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Vitreous body
Last reviewed: 20.11.2021
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The vitreous humor is a transparent, colorless, gel-like substance that fills the cavity of the eyeball. The front of the vitreous body is: the lens, the zonular ligament and the ciliary processes, and the back and sides are the retina. The vitreous is the most voluminous structure of the eye, which is 55% of the inner contents of the eye. In an adult, the normal vitreous mass is 4 grams, the volume is 3.5-4 ml.
The vitreous body is spherical, somewhat flattened in the sagittal direction. Its posterior surface is directly attached to the retina, to which the vitreous is fixed only at the optic nerve disk and in the dentate line near the flat part of the ciliary body. This region in the form of a belt 2-2.4 mm wide is called the base of the vitreous.
The vitreous body has the following structures: the vitreous proper, the border membrane and the vitreous (cloquet) canal, which is a tube 1-2 mm in diameter, extending from the optic disk to the posterior part of the lens, not reaching the posterior crust of the lens. In the embryonic period of a person, through the cloquet, the canal passes the artery of the vitreous, which disappears at the time of birth.
Thanks to the use of modern intravital methods for studying the vitreous body it was possible to establish that it has a fibrillar structure and that the interfibrillar spaces are filled with a liquid, viscous, amorphous substance. The fact that the naked vitreous body does not spread and can retain its shape even when a load is applied to it indicates that it has its own outer membrane. A number of authors consider it to be the thinnest, transparent independent shell. However, the more popular point of view is that this is the denser layer of the vitreous humor formed by the thickening of the outer layers of the vitreous body and the condensation of the fibrils.
From the point of view of the chemical structure, the vitreous is a hydrophilic gel of organic nature, 98.8% of which is water and 1.12% is a dry residue that contains proteins, amino acids, glucose, urea, creatinine, potassium, magnesium, sodium, phosphates, Chlorides, sulfates, cholesterol and other substances. Proteins, which make up 3.6% of the dry residue, are represented by vitroquine and mucin and provide a viscosity of the vitreous, which is several tens of times greater than the viscosity of water.
Normally, the vitreous does not have fibrinolytic activity. But it has been experimentally proved that in cases of hemorrhage in the vitreous body, its thromboplastic activity is significantly increased, which is aimed at stopping bleeding. Due to the presence of antifibrinolytic properties in the vitreous humor, fibrin does not resolve for a long time, and this promotes cellular proliferation and the formation of connective tissue opacities.
The vitreous body has the properties of colloidal solutions, and it can be considered as a structural, but poorly differentiated connective tissue. Vessels and nerves in vitreous matter are alienated. Vital activity and constancy of the vitreous environment is provided by osmosis and diffusion of organic substances from the intraocular fluid through the glassy film, which has a directional permeability.
Microscopically, the vitreous body consists of bands of various forms of tender-gray color with interspersed dot and clavate formations of whitish color. With the movement of the eye, these structural formations "swing". Between the tapes and patches are colorless, transparent areas. With time, floating opacities and vacuoles may appear in the vitreous humor. The vitreous humor can not regenerate and, with partial loss, begins to be replaced by the intraocular fluid.
The presence of a liquid in the vitreous body is confirmed by the results of radiographic studies: the movement of indifferent paints or radionuclide isotopes introduced extraocularly in the vitreal masses has been established. The fluid produced by the ciliary body enters the vitreous body, from where it moves along the outflow path anteriorly, into the anterior chamber and posteriorly into the perivascular spaces of the optic nerve. In the first case, the liquid mixes with the chamber moisture and is withdrawn together with it, in the second of the posterior parts of the vitreous that borders the optical part of the retina, the fluid flows along the perivascular spaces of the retinal vessels. Knowledge of the peculiarities of the circulation of the intraocular fluid allows us to present the character of the distribution of medicinal substances in the cavity of the eye.
The vitreous humor has a low bactericidal activity. Leukocytes are found in it after some time after infection. In the opinion of several authors, the antigenic properties of the vitreous humor do not differ from those of the blood proteins.
The main functions of the vitreous:
- maintaining the shape and tone of the eyeball;
- conducting light beams;
- participation in intraocular metabolism;
- ensuring contact of the retina with the choroid of the eye