Choroid

Choroida (from Latin chorioidea) - the actual choroid, the posterior part of the vascular tract of the eye, located from the dentate line to the optic nerve.

The thickness of the actual choroid at the posterior pole of the eye is 0.22-0.3 mm and decreases towards the dentate line to 0.1-0.15 mm. Vessels of the choroid are branches of the posterior short ciliary arteries (orbital branches of the orbital artery), posterior long ciliary arteries moving from the dentate line to the equator, and the anterior ciliary arteries, which, as a continuation of the muscular arteries, send branches to the anterior part of the choroid, where they anastomose with branches of the short posterior ciliary arteries.

The posterior short ciliary arteries perforate the sclera and penetrate the space located between the sclera and the choroid in the suprachoroidal region around the optic nerve disk. They break up into a large number of branches, which form the vascular envelope itself. Around the disc of the optic nerve is formed the vascular ring of Cinna-Galler. In some cases, there is an additional branch to the area of the macula (a.cilioretinalis) visible on the optic nerve disk or on the retina, which plays an important role in the occurrence of embolism of the central artery of the retina.

Four discs are distinguished in the choroid: supervascular, vascular, vascular capillary and basal complex.

The supervascular plate 30 μm thick is the outermost layer of the choroid, adjacent to the sclera. It is formed by a loose fibrous connective tissue, contains a large number of pigment cells. In pathological conditions, the space between the thin fibers of this layer can be filled with liquid or blood. One such condition is the hypotension of the eye, which is often accompanied by fluid transudation into the suprachoroidal space.

The vascular plate consists of intertwining arteries and veins, between which lie a loose fibrous connective tissue, pigment cells, individual bundles of smooth myocytes. Outside, there is a layer of large vessels (the Haller layer), behind it lies a layer of medium vessels (the Zattler layer). Vessels anastomose with each other, forming a dense plexus.

The vascular capillary plate, or layer of chorio capillaries, is a system of intertwined capillaries formed by vessels of relatively large diameter with holes in the walls for passage of liquid, ions and small protein molecules. The capillaries of this layer are characterized by an uneven caliber and the ability to simultaneously transmit up to 5 erythrocytes. Between the capillaries are located flattened fibroblasts.

The basal complex, or Bruch's membrane, is a very thin plate (1-4 μm thick), located between the choroid and the retinal pigment epithelium. Three layers are distinguished in this plate: the outer collagen layer with the zone of thin elastic fibers; the fibrous (fibrous) collagen layer and the cuticular layer, which is the basal membrane of the retinal pigment epithelium.

With age, the Bruch's membrane gradually thickens, lipids are deposited in it, its permeability to liquids decreases. Older people often find focal segments of calcification.

Actually, the vascular membrane has the highest capacity for transmitting fluid (perfusion), and its venous blood contains a large amount of oxygen.

Functions of the choroid:

• provides nutrition of retinal pigment epithelium, photoreceptors and outer plexiform layer of the retina;
• supplies the retina of substances that promote the photochemical transformation of the visual pigment;
• participates in maintaining the intraocular pressure and temperature of the eyeball;
• is a filter for the thermal energy that occurs when light is absorbed.

[1], [2], [3], [4], [5], [6], [7]