eye or inflammation in the eye (uveitis).

Pathology

The collagen fibers of the vitreous are held apart by electrical charges. With aging, these charges tend to reduce, and the fibers may clump together. Similarly, the gel may liquefy, a condition known as syneresis, leading to cells and other organic clusters to float freely within the vitreous humour. These commonly lead to floaters which are perceived in the visual field as spots or fibrous strands. Floaters are generally harmless, but the sudden onset of recurring floaters may signify a posterior vitreous detachment (PVD) or other diseases of the eye.

Clinical significance

The metabolic exchange and equilibration between systemic circulation and vitreous humour is so slow that vitreous humour is sometimes the fluid of choice for postmortem analysis of glucose levels or substances which would be more rapidly diffused, degraded, excreted, or metabolised from the general circulation.

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