Vitreous humour

The vitreous humour (UK spelling) or vitreous humor (US 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".

Composition, properties and function
The vitreous is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye. It is present at birth and does not change much over the course of aging. It is produced by cells in the non-pigmented portion of the ciliary body deriven from embryonic mesenchyme cells which then degenerate after birth. Its composition is similar to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98-99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the glycosaminoglycan hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye. The lens, on the other hand, is tightly packed with cells. 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.

Although the vitreous is in contact with the retina and helps to keep it in place by pressing it against the choroid, it does not adhere to the retina, except at the optic nerve disc, AKA: papilla nervi optici (where the retina sends about 1.2 million nerve fibres (axons) to the brain). It is also connected to the Ora serrata (where the retina ends anteriorly), at the Wieger-band, the dorsal side of the lens. It is however, not connected at the macula, the tiny spot in the retina which gives us our "detail" and central vision.

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 occur after injuries to the eye or inflammation in the eye (uveitis).

Pathology
The collagen fibres of the vitreous are held apart by electrical charges. With aging, these charges tend to reduce, and the fibres may clump together. Similarly, the gel may liquefy, a condition known as synaeresis, allowing cells and other organic clusters to float freely within the vitreous humour. These allow 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 metabolized from the general circulation.