The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear

gel A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still di ...

that fills the space between the lens and the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...

of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humor (also spelled humour, from Latin meaning liquid) or simply "the vitreous". Vitreous fluid or "liquid vitreous" is the liquid component of the vitreous gel, found after a vitreous detachment. It is not to be confused with the aqueous humor, the other fluid in the eye that is found between the

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

and lens.

Structure

The vitreous humor is a transparent, colorless, gelatinous mass that fills the space in the eye between the lens and the

. It is surrounded by a layer of collagen called the vitreous membrane (or hyaloid membrane or vitreous cortex) separating it from the rest of the eye. It makes up four-fifths of the volume of the eyeball. The vitreous humour is fluid-like near the centre, and gel-like near the edges. The vitreous humour is in contact with the vitreous membrane overlying the

. Collagen fibrils attach the vitreous at the optic nerve disc and the

ora serrata The ora serrata is the serrated junction between the choroid and the ciliary body. This junction marks the transition from the simple, non-photosensitive area of the ciliary body to the complex, multi-layered, photosensitive region of the retina. ...

(where the retina ends anteriorly), at the Wieger-band, the dorsal side of the lens. The vitreous also firmly attaches to the lens capsule, retinal vessels, and the

macula The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal av ...

, the area of the retina which provides finer detail and central vision. Aquaporin 4 in Müller cells in rats transports water to the vitreous body.

Anatomical features

The vitreous has many anatomical landmarks, including the hyaloid membrane, Berger's space, space of Erggelet, Wieger's ligament,

Cloquet's canal Hyaloid canal (Cloquet's canal and Stilling's canal) is a small transparent canal running through the vitreous body from the optic nerve disc to the lens. It is formed by an invagination of the hyaloid membrane, which encloses the vitreous body. ...

and the space of Martegiani. Surface features: * Patella fossa: Shallow saucer-like concavity anteriorly, in which the lens rests, separated by Berger's space * (Wieger's ligament): Circular thickening of vitreous 8-9mm in diameter, delineates the patella fossa * Anterior hyaloid: Vitreous surface anterior to

. Continuous with and invests in the zonular fibres, and extends forward between the ciliary processes * Vitreous base: Denser cortical area of vitreous. Firmly attached to the posterior 2mm of the pars plana, and the anterior 2-4mm of

* Posterior hyaloid surface: Closely applied to retinal internal limiting membrane. Firm attachment sites: Along blood vessels and at sites of retinal degeneration * Space of Martegioni: A funnel shaped space overlying the optic disc with condensed edge *

: A 1–2 mm wide canal within the vitreous, from the space of Martegioni to the space of Berger, along an S-shaped course mainly below the horizontal. * Mittendorf's dot: A small circular opacity on the posterior lens capsule, which represents the site of attachment of the hyaloid artery before it subsequently regressed. * Bergmeister's papilla: A tuft of fibrous tissue at the optic disc, which represents a remnant of the sheath associated with the hyaloid artery before it subsequently regressed. Internal structures of the vitreous * The vitreous body at birth is homogenous with a finely striated pattern. * With early aging the vitreous develops narrow transvitreal "channels". * The cortex is denser than the centre with development. * From adolescence, vitreous tracts form from anterior to posterior. * These vitreous tracts are fine sheet-like condensations of vitreous. Named tracts * Retrolental tract: Extends posteriorly from the hyaloideocapsular ligament into central vitreous * Coronary tract: External to the retrolental tract, and excluding posteriorly from a circular zone overlying the posterior 1/3rd of the ciliary processes * Median tract: Extends back from a circular zone external to the coronary tract, at the anterior margin of the vitreous base * Preretinal tract: Extends back from the

and vitreous base

Biochemical properties

Its composition is similar to that of the

, but the vitreous contains very few cells. It is composed mostly of phagocytes, which remove unwanted cellular debris in the

visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a poin ...

, and hyalocytes, which turn over the hyaluronan. The vitreous humour contains no blood vessels, and 98–99% of its volume is water (as opposed to only 75% in the cornea). In addition to water, the vitreous consists of salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibrils with

glycosaminoglycan Glycosaminoglycans (GAGs) or mucopolysaccharides are long, linear polysaccharides consisting of repeating disaccharide units (i.e. two-sugar units). The repeating two-sugar unit consists of a uronic sugar and an amino sugar, except in the case ...

, hyaluronan,

opticin Opticin is a protein that in humans is encoded by the ''OPTC'' gene. Opticin belongs to class III of the small leucine-rich repeat protein (SLRP) family. Members of this family are typically associated with the extracellular matrix. Opticin i ...

, and a wide array of proteins. Despite having little solid matter, the fluid is substantial enough to fill the eye and give it its spherical shape. This contrasts with the aqueous humour, which is more fluid, and the lens, on the other hand, which is elastic in nature and is tightly packed with cells. The vitreous humour has a

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...

two to four times that of water, giving it a gelatinous consistency. It has a

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, ...

of 1.336.

Development

The vitreous fluid is not present at birth (the eye being filled with only the gel-like vitreous ''body''), but found after age 4-5, and increases in size thereafter. Produced by cells in the non-pigmented portion of the

ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...

, the vitreous humour is derived from embryonic mesenchyme cells, which degenerate after birth. The nature and composition of the vitreous humour changes over the course of life. In adolescence, the vitreous cortex becomes more dense and vitreous tracts develop; and in adulthood, the tracts become better defined and sinuous. Central vitreous liquefies, fibrillar degeneration occurs, and the tracts break up ( syneresis). Coarse strands develop with ageing. The gel volume decreases with age, and the liquid volume increases. The cortex may disappear at sites, allowing liquid vitreous to extrude adjacently into the potential space between vitreous cortex and retina (vitreous detachment).

Clinical significance

Injury

If the vitreous pulls away from the retina, it is known as a vitreous detachment. As the human body ages, 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). 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 A posterior vitreous detachment (PVD) is a condition of the eye in which the vitreous membrane separates from the retina. It refers to the separation of the posterior hyaloid membrane from the retina anywhere posterior to the vitreous base (a 3� ...

or other diseases of the eye.

Posterior vitreous detachment A posterior vitreous detachment (PVD) is a condition of the eye in which the vitreous membrane separates from the retina. It refers to the separation of the posterior hyaloid membrane from the retina anywhere posterior to the vitreous base (a 3� ...

: Once liquid vitreous enters the sub-hyaloid space between the vitreous cortex and the retina, it may strip the vitreous cortex off the

with each eye movement (see Saccade).

Postmortem and forensic

After death, the vitreous resists

putrefaction Putrefaction is the fifth stage of death, following pallor mortis, algor mortis, rigor mortis, and livor mortis. This process references the breaking down of a body of an animal, such as a human, post-mortem. In broad terms, it can be view ...

longer than other body fluids. Within the hours, days and weeks after death, the vitreous potassium concentration rises, at such a predictable speed that vitreous potassium levels are frequently used to estimate the time since death ( post-mortem interval) of a corpse. The metabolic exchange and equilibration between

systemic circulation The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...

and vitreous humour is so slow that vitreous humour is sometimes the fluid of choice for postmortem analysis of

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

levels or substances which would be more rapidly diffused, degraded, excreted or metabolized from the general circulation. According to Jewish religion, extracting the vitreous fluid for forensic chemical analysis is preferred to blood analysis (in case a forensic or post-mortem toxicology test is deemed necessary). This avoids the loss of even a few droplets of blood from the body prior to burial.

Additional images

File:Three Main Layers of the Eye.png, Structures of the eye labeled File:Three Internal chambers of the Eye.svg, This image shows another labeled view of the structures of the eye

References

External links

The Eye - explanations and diagrams from a McGill University Web site
{{DEFAULTSORT:Vitreous Humour Body fluids Human eye anatomy