Cornea

Cornea (cornea) - anterior section of the outer capsule of the eyeball. The cornea is the main refractive medium in the optic system of the eye.

The cornea occupies 1/6 of the area of the outer capsule of the eye, it has the shape of a convex-concave lens. At its center, its thickness is 450-600 μm, and on the periphery it is 650-750 μm. Due to this, the radius of curvature of the outer surface is greater than the radius of curvature of the inner surface and is on average 7.7 mm. The horizontal diameter of the cornea (11 mm) is slightly larger than the vertical (10 mm). Limb - a translucent line of the corneal transition into the sclera has a width of about 1 mm. The inner part of the limb zone is transparent. This feature makes the cornea look like a watch glass inserted into an opaque mandrel.

By 10-12 years of life, the shape of the cornea, its size and optical power reach the parameters characteristic of an adult. In the elderly, on the periphery of the concentric limbus from the deposition of salts and lipids, an opaque ring is sometimes formed, the so-called senile arc, or the so-called arcus senilis.

In the thin structure of the cornea, 5 layers are distinguished, performing certain functions. In the transverse section, one can see that 9/10 of the thickness of the cornea occupies its own substance - the stroma. Front and back it is covered with elastic membranes, which are respectively the anterior and posterior epithelium.

The diameter of the cornea averages 11.5 mm (vertical) and 12 mm (horizontal). The cornea consists of the following layers:

1. The epithelium (multilayered, scaly and nonkerberry) includes: Monolayer of basal prismatic cells linked to the underlying basal membrane with the help of Iulolesmosomes.
   • Two or three rows of detached pterygoid cells.
   • Two layers of scaly superficial cells.
   • The surface of the outer cells is increased due to micro-folds and microvilli, which promote the adhesion of mucin. Within a few days, the surface cells slough. Due to the extremely high ability of the epithelium to regenerate, scars are not formed in it.
   • Epithelial stem cells, located mainly in the upper and lower limbs, are necessary to maintain the normal state of the corneal epithelium. This zone also plays the role of a barrier that prevents the growth of the conjunctiva on the cornea. Dysfunction or deficiency of limbal stem cells can lead to chronic epithelial defects, proliferation of the conjunctival epithelium on the surface of the cornea and vascularization.
2. The Bowman's membrane is an acellular superficial layer of the stroma, the damage of which leads to scar formation.
3. The stroma occupies about 90% of the entire thickness of the cornea and consists mainly of correctly oriented collagen fibers, the space between them is filled with the basic substance (chondroitin sulfate and keratan sulfate) and modified fibroblasts (keratocytes).
4. The Descemet's membrane consists of a network of thin collagen fibers and includes an anterior connective zone that develops in utero and a posterior non-bonding zone covered with a layer of endothelium throughout life.
5. Endothelium consists of a monolayer of hexagonal cells and plays a crucial role in maintaining the state of the cornea and preventing it from swelling under the influence of IOP, but does not have the ability to regenerate. With age, the number of cells gradually decreases; The remaining cells, increasing, fill the vacant space.

The cornea is abundantly innervated by the nerve endings of the first branch of the trigeminal nerve. Allocate the subepithelial and stromal nerve plexuses. Corneal edema is the cause of color aberrations and the appearance of the "rainbow circles" symptom.

The non-coronetive anterior epithelium of the cornea consists of several rows of cells. The innermost of them - a layer of high prismatic basal cells with large nuclei called germinative, i.e., embryonic. Due to the rapid multiplication of these cells, the epithelium is renewed, the defects on the surface of the cornea are closed. The two outer layers of the epithelium consist of sharply flattened cells, in which even the nuclei are parallel to the surface and have a flat outer edge. This ensures an ideal smoothness of the cornea. Between the integumentary and basal cells there are 2-3 layers of multifaceted cells securing the entire structure of the epithelium. Mirror smoothness and shine of the cornea is imparted by a tear fluid. Thanks to the blinking movements of the eyelids, it mixes with the secret of the meibomian glands and the emulsion formed thin layer covers the corneal epithelium in the form of a pre-corneal film that smooths the optical surface and protects it from drying out.

The integumentary epithelium of the cornea has the ability to quickly regenerate, protecting the cornea from the adverse effects of the external environment (dust, wind, temperature changes, suspended and gaseous toxic substances, thermal, chemical and mechanical injuries). Extensive post-traumatic uninfected erosions in a healthy cornea are closed in 2-3 days. Epithelialization of a small cell defect can be seen even in the cadaveric eye in the first hours after death, if an isolated eye is placed in the conditions of the thermostat.

Under the epithelium there is a thin (8-10 μm) structureless anterior border membrane - the so-called Bowman's membrane. This is the hyalineized upper part of the stroma. On the periphery, this membrane terminates, not reaching 1 mm to the limb. A sturdy membrane retains the shape of the cornea at impacts, but it is not resistant to the action of microbial toxins.

The thickest layer of the cornea is the stroma. Stroma of the cornea consists of the finest plates, built of collagen fibers. Plates are located parallel to each other and the surface of the cornea, but in each plate reveals its direction of the collagen fibrils. This structure ensures the strength of the cornea. Every ophthalmic surgeon knows that making a puncture in the cornea with a not very sharp blade is difficult or even impossible. At the same time, foreign bodies flying at high speed pierce it through and through. Between the corneal plates there is a system of communicating slots in which the keratocytes (corneal bodies) are located, which are multistep flat cells - fibrocytes, which form a fine syncytium. Fibrocytes are involved in wound healing. In addition to such fixed cells, there are wandering cells in the cornea - leukocytes, the number of which rapidly increases in the focus of inflammation. Corneal plates are bonded together by a gluing agent containing a sulfurohaluronic acid sulfide salt. Mucoid cement has the same refractive index with corneal fiber fibers. This is an important factor that ensures the transparency of the cornea.

From the inside, a flexible posterior marginal plate is attached to the stroma - the so-called Descemet's membrane, which has thin fibrils from a substance like collagen. Near the limb Descemet's shell thickens, and then is divided into fibers that cover the inside of the trabecular apparatus of the iris-corneal angle. Descemet's coat is loosely associated with the stroma of the cornea and as a result of a sharp decrease in the intraocular pressure it forms folds. With the through crossing of the cornea, the Descemet's membrane contracts and often moves away from the edges of the incision. When these wound surfaces are compared, the edges of the elastic posterior border plate are not in contact, therefore, the restoration of the integrity of the descemet coat is delayed for several months. This affects the strength of the corneal scar in general. With burns and purulent ulcers, the substance of the cornea is rapidly destroyed and only Descemet's membrane can withstand the action of chemical and proteolytic agents for so long. If on the background of the ulcerative defect there is only Descemet's membrane, then under the influence of intraocular pressure it protrudes forward in the form of a vesicle (descemetocele).

The inner layer of the cornea is the so-called posterior epithelium (formerly called the endothelium or descemet epithelium). The inner layer of the cornea consists of a single row of flat hexahedral layers that are attached, which are attached to the basal membrane by cytoplasmic outgrowths. Thin processes allow these cells to stretch and contract with the changes in intraocular pressure, and stay in their places. At the same time, the body cells do not lose contact with each other. At the extreme periphery, the posterior epithelium, together with the descemet coat, covers the corneoscleral trabeculae of the filtration zone of the eye. There is a hypothesis that these cells are of glial origin. They do not exchange, so they can be called long-livers. The number of cells decreases with age. Cells of the posterior epithelium of the cornea under normal conditions are not capable of complete regeneration. The replacement of defects occurs by the closing of neighboring cells, which leads to their stretching and increase in size. Such a substitution process can not be infinite. Normally, a person aged 40-60 years in 1 mm2 of the posterior epithelium of the cornea contains between 2200 and 3200 cells. When their number decreases to 500-700 per mm2, edematous degeneration of the cornea may develop. In recent years, there have been reports that under special conditions (the development of intraocular tumors, gross on the destruction of tissue nutrition), it is possible to detect the true division of single cells of the posterior epithelium of the cornea at the periphery.

The monolayer of the cells of the posterior epithelium of the cornea serves as a double-action pump that ensures the supply of organic substances to the stroma of the cornea and displays metabolic products, and is distinguished by selective permeability for various ingredients. The posterior epithelium protects the cornea from excessive impregnation with intraocular fluid.

The appearance of even small piercing between the cells leads to edema of the cornea and a decrease in its transparency. Many features of the structure and physiology of cells of the posterior epithelium have become known in recent years in connection with the appearance of the method of intravital mirror biomicroscopy.

The cornea has no blood vessels, so the metabolic processes in the cornea are very slow. Exchange processes occur due to the moisture of the anterior chamber of the eye, tear fluid and small vessels of the pericorneal loop network, which is located around the cornea. This network is formed from branches of conjunctival, ciliary and episcleral vessels, so the cornea reacts to inflammatory processes. In conjunctiva, sclera, iris and ciliary body. A thin network of capillary vessels along the circumference of the limbus comes into the cornea only 1 mm.

Despite the fact that there are no blood vessels in the cornea, it has abundant innervation, which is represented by trophic, sensitive and vegetative nerve fibers.

The processes of metabolism in the cornea are regulated by trophic nerves, departing from the trigeminal and facial nerves.

High sensitivity of the cornea is provided by a system of long ciliary nerves (from the orbital branch of the trigeminal nerve), which form around the cornea perilymbalnoe nerve plexus. Entering the cornea, they lose the myelin sheath and become invisible. The cornea has three tiers of nerve plexuses - in the stroma, under the basal membrane and subepithelial. Closer to the surface of the cornea, the nerve endings become thinner, and their interlacing is more dense.

Each cell of the anterior epithelium of the cornea has a separate nerve end. This fact explains the high tactile sensitivity of the cornea and pronounced pain in exposing sensitive endings (erosion of the epithelium). The high sensitivity of the cornea is the basis of its protective function: so, with a slight touch to the surface of the cornea, as well as when wind blows, an unconditional corneal reflex arises - eyelids close up, the eyeball turns upward, thus removing the cornea from danger, dust particles. The afferent part of the arc of the corneal reflex is carried by the trigeminal nerve, the efferent part is the facial nerve. The loss of the corneal reflex occurs with severe brain damage (shock, coma). The disappearance of the corneal reflex is an indicator of the depth of anesthesia. The reflex disappears with some lesions of the cornea and upper cervical spinal cord.

The rapid response of the vessels of the marginal loop network to any irritation of the cornea arises with the help of the sympathetic and parasympathetic nerves that are present in the perimelbital neural plexus. They are divided into 2 endings, one of them passes to the walls of the vessel, and the other penetrates into the cornea and contacts the branched network of the trigeminal nerve.

Normally, the cornea is transparent. This property is due to the special structure of the cornea and the absence of blood vessels. The convex-concave shape of the transparent cornea provides its optical properties. The refractive power of light rays is individual for each eye and ranges from 37 to 48 D, most often 42-43 D. The central optical zone of the cornea is almost spherical. To the periphery, the cornea flattened unevenly in different meridians.

Functions of the cornea:

• as the outer capsule of the eye performs a supporting and protective function due to strength, high sensitivity and the ability to quickly regenerate the anterior epithelium;
• As an optical medium performs the function of light transmission and refraction due to its transparency and characteristic shape.

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