Layers of the cornea of ​​the eye. Purpose of the cornea

The anterior epithelium near the cornea is directly connected to the conjunctiva and has an absolutely smooth surface. The epithelium is equipped with numerous nerve endings, which makes this layer sensitive to external influences. The surface layers of the epithelium are permeable to liquids and gases, this property is used to treat diseases with the help of drops.

The posterior epithelium is an endothelium consisting of polygonal cells. The thickness of the cornea where the endothelium is located is in the range of 0.4-0.5 microns. The endothelium along the periphery of the cornea passes to the trabecular meshwork.

The endothelium is examined using mirror microscopy, this allows you to determine the structure and location of epithelial cells. The endothelium of the cornea is able to recover due to the migration and expansion of cells. After 60 years, the endothelium changes - the number of cells in this layer decreases, but they become larger in size. If we compare the number of cells at different ages, then at the age of 16 there will be 3700 cells in the posterior epithelium, and closer to 80 it will decrease to about 2330 cells. Damaged endothelium, diseases of this layer or malnutrition also lead to a decrease in the number of cells in the posterior epithelium. Injured endothelium leads to the penetration of fluid from the anterior chamber into all layers of the membrane, which, in turn, causes its clouding and leads to malnutrition of the cornea.

However, these are not all layers of the cornea. The tear film should be mentioned, it also plays a role in the functioning of the entire apparatus of the eye.

Functions of the cornea

The function performed by the cornea for the entire eye can be compared to the work that the lens does when photographing. It is possible to draw an analogy between the structure of the cornea and the device of a lens, which continuously collects all the scattered rays of a stream of light and focuses them in the right direction.

That is why we can say that all layers of the cornea are the main refractive medium of the human eye. The shape of this shell is convex with a shiny and smooth surface. The cornea is located in the sclera like glass in a watch.

The cornea of ​​a healthy eye has a refractive power that ranges from 40 to 44 diopters.

Corneal thickness

The thickness of the cornea along the periphery is from 1 to 1.2 mm, in the central part this size is smaller - 0.8–0.9 mm. The diameter of the cornea horizontally reaches an average of 11 to 12 mm, vertically this figure is less (from 10.5 to 11 mm). The radius of curvature of the cornea reaches an average of 7.8 mm. The thickness of the cornea in the place where its stroma passes is 9/10 of the total size. The curvature of the cornea is important in the selection of lenses, this value is determined using ophthalmometry.

The main properties of the cornea of ​​​​the eye include transparency, high sensitivity, sphericity, the absence of blood vessels, and specularity. All layers of the cornea protect the inner membranes of the eye from damage and the penetration of bacteria.

Corneal diseases

Almost all diseases of the cornea are inflammatory in nature. Inflammation of the eyelids or other parts of the eye can quickly move to the cornea. Corneal diseases are also caused by external causes - infectious pathogens, unfavorable environmental conditions, the influence of allergens, tobacco smoke, chemicals. All these factors affect the change in the functions of the cornea, which leads to its clouding.

Sometimes clouding of the cornea is congenital. This occurs as a result of infectious diseases of a woman during pregnancy. Various infections lead to abnormal development of the cornea, as a result of which its structure is disturbed, and the child is already born with a pathology of vision.

Corneal diseases caused by a fungus are especially dangerous. It is possible to infect with a fungus when the cornea is damaged by an object containing spores of this microorganism. Fungal diseases are difficult to treat.

Treatment of diseases of the cornea

When corneal diseases are detected, a wide variety of therapies are selected. In diseases caused by infectious pathogens, it is necessary to use appropriate antibacterial local and systemic drugs. The thinning of the cornea, the formation of scars on it, in addition to conservative treatment, will also require surgical intervention.

The cornea is a very important part of the human eye apparatus and you should not self-medicate it. At the slightest deterioration in vision or other pathologies, you should immediately consult a doctor. Timely therapy in most cases passes quickly and helps to avoid vision loss.

The cornea is the anterior part of the eye capsule, which is the main component of the refractive system of the organs of vision. The cornea looks like a convex-concave lens. Due to the fact that it has a different thickness at the periphery and in the center, different curvature of the inner and outer planes of this component of the refractive system is achieved. The refractive power of the cornea of ​​the eye is 40 diopters. The radius of curvature is approximately 7.8 mm. The cornea is absolutely transparent, there are no blood vessels in it. Its diameter is 11 mm vertically and 12 mm horizontally. The normal thickness of the stratum corneum at the periphery is 700 µm, in the center 550 µm.

The cornea is an organic biconvex lens that is attached to the sclera of the eye with thin fibrous fibers. The place where the cornea meets the sclera is called the limbus. .

The cornea consists of 6 layers.

1. Stroma. It is the thickest and largest layer of the cornea, covering more than 90% of its area. Formed by collagen fibers, fibrocytes, keratites, leukocytes. Responsible for the strength of the cornea. It is filled with the thinnest plates of collagen fibers. The plates are parallel to each other, but the direction of the fibers in each plate is different, which ensures strength.
2. Epithelium. Performs a protective function. It tolerates mechanical damage very well. Stratified squamous non-keratinizing epithelium can be regenerated in a day and no traces remain. The anterior and posterior epithelium retain moisture in the stroma. If their function is impaired, the cornea swells and loses its transparency.
3. Bowman's membrane (membrane). Acellular superficial layer of the stroma. Maintains the natural shape of the cornea.
4. Endothelium. The posterior and inner layer of the cornea, which plays an important role in its nutrition and maintenance, prevents its swelling as a result of increased intraocular pressure. The transparency of the cornea is due to this layer. The endothelium is made up of hexagonal cells. Performs a pumping function, providing corneal cells with nutrients from the intraocular fluid. Various eye pathologies weaken this layer, as a result, the number of its cells decreases. The fewer endothelial cells, the less becomes the transparency of the cornea and it swells more. The endothelium consists of a single layer of cells and does not allow the cornea to swell. The older a person is, the thinner the endothelium becomes. He is unable to regenerate.
5. Descemet's layer. Collagen sheath, which is a protective barrier, preventing the penetration of infection into the rest of the eye structures. Provides eye protection from internal and external adverse effects.
6. Dua (Dua) layer. The thickness (norm) of this component of the stratum corneum is approximately 15 microns. The Dua layer is high strength, which can withstand pressure from 150 to 200 kPa. Located between the Descemet's membrane and the stroma.

The layers of the cornea.

The cornea differs from other eye structures in the absence of blood vessels in it, which provide tissue cells with oxygen and nutrients. This feature of the structure leads to a slowdown in metabolic processes in the corneal layers. Metabolism and nutrition of the cornea is carried out thanks to a thin network of capillaries that are located around the cornea and penetrate into it by 1 mm.

The innervation of the cornea is provided by the trigeminal nerve.

Functions

The functions performed by the cornea are determined by its location and anatomy.

The main functions are:

1. Refraction of light rays. The anatomical structure of the cornea is an optical lens that collects into focus the light rays entering the eye from different directions. In connection with this function, it is one of the most important components of the refractive system of the eyeball.
2. Protection. The corneal layer performs a protective function, from fine particles of dust, dirt, which are constantly in the air. It is also highly sensitive to light and reacts quickly to temperature changes. As a result, with any (even the mildest) injury, the eye closes reflexively, the cornea does not allow it to open for some time due to a sharp increase in photosensitivity, and at this time a large amount of tear fluid is released, which allows you to get rid of the foreign body.

Diseases

The cornea, performing its protective functions, is often subjected to strong influence, which results in various pathologies. Diseases of the cornea of ​​the eye are divided into groups.

Keratitis (inflammatory diseases)

Most often, pathologies that affect the cornea of ​​\u200b\u200bthe eye are inflammatory. Pathologies are caused by infectious agents, exposure to damaging particles, chemicals. Each of these factors has a detrimental effect on the cornea, reducing its light transmission and changing its properties.

traumatic

Occur due to chemical, mechanical damage. Epithelial cells are damaged, their ability to regenerate is impaired.

With mechanical, thermal injury to the cornea, purulent often develops, which quickly destroys it. The longest in such a disease is the Descemet's membrane, which for a long time withstands the action of destructive factors.

Dystrophic changes

It develops due to metabolic disorders in the body. Symptoms do not appear for a long time, but then the person is worried about dry eyes, blurred vision.

anomalies

Some people have an abnormal development of the cornea, which is expressed in a change in the degree of its transparency, a change in size and shape.

Anomalies include:

• giant cornea (megalocornea). This is mainly a hereditary congenital disease, but sometimes at a young age, due to the development of uncompensated glaucoma, megalocornea appears as an acquired disease;
• small cornea (microcornea). The anomaly can be unilateral or bilateral. Due to the reduced size of the cornea, the eyeball also decreases;
• . A change in the shape of the stratum corneum, in which it becomes thinner and begins to stretch in the form of a cone. The anomaly refers to genetic, develops, as a rule, in both eyes, but with some difference in time;
• keratoglobus. An anomaly in which the cornea has a spherical shape. The appearance of this anomaly contributes to the weakness of the elasticity of the cornea.

Research methods

To detect changes in the cornea in various diseases, a number of diagnostic procedures are performed. Based on their results, the specialist will prescribe treatment.

The main methods for examining the cornea:

1. Biomicroscopy. Examination of the stratum corneum using a microscope and an illuminator.
2. Keratometry. Determination of the curvature of the radius of the cornea.
3. pachymetry. The thickness of the cornea is examined using a special ultrasound probe.
4. Topography. With the help of topography, the entire surface of the cornea is examined, its shape (asphericity or eccentricity) and refractive power are accurately determined.
5. Microbiological research. Take a scraping from the surface layers of the cornea.
6. Biopsy. Corneal tissue is taken for examination when culture and scraping are insufficient for diagnosis.
7. Mirror microscopy. Analysis of the shape of cells and determination of their number in the endothelial layer. The norm is 3000 cells per square millimeter.

Treatment

The method of treatment is chosen depending on the disease, the clinical picture, the general condition of the person. Infectious lesions are treated with antibacterial drops. If the cornea has changed its shape and refractive power, optical correction is performed using glasses or lenses. To suppress inflammation, drugs with glucocorticoids are prescribed; in the infectious process, antiviral, antimycotic agents are used. If the injuries are superficial, agents are used that accelerate the restoration of epithelial tissues.

With the ineffectiveness of conservative treatment, extensive damage to the cornea, progressive deterioration of vision, congenital anomalies, surgery is performed. This can be (donor cornea transplantation) or keratoprosthetics (installation of an artificial cornea).

The cornea is part of the refractive system of the eye. If its structure is disturbed, dysfunction develops, then the entire optical system suffers.

Only timely attention to symptoms that indicate problems with this important layer of the eyeball and diagnosis will allow timely treatment and save vision.

The anterior transparent part of the fibrous membrane of the eye is called the cornea. The functions of the cornea of ​​​​the eye are protection and refraction of light. It is a convex-concave lens and looks like a convex watch glass, being an important part of the optical system of the organs of vision. It occupies about 17% of the area, the rest of the opaque part is called the sclera. The place of transition of the sclera to the cornea is called the limbus.

Anatomy and structure

The cornea of ​​the eye consists of 6 layers:

• The anterior epithelium is a stratified squamous non-keratinized epithelium.
• Bowman's membrane is a derivative of the ground substance.
• Stroma - makes up 90% of the volume. The structure is layered, each of the layers has a different arrangement of collagen fibers.
• Dua's layer - the thinnest transparent layer, located between the stroma and the Descemet's membrane, is highly durable, was discovered in 2013.
• Descemet's membrane is a derivative of the endothelium.
• The posterior epithelium (endothelium) is a single-layer epithelium lining the inner surface.

Food sources of the cornea. The structure of the cornea has a peculiarity - the absence of blood vessels, thanks to which it is transparent, but the metabolism in it is slowed down. The cornea is nourished by lacrimal fluid, the aqueous humor of the anterior chamber of the eye. The ciliary nerves are also important, which ensure the normal functional state of the cornea.

Innervation of the cornea. Sensitive innervation is carried out by the first branch of the trigeminal nerve, from which the ciliary nerves approach the cornea. For the protective corneal reflex, the close connection of the trigeminal and facial nerves is responsible, which carries out the reaction of closing the eyelids in response to the slightest irritation.

Main functions

The cornea performs two main functions - it protects the eye and collects and refracts light rays.

• Protective. The thin cornea is a mechanical barrier between the internal environment of the eye and the surrounding space. The tear that bathes the cornea contains lysozyme, which also has protective properties.
• Light refraction. Collects and refracts the rays of light falling on the surface of the eye, directing them through the pupil into the lens. The normal refractive power of the cornea is about 40 diopters. The thickness of this lens in the central part is 450-600 microns, in the peripheral part - 600-750 microns. The diameter of the cornea is 11.5-12 mm, the radius of curvature is on average 7.8 mm.

Normally, the human cornea looks completely transparent, moist, smooth, shiny and sensitive.

Diseases

• Anomalies are changes in shape and size.
• Keratitis is inflammation.
• Dystrophies are diseases caused by metabolic disorders.
• Tumors.

Common anomalies

One of the most common pathologies of the visual system is keratoconus, in which the stroma is thinned and deformed.

The most common anomalies in the development of the cornea:

• Megalocornea is a giant cornea with a diameter of more than 12 mm.
• Microcornea is a small cornea, less than 10 mm in diameter.
• Embryotoxon - ring opacity.
• The conical shape is a hereditary disease, thinning of the stroma and deformation in the form of a cone.
• Acute keratoconus is a conical deformity due to cracks in the Descemet's membrane.
• Weakness of the elastic skeleton, a condition preceding keratoconus, is characterized by progressive irregular astigmatism.
• Keratoglobus is a spherical change, genetically determined.

Acquired diseases

Keratitis is an inflammatory disease of the cornea. Keratitis is traumatic and infectious. Depending on the pathogen, bacterial, viral, fungal keratitis (keratomycosis) are distinguished. Endogenous keratitis is caused by concomitant human diseases, such as tuberculosis, syphilis, beriberi, trigeminal nerve damage, and others. Possible outcomes of the disease: cloudy opacity, spot (limited white opacity), walleye (dense opaque scar).

Corneal dystrophy is manifested by its clouding and is of two types: hereditary and acquired.

Corneal dystrophy is a non-inflammatory disease that leads to a decrease in transparency. The reason is metabolic disorders: amyloid, hyaline, lipids, uric acid and others. Dystrophies are:

• hereditarily conditioned (primary);
• acquired (secondary) dystrophies - occur against the background of the underlying disease: neurotrophic - occur with loss of sensitivity, recurrent erosion - after traumatic injury, Kaiser-Fleischner's ring in Wilson's disease and impaired copper metabolism, age-related degeneration - senile arc, Vogt's limbal girdle.

Date: 28.12.2015

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• Anatomy of the cornea
• Functions performed by the cornea of ​​the eye
• Anomalies in the development of the cornea
• Diseases of the cornea of ​​the eye
• Diagnosis and treatment of diseases of the cornea

The cornea of ​​the eye is the anterior part of the capsule of the organ of vision, which has a certain degree of transparency. In addition, this department is a component of the main refractive system.

Anatomy of the cornea

The cornea covers about 17% of the entire area of ​​the outer capsule of the visual organ. It has a structure in the form of a convex-concave lens. The thickness of the cornea in the center is about 450-600 microns, and closer to the periphery - 650-750 microns. Due to the difference in the thickness of the cornea, a different curvature of the outer and inner planes of this element of the optical system is achieved. The refractive index of light rays is 1.37, and the refractive power of the cornea is 40 diopters. The thickness of the stratum corneum is 0.5 mm in the center, and 1-1.2 mm on the periphery.

The radius of curvature of the cornea of ​​the eyeball is approximately 7.8 mm. The performance of the light-refracting function of the cornea of ​​​​the eye depends on the indicator of the curvature of the cornea.

The main substance of the cornea is a transparent connective tissue stroma and corneal bodies. Adjacent to the stroma are two lamellae, called marginal lamellae. The anterior plate is a derivative formed from the main substance of the cornea. The posterior lamina is formed by alteration of the endothelium that covers the posterior surface of the cornea. The anterior surface of the cornea is covered with a layer of stratified epithelium. The structure of the cornea of ​​the eyeball includes six layers:

• anterior epithelial layer;
• anterior border membrane;
• the main substance is the stroma;
• Dua layer - highly transparent layer;
• posterior border membrane;
• corneal endothelial layer.

All layers of the cornea have a structure whose main function is to refract the light beam entering the eye. The mirror surface and the characteristic luster of the surface are provided by the lacrimal fluid.

The lacrimal fluid, mixing with the secretion of the glands, wets the epithelium with a thin layer, protecting it from drying out, and at the same time levels the optical surface. A characteristic difference between the cornea of ​​the eye and other tissues of the eyeball is the absence of blood vessels in it, which nourish the tissues and supply the cells with oxygen. This feature of the structure leads to the fact that the metabolic processes in the cells that make up the stratum corneum are greatly slowed down. These processes occur due to the presence of moisture in the anterior chamber of the eye, tear fluid and the vascular system located around the cornea. A thin network of capillaries enters the corneal layers by only 1 mm.

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Functions performed by the cornea of ​​the eye

The functions of the cornea are determined by its structure and anatomical location in the structure of the eyeball, the main functions are:

• protective;
• function of light refraction in the optical system of the organ of vision.

Anatomically, the cornea is an optical lens, that is, it collects and focuses a beam of light that comes from different sides to the surface of the cornea.

In connection with the main function performed, it is an integral part of the optical system of the eye, which ensures the refraction of rays in the eyeball. Geometrically, the cornea is a convex sphere that performs a protective function.

The cornea protects the eye from external influences and is constantly exposed to environmental influences. In the process of performing the functions assigned to the corneal layer, it is constantly exposed to the influence of dust and small suspended particles floating in the air. In addition, such a department of the optical system of the eye has a high photosensitivity and responds to temperature effects. In addition to those listed, the stratum corneum has a number of other properties on which the normal operation of the human visual apparatus largely depends.

The protective function consists in a high degree of perception and sensitivity. The sensitivity of the corneal surface lies in the fact that when foreign objects, dust particles and small debris get on it, a person turns on a reflex response to irritation, which is expressed in a sharp closing of the eyelids.

When the surface of this section of the optical system of the eyeball is irritated, a sharp wrinkling of the eyes occurs, this reaction is a response to the effects of damaging and irritating factors that can provoke damage to the organ. In addition, when an irritating factor acts on the cornea, the appearance of photophobia in the form of a protective reaction, increased lacrimation can be observed. By increasing lacrimation, the eyeball cleanses its surface of small irritating particles of dust and dirt.

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Anomalies in the development of the cornea

Abnormal development of the cornea is expressed, as a rule, by a change in its size, degree of translucency and shape.

The most common developmental anomalies are:

• megalocornea;
• microcornea;
• embryotoxon;
• conical cornea;
• weakness of the elastic framework of the cornea;
• acute keratoconus;
• keratoglobus.

Megalocornea, or giant cornea, is most often a hereditary anomaly. There are cases of the development of a large cornea, not only congenital, but also acquired. Acquired megalocornea increases in size in the presence of an uncompensated form of glaucoma in the body at a young age.

Microcornea - a small cornea, an anomaly occurs in one-sided and two-sided form. The eyeball in the case of the development of such an anomaly is also reduced in size. With the development of megalo- and microcornea in the body, a high degree of probability of developing glaucoma appears. As an acquired pathology, a decrease in the size of the cornea contributes to the development of subatrophy of the eyeball. These diseases of the cornea cause it to lose its transparency.

Embryotoxon is an annular opacification of the cornea, which is located concentrically to the limbus and resembles the senile arc in appearance. Such an anomaly does not require treatment.

Keratoconus is a genetically determined anomaly in the development of the cornea, which is manifested by changes in shape. The cornea becomes thinner and it is stretched in the form of a cone. One of the signs of the development of an anomaly is the loss of normal elasticity. Most often, this process develops in the form of a bilateral anomaly, however, the development of the process does not occur simultaneously on both organs of vision.

The weakness of the elastic skeleton of the cornea is an anomaly, the progression of which provokes the occurrence and progression of irregular astigmatism. This type of anomaly is a harbinger of the development of keratoconus in the organ of vision.

Acute keratoconus develops in a person in the event of cracks in the thickness of the Descemet's membrane.

Keratoglobus is a spherical cornea. The reason for the appearance and progression of such an anomaly is the weakness of elastic properties due to genetic disorders.

Cornea of ​​the eye- this is the anterior shell of the eye, which does not have blood vessels, so it is absolutely transparent, while it is well innervated.

The cornea of ​​the eye is the main part of the refractive apparatus of the eye with a refractive power of 40 diopters. The corneal diameter is 11 mm vertically and 12 mm horizontally, the thickness is 550 µm in the center and 700 µm at the periphery. The radius of curvature of the cornea is equal to 7.8 mm. The diameter that the cornea of ​​​​the eye has from the moment of birth may increase slightly, from the moment of increase, this indicator becomes a constant value.

Layers of the cornea

Analyzing the structure of the cornea of ​​the eye, it should be noted that until 2013 it was believed thatcorneahas only 5 layers. Now after openings in 2013 6 layers are distinguished in the cornea.

In the structure of the cornea, the eye is divided into 6 layers:

The epithelial layer is a squamous, stratified, non-keratinized epithelium. Performs a protective function. Resistant to mechanical damage and recovers quickly.

Bowman's membrane is the surface layer of the stroma that does not have cells. After damage, scars remain.

- The stroma of the cornea of ​​the eye - occupies the largest area, which is 90% of the thickness of the cornea.

- The Dua layer - only about 15 microns thick, is highly durable, withstanding a pressure of 150-200 kPa, and is located between the stroma and the Descemet's membrane.

- Descemet's membrane - the structure of this membrane is made up of collagen fibers. It is a protective barrier, preventing the spread of infection.

— Endothelium is the inner or back layer of the cornea, which plays an important role in its nutrition and is responsible for the transparency of the cornea, and also participates in maintaining its condition, protects the cornea from swelling under the influence of intraocular pressure. Over time, the number of endothelial cells decreases, and various eye diseases accelerate this process. The fewer endothelial cells, the stronger the swelling of the cornea and the less transparency.

Functions of the cornea

The cornea of ​​the eye is the first barrier to the harmful effects of the environment - dust, wind, mechanical particles, chemical particles, etc. The protective function of the cornea of ​​the eye is expressed by its high sensitivity. When the cornea is irritated by a foreign body, the person reflexively closes the eyelids, the eye rolls up, and at this moment an abundant release of tears begins, washing out the foreign body, at the same time sensitivity to light increases, thus, the cornea protects itself from damage.

The cornea of ​​the eye and its research methods

- In order to determine all changes in the cornea in diseases, they use a microscope and an illuminator, this research method is called - Biomicroscopy of the cornea .

— Keratometry- allows you to measure the radius of curvature of the cornea.

- Using an ultrasonic sensor, the thickness of the cornea is measured, this research method is called - pachymetry.

– Examination of the entire surface of the cornea, an accurate determination of its shape, as well as its refractive power, allows you to do – Topography of the cornea.

— Microbiological research is a scraping from the surface of the cornea.

— Corneal biopsy- This is a research method in which the tissue of the body or its cells is taken. It is used only when the results of scraping and sowing for diagnosis were not enough.

Diseases of the cornea

- Keratitis;
- Keratoconus;
- Keratomalacia;
- corneal dystrophy;
- Bullous keratopathy.