Chorioidea

Choroid (from Latin chorioidea) is the vascular membrane itself, the posterior part of the vascular tract of the eye, located from the dentate line to the optic nerve.

The thickness of the choroid itself 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. The vessels of the choroid are branches of the posterior short ciliary arteries (orbital branches of the ophthalmic artery), the posterior long ciliary arteries, directed from the dentate line to the equator, and the anterior ciliary arteries, which, being a continuation of the muscular arteries, send branches to the anterior part of the choroid, where they anastomose with the branches of the short posterior ciliary arteries.

The posterior short ciliary arteries perforate the sclera and penetrate into the suprachoroidal space around the optic nerve head, located between the sclera and the choroid. They split into a large number of branches, which form the choroid proper. The vascular ring of Zinn-Haller is formed around the optic nerve head. In some cases, there is an additional branch to the macula area (a. cilioretinalis), visible on the optic nerve head or on the retina, which plays an important role in the case of embolism of the central retinal artery.

In the vascular membrane, four plates are distinguished: supravascular, vascular, vascular-capillary and basal complex.

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

The vascular plate consists of intertwined arteries and veins, between which are located loose fibrous connective tissue, pigment cells, individual bundles of smooth myocytes. On the outside is a layer of large vessels (Haller's layer), behind it lies a layer of medium vessels (Sattler's layer). The vessels anastomose with each other, forming a dense plexus.

The vascular-capillary plate, or choriocapillary layer, is a system of intertwined capillaries formed by relatively large diameter vessels with openings in the walls for the passage of fluid, ions, and small protein molecules. The capillaries of this layer are characterized by an uneven caliber and the ability to pass up to 5 red blood cells at a time. Flattened fibroblasts are located between the capillaries.

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. This plate has three layers: an outer collagen layer with a zone of thin elastic fibers; an inner fibrous collagen layer; and a cuticular layer, which is the basal membrane of the retinal pigment epithelium.

With age, Bruch's membrane gradually thickens, lipids are deposited in it, and its permeability to liquids decreases. Focal calcification segments are often found in older people.

The choroid itself has the highest capacity for fluid permeability (perfusion), and its venous blood contains a large amount of oxygen.

Functions of the choroid:

• provides nutrition to the retinal pigment epithelium, photoreceptors and the outer plexiform layer of the retina;
• supplies the retina with substances that facilitate photochemical transformations of visual pigment;
• participates in maintaining intraocular pressure and temperature of the eyeball;
• is a filter for thermal energy generated by the absorption of light.

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