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Cornea

Medical expert of the article

Dermatologist, oncodermatologist
, medical expert
Last reviewed: 07.07.2025

The cornea is the anterior part of the outer capsule of the eyeball. The cornea is the main refractive medium in the optical system of the eye.

The cornea occupies 1/6 of the area of the outer capsule of the eye, has the shape of a convex-concave lens. In the center, its thickness is 450-600 µm, and on the periphery - 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). The limbus - a translucent line of transition of the cornea to the sclera is about 1 mm wide. The inner part of the limbus zone is transparent. This feature makes the cornea look like a watch glass inserted into an opaque frame.

By the age of 10-12 years, the shape of the cornea, its size and optical power reach the parameters characteristic of an adult. In old age, an opaque ring sometimes forms along the periphery concentric with the limbus from the deposition of salts and lipids - 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. On the cross-section it is seen that 9/10 of the thickness of the cornea is occupied by its own substance - the stroma. In front and behind it is covered with elastic membranes, on which the anterior and posterior epithelium are located, respectively.

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

  1. The epithelium (stratified, squamous and nonkeratinizing) consists of: A monolayer of basal prismatic cells, linked to the underlying basement membrane by ioloulesmosomes.
    • Two to three rows of branched wing-shaped cells.
    • Two layers of squamous superficial cells.
    • The surface of the outer cells is increased by microfolds and microvilli, which facilitate the adhesion of mucin. Within a few days, the surface cells are exfoliated. Due to the extremely high regenerative capacity of the epithelium, scars do not form in it.
    • Epithelial stem cells, located primarily at the superior and inferior limbus, are essential for maintaining the normal corneal epithelium. This area also acts as a barrier to prevent conjunctival growth onto the cornea. Dysfunction or deficiency of limbal stem cells can lead to chronic epithelial defects, conjunctival epithelial growth onto the corneal surface, and vascularization.
  2. Bowman's membrane is an acellular superficial layer of the stroma, damage to 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 which is filled with the main substance (chondroitin sulfate and keratan sulfate) and modified fibroblasts (keratocytes).
  4. Descemet's membrane consists of a network of fine collagen fibers and includes an anterior connecting zone, which develops in utero, and a posterior non-connecting zone, which is covered by a layer of endothelium throughout life.
  5. The endothelium consists of a monolayer of hexagonal cells and plays a vital role in maintaining the condition 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 in size, fill the vacated space.

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

The non-keratinizing anterior corneal epithelium consists of several rows of cells. The innermost of them is a layer of tall prismatic basal cells with large nuclei called germinative, i.e. embryonic. Due to the rapid proliferation of these cells, the epithelium is renewed, and 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 located parallel to the surface and have a flat outer edge. This ensures the ideal smoothness of the cornea. Between the integumentary and basal cells there are 2-3 layers of multi-branched cells that hold the entire structure of the epithelium together. The lacrimal fluid gives the cornea a mirror-like smoothness and shine. Due to the blinking movements of the eyelids, it mixes with the secretion of the meibomian glands and the resulting emulsion covers the corneal epithelium with a thin layer in the form of a precorneal film, which evens out the optical surface and protects it from drying out.

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

Under the epithelium is a thin (8-10 µm) structureless anterior border membrane - the so-called Bowman's membrane. This is the hyalinized upper part of the stroma. At the periphery, this membrane ends, not reaching 1 mm to the limbus. The strong membrane maintains the shape of the cornea when struck, but it is not resistant to the action of microbial toxins.

The thickest layer of the cornea is the stroma. The corneal stroma consists of the thinnest plates built from collagen fibers. The plates are located parallel to each other and to the surface of the cornea, but each plate has its own direction of collagen fibrils. This structure provides the strength of the cornea. Every ophthalmic surgeon knows that it is quite difficult or even impossible to make a puncture in the cornea with a not very sharp blade. At the same time, foreign bodies flying off at high speed pierce it right through. Between the corneal plates there is a system of communicating slits in which keratocytes (corneal corpuscles) are located, which are multi-branched flat cells - fibrocytes, making up a thin syncytium. Fibrocytes take part in wound healing. In addition to such fixed cells, wandering cells - leukocytes are present in the cornea, the number of which quickly increases in the inflammation focus. The corneal plates are bonded together by an adhesive containing sulphurous salt of sulphohyaluronic acid. The mucoid cement has the same refractive index as the fibres of the corneal plates. This is an important factor in ensuring the transparency of the cornea.

From the inside, the elastic posterior border plate, the so-called Descemet's membrane, adjoins the stroma, containing thin fibrils of a substance similar to collagen. Near the limbus, Descemet's membrane thickens and then divides into fibers that cover the trabecular apparatus of the iridocorneal angle from the inside. Descemet's membrane is loosely connected to the corneal stroma and forms folds as a result of a sharp decrease in intraocular pressure. When the cornea is cut through, Descemet's membrane contracts and often moves away from the edges of the incision. When these wound surfaces are aligned, the edges of the elastic posterior border plate do not touch, so restoration of the integrity of Descemet's membrane is delayed for several months. The strength of the corneal scar as a whole depends on this. In burns and purulent ulcers, the corneal substance is quickly destroyed and only Descemet's membrane can withstand the action of chemical and proteolytic agents for so long. If only Descemet's membrane remains against the background of an ulcerative defect, then under the influence of intraocular pressure it protrudes forward in the form of a bubble (descemetocele).

The inner layer of the cornea is the so-called posterior epithelium (previously called endothelium or Descemet's epithelium). The inner layer of the cornea consists of a single-row layer of flat hexagonal cells that are attached to the basal membrane by means of cytoplasmic processes. Thin processes allow these cells to stretch and contract with intraocular pressure changes and remain in place. At the same time, the cell bodies do not lose contact with each other. At the extreme periphery, the posterior epithelium, together with Descemet's membrane, 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. Under normal conditions, the cells of the posterior corneal epithelium are not capable of complete regeneration. Defects are replaced by the closure of adjacent cells, which leads to their stretching and increase in size. Such a process of substitution cannot be endless. Normally, a person aged 40-60 years old has from 2200 to 3200 cells per 1 mm2 of the posterior corneal epithelium. When their number decreases to 500-700 per 1 mm2, edematous corneal dystrophy may develop. In recent years, there have been reports that under special conditions (development of intraocular tumors, severe disruption of tissue nutrition), true division of individual cells of the posterior corneal epithelium can be detected on the periphery.

The monolayer of posterior corneal epithelium cells functions as a dual-action pump, which supplies organic substances to the corneal stroma and removes metabolic products, and is characterized by selective permeability for various ingredients. The posterior epithelium protects the cornea from excessive saturation with intraocular fluid.

The appearance of even small gaps between cells leads to corneal edema and a decrease in its transparency. Many features of the structure and physiology of posterior epithelial cells have become known in recent years due to the emergence of the method of intravital mirror biomicroscopy.

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

Despite the fact that the cornea has no vessels, it has abundant innervation, which is represented by trophic, sensory and autonomic nerve fibers.

Metabolic processes in the cornea are regulated by trophic nerves extending from the trigeminal and facial nerves.

The high sensitivity of the cornea is provided by the system of long ciliary nerves (from the ophthalmic branch of the trigeminal nerve), which form a perilimbal nerve plexus around the cornea. Entering the cornea, they lose their 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 interweaving denser.

Each cell of the anterior corneal epithelium has a separate nerve ending. This fact explains the high tactile sensitivity of the cornea and the sharply expressed pain when sensitive endings are exposed (erosion of the epithelium). High sensitivity of the cornea underlies its protective function: thus, when the surface of the cornea is lightly touched, as well as when a gust of wind blows, an unconditional corneal reflex occurs - the eyelids close, the eyeball turns upward, thus moving the cornea away from danger, and lacrimal fluid appears, washing away dust particles. The afferent part of the corneal reflex arc is carried by the trigeminal nerve, the efferent part - by the facial nerve. Loss of the corneal reflex occurs in severe brain damage (shock, coma). Disappearance of the corneal reflex is an indicator of the depth of anesthesia. The reflex disappears in some lesions of the cornea and upper cervical parts of the spinal cord.

The rapid reaction of the vessels of the marginal loop network to any irritation of the cornea occurs with the help of sympathetic and parasympathetic nerves, which are present in the perilimbal nerve plexus. They are divided into 2 endings, one of which passes to the walls of the vessel, and the other penetrates 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 diopters, most often amounting to 42-43 diopters. The central optical zone of the cornea is almost spherical. Towards the periphery, the cornea flattens unevenly in different meridians.

Functions of the cornea:

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

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