The lens is a biological formation that is part of the optical system in the organ of vision, which is involved in the process of accommodation. It looks like a biconvex lens, the refractive power of which is approximately 20D on average, in the state of accommodation, the optical power increases significantly, often reaching 30-33D. The lens is placed inside the eyeball in the frontal plane between the iris and the vitreous body. Together with the iris, they make up the iris lens diaphragm, which divides the eyeball into anterior and posterior sections.

The lens has an anterior and a posterior surface. In this case, the line limiting the transition of the front surface to the back is usually called the equator. The center of the anterior lens surface is called the anterior pole, the center of the posterior surface is called the posterior pole. The line that connects both poles is called the lens axis.

Dimensions and curvature of the lens

The radius of curvature of the anterior lens surface at rest of accommodation is 10 mm, and that of the posterior surface is 6 mm. The length of the lens axis is usually 3.6 mm. A narrow gap delimiting the posterior lens surface from the vitreous body forms the retrolenticular or retrolenticular space. In the eye, the lens is held by a ligament of zon, which is formed by thin fibers. They are attached to it in the equatorial region. The other ends of the zinn ligament are attached to the processes of the ciliary body.

The lens capsule is the membrane that covers it, which is a transparent and elastic eye tissue. The part of the capsule covering the anterior surface of the lens is called the anterior capsule, the second part is called the posterior capsule. The tissue thickness of the anterior capsule can range from 11 µm to 15 µm, and that of the posterior capsule can range from 4 µm to 5 µm. Under the surface of the anterior capsule, a single-layer cuboidal epithelium is located, reaching the equator of the lens, and in this place, its cells become more elongated.

Layers of the lens

The germinal zone or growth zone of the lens is the equatorial zone of its anterior capsule, it is here that young lens fibers are formed from its epithelial cells during a person's life.

The lens fibers are placed in the same plane and are interconnected by a certain adhesive substance, forming radial plates. The glued ends of the fibers of adjacent plates form seams on the anterior and posterior surfaces of the lens. When connected to each other, these seams create a lens star. The outer layers of its substance (subcapsular layers) adjacent to the lens capsule form the lens cortex, and the deep layers form its nuclear zone.

Proteins of the lens

The anatomical feature of the lens is the complete absence of lymphatic and blood vessels, as well as nerve fibers in it. The lens consists of a protein substrate and water. Moreover, the proportion of water is approximately 65%, and proteins - almost 35%.

Normally, the lens substance includes nucleoprotein, mucoprotein, compounds of calcium, potassium, sodium, phosphorus, sulfur, magnesium, chlorine, traces of copper, manganese, iron, boron and zinc. Participants in its redox processes are the tripeptide glutathione and ascorbic acid. The lens also contains lipids, vitamins (A, B1, B2, PP) and other substances necessary for a full-fledged metabolism.

Metabolism takes place in the lens slowly through diffusion and osmosis. In this case, the lens capsule is assigned the function of a semi-permeable biological membrane. Substances required for the normal function of the lens are brought by the intraocular fluid that bathes the lens.

Age-related changes in the lens

The size, shape, transparency, as well as the consistency of the lens undergo changes throughout human life. So, in newborns, the lens has an almost spherical shape, soft texture and almost absolute transparency without color. In an adult, the shape of the lens transforms into a biconvex lens with a flat anterior surface. Its color becomes yellowish, but the transparency is preserved. The intensity of yellow in the hue of the lens increases with age.

By the age of 40-45, the core of the human lens becomes dense and it loses its former elasticity. By this age, there is a weakening of accommodation and presbyopia develops.

By about the age of 60, the ability to accommodate is almost completely lost. This is due to severe sclerosis of the lens nucleus - phacosclerosis. At this age, due to natural aging - deterioration and slowing down of metabolism, tissue respiration and energy metabolism, in different layers of the lens, opacities of varying severity and magnitude may appear, which are called senile cataracts. This disease is detected by a study using a slit lamp when dilating the pupil with mydriatic drugs.

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Anatomy lens

lens is an essential part optical system of the eye , which also includes the cornea, fluid of the anterior and posterior chambers, and vitreous body .

lens located inside eye apples between the iris and vitreous body . It looks like a biconvex refractive power lenses about 20 diopters. In an adult, the diameter lens is 9-10 mm, thickness - from 3.6 to 5 mm, depending on accommodation (concept accommodation will be discussed below). IN lens distinguish between the anterior and posterior surfaces, the line of transition of the anterior surface to the posterior is called the equator lens .

In my place lens it is held by the fibers of the zinn ligament that supports it, which is attached circularly in the region of the equator lens on one side and to the processes ciliary body with another. Partially crossing each other, the fibers are firmly woven into lens capsule . Via Viger's ligament originating from the posterior pole lens , it is strongly associated with vitreous body . From all sides lens washed by the aqueous humor produced by the processes ciliary body .

Support apparatus lens

Examining the lens under a microscope, the following structures can be distinguished in it: capsule lens , epithelium lens and the actual substance lens.

Microscopic structure lens (lens cut)

lens capsule . From all sides lens covered with a thin elastic shell - capsule . Part capsules covering its front surface is called the front lens capsule ; plot capsules covering the back surface - back lens capsule . Front thickness capsules is 11-15 microns, back - 4-5 microns.

Under the front lens capsule one layer of cells - epithelium lens , which extends to the equatorial region, where the cells become more elongated. Equatorial zone of the anterior capsules is a growth zone (germinal zone), since throughout a person’s life, fibers are formed from its epithelial cells lens .

fibers lens located in the same plane are interconnected by an adhesive and form plates oriented in the radial direction. The soldered ends of the fibers of neighboring plates form on the front and back surfaces crystalline lens seams, which, when joined together like orange slices, form the so-called lens "star". Layers of fibers adjacent to capsule , form its bark, deeper and denser - the core lens .

feature lens is the absence in it of blood and lymphatic vessels, as well as nerve fibers. Nutrition lens carried out by diffusion or active transport through capsule nutrients and oxygen dissolved in the intraocular fluid. Includes lens from specific proteins and water (the latter accounts for about 65% of the mass lens ).

Transparency state lens is determined by the peculiarity of its structure and the peculiarity of metabolism. Preservation of transparency lens It is ensured by the balanced physicochemical state of its proteins and membrane lipids, the content of water and ions, the intake and release of metabolic products.

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Description

Particular attention was paid to the structure of the lens at the earliest stages of microscopy. It was the lens that was first examined microscopically by Leeuwenhoek, who pointed out its fibrous structure.

Shape and size

(Lens) is a transparent, disc-shaped, biconvex, semi-solid formation located between the iris and the vitreous body (Fig. 3.4.1).

Rice. 3.4.1. The relationship of the lens with the surrounding structures and its shape: 1 - cornea; 2- iris; 3- lens; 4 - ciliary body

The lens is unique in that it is the only "organ" of the human body and most animals, consisting from the same cell type at all stages- from embryonic development and postnatal life up to death. Its essential difference is the absence of blood vessels and nerves in it. It is also unique in terms of the characteristics of metabolism (anaerobic oxidation predominates), chemical composition (the presence of specific proteins - crystallins), and the lack of tolerance of the body to its proteins. Most of these features of the lens are associated with the nature of its embryonic development, which will be discussed below.

Anterior and posterior surfaces of the lens unite in the so-called equatorial region. The equator of the lens opens into the posterior chamber of the eye and is attached to the ciliary epithelium with the help of the ligament of zon (ciliary girdle) (Fig. 3.4.2).

Rice. 3.4.2. The ratio of the structures of the anterior part of the eye (scheme) (no Rohen; 1979): a - a section passing through the structures of the anterior part of the eye (1 - cornea: 2 - iris; 3 - ciliary body; 4 - ciliary girdle (zinn ligament); 5 - lens); b - scanning electron microscopy of the structures of the anterior part of the eye (1 - fibers of the zonular apparatus; 2 - ciliary processes; 3 - ciliary body; 4 - lens; 5 - iris; 6 - sclera; 7 - Schlemm's canal; 8 - anterior chamber angle)

Due to the relaxation of the ligament of zon, during the contraction of the ciliary muscle, the lens is deformed (an increase in the curvature of the anterior and, to a lesser extent, the posterior surfaces). In this case, its main function is performed - a change in refraction, which makes it possible to obtain a clear image on the retina, regardless of the distance to the object. At rest, without accommodation, the lens gives 19.11 of the 58.64 diopters of the refractive power of the schematic eye. To fulfill its primary role, the lens must be transparent and elastic, which it is.

The human lens grows continuously throughout life, thickening by about 29 microns per year. Starting from the 6-7th week of intrauterine life (18 mm embryo), it increases in the anterior-posterior size as a result of the growth of primary lens fibers. At the stage of development, when the embryo reaches a size of 18-24 mm, the lens has an approximately spherical shape. With the appearance of secondary fibers (embryo size 26 mm), the lens flattens and its diameter increases. Zonular apparatus, which appears when the length of the embryo is 65 mm, does not affect the increase in the diameter of the lens. Subsequently, the lens rapidly increases in mass and volume. At birth, it has an almost spherical shape.

In the first two decades of life, the increase in the thickness of the lens stops, but its diameter continues to increase. The factor contributing to the increase in diameter is core compaction. Tension of the ligament of Zinn contributes to a change in the shape of the lens.

The diameter of the lens (measured at the equator) of an adult is 9-10 mm. Its thickness at the time of birth in the center is approximately 3.5-4.0 mm, 4 mm at 40 years old, and then slowly increases to 4.75-5.0 mm by old age. The thickness also changes in connection with a change in the accommodative ability of the eye.

In contrast to the thickness, the equatorial diameter of the lens changes to a lesser extent with age. At birth, it is 6.5 mm, in the second decade of life - 9-10 mm. Subsequently, it practically does not change (Table 3.4.1).

Table 3.4.1. Lens dimensions (according to Rohen, 1977)

The anterior surface of the lens is less convex than the posterior (Fig. 3.4.1). It is a part of a sphere with a radius of curvature equal to an average of 10 mm (8.0-14.0 mm). The anterior surface is bordered by the anterior chamber of the eye through the pupil, and along the periphery by the posterior surface of the iris. The pupillary edge of the iris rests on the anterior surface of the lens. The lateral surface of the lens faces the posterior chamber of the eye and is attached to the processes of the ciliary body by means of the ligament of cinnamon.

The center of the anterior surface of the lens is called anterior pole. It is located approximately 3 mm behind the posterior surface of the cornea.

The posterior surface of the lens has a greater curvature (the radius of curvature is 6 mm (4.5-7.5 mm)). It is usually considered in combination with the vitreous membrane of the anterior surface of the vitreous body. However, between these structures there is slit-like space made by liquid. This space behind the lens was described by Berger in 1882. It can be observed using a slit lamp.

Lens equator lies within the ciliary processes at a distance of 0.5 mm from them. The equatorial surface is uneven. It has numerous folds, the formation of which is due to the fact that a zinn ligament is attached to this area. The folds disappear with accommodation, i.e., when the tension of the ligament stops.

Refractive index of the lens is equal to 1.39, i.e., somewhat larger than the refractive index of chamber moisture (1.33). It is for this reason that, despite the smaller radius of curvature, the optical power of the lens is less than that of the cornea. The contribution of the lens to the refractive system of the eye is approximately 15 out of 40 diopters.

At birth, the accommodative force, equal to 15-16 diopters, decreases by half by the age of 25, and at the age of 50 it is only 2 diopters.

Biomicroscopic examination of the lens with a dilated pupil reveals features of its structural organization (Fig. 3.4.3).

Rice. 3.4.3. The layered structure of the lens during its biomicroscopic examination in individuals of different ages (according to Bron et al., 1998): a - age 20 years; b - age 50 years; b - age 80 years (1 - capsule; 2 - first cortical light zone (C1 alpha); 3 - first zone of separation (C1 beta); 4 - second cortical light zone (C2): 5 - light scattering zone of the deep cortex (C3 ); 6 - light zone of the deep cortex; 7 - lens nucleus. There is an increase in the lens and increased light scattering

First, the multi-layered lens is revealed. The following layers are distinguished, counting from front to center:

• capsule;
• subcapsular light zone (cortical zone C 1a);
• light narrow zone of inhomogeneous scattering (C1);
• translucent zone of the cortex (C2).

These zones make up the superficial cortex of the lens. There are two more deeply located zones of the cortex. They are also called pernuclear. These zones fluoresce when the lens is illuminated with blue light (C3 and C4).

lens nucleus considered as its prenatal part. It also has layering. In the center is a light zone, called the "embryonic" (embryonic) nucleus. When examining the lens with a slit lamp, the sutures of the lens can also be found. Specular microscopy at high magnification allows you to see epithelial cells and lens fibers.

The following structural elements of the lens are determined (Fig. 3.4.4-3.4.6):

Rice. 3.4.4. Scheme of the microscopic structure of the lens: 1 - lens capsule; 2 - epithelium of the lens of the central sections; 3- lens epithelium of the transition zone; 4- epithelium of the lens of the equatorial region; 5 - embryonic nucleus; 6-fetal nucleus; 7 - the core of an adult; 8 - bark

Rice. 3.4.5. Features of the structure of the equatorial region of the lens (according to Hogan et al., 1971): 1 - lens capsule; 2 - equatorial epithelial cells; 3- lens fibers. As the proliferation of epithelial cells located in the region of the lens equator, they shift to the center, turning into lens fibers

Rice. 3.4.6. Features of the ultrastructure of the lens capsule of the equatorial region, the ligament of zon and the vitreous body: 1 - vitreous body fibers; 2 - fibers of the zinn ligament; 3-precapsular fibers: 4-capsule lens

1. Capsule.
2. Epithelium.
3. fibers.

lens capsule(capsula lentis). The lens is covered on all sides by a capsule, which is nothing more than a basement membrane of epithelial cells. The lens capsule is the thickest basement membrane of the human body. The capsule is thicker in front (15.5 µm in front and 2.8 µm behind) (Fig. 3.4.7).

Rice. 3.4.7. The thickness of the lens capsule in different areas

The thickening along the periphery of the anterior capsule is more pronounced, since the main mass of the zonium ligament is attached in this place. With age, the thickness of the capsule increases, which is more pronounced in front. This is due to the fact that the epithelium, which is the source of the basement membrane, is located in front and participates in the remodulation of the capsule, which is noted as the lens grows.

The ability of epithelial cells to form capsules persists throughout life and manifests itself even under conditions of cultivation of epithelial cells.

The dynamics of changes in the thickness of the capsule is given in table. 3.4.2.

Table 3.4.2. Dynamics of changes in the thickness of the lens capsule with age, µm (according to Hogan, Alvarado, Wedell, 1971)

This information may be needed by surgeons performing cataract extraction and using a capsule for attaching posterior chamber intraocular lenses.

The capsule is pretty powerful barrier to bacteria and inflammatory cells, but freely passable for molecules whose size is commensurate with the size of hemoglobin. Although the capsule does not contain elastic fibers, it is extremely elastic and is almost constantly under the influence of external forces, i.e., in a stretched state. For this reason, the dissection or rupture of the capsule is accompanied by twisting. The property of elasticity is used when performing extracapsular cataract extraction. Due to the contraction of the capsule, the contents of the lens are removed. The same property is also used in laser capsulotomy.

In a light microscope, the capsule looks transparent, homogeneous (Fig. 3.4.8).

Rice. 3.4.8. Light-optical structure of the lens capsule, the epithelium of the lens capsule and the lens fibers of the outer layers: 1 - lens capsule; 2 - epithelial layer of the lens capsule; 3 - lens fibers

In polarized light, its lamellar fibrous structure is revealed. In this case, the fiber is located parallel to the surface of the lens. The capsule also stains positively during the PAS reaction, which indicates the presence of a large amount of proteoglycans in its composition.

The ultrastructural capsule has relatively amorphous structure(Fig. 3.4.6, 3.4.9).

Rice. 3.4.9. Ultrastructure of the ligament of zon, lens capsule, epithelium of the lens capsule and lens fibers of the outer layers: 1 - zinn ligament; 2 - lens capsule; 3- epithelial layer of the lens capsule; 4 - lens fibers

Insignificant lamellarity is outlined due to the scattering of electrons by filamentary elements that fold into plates.

Approximately 40 plates are identified, each of which is approximately 40 nm thick. At a higher magnification of the microscope, delicate collagen fibrils with a diameter of 2.5 nm are revealed.

In the postnatal period, there is some thickening of the posterior capsule, which indicates the possibility of secretion of basal material by the posterior cortical fibers.

Fisher found that 90% of the loss of elasticity of the lens occurs as a result of a change in the elasticity of the capsule.

In the equatorial zone of the anterior lens capsule with age, electron-dense inclusions, consisting of collagen fibers with a diameter of 15 nm and with a period of transverse striation equal to 50-60 nm. It is assumed that they are formed as a result of the synthetic activity of epithelial cells. With age, collagen fibers also appear, the striation frequency of which is 110 nm.

The sites of attachment of the ligament of zon to the capsule are named. Berger plates(Berger, 1882) (another name is the pericapsular membrane). This is a superficially located layer of the capsule, having a thickness of 0.6 to 0.9 microns. It is less dense and contains more glycosaminoglycans than the rest of the capsule. The fibers of this fibrogranular layer of the pericapsular membrane are only 1–3 nm thick, while the thickness of the fibrils of the zinn ligament is 10 nm.

found in the pericapsular membrane fibronectin, vitreonectin and other matrix proteins that play a role in the attachment of ligaments to the capsule. Recently, the presence of another microfibrillary material, namely fibrillin, has been established, the role of which is indicated above.

Like other basement membranes, the lens capsule is rich in type IV collagen. It also contains collagen types I, III and V. Many other extracellular matrix components are also found - laminin, fibronectin, heparan sulfate and entactin.

Permeability of the lens capsule human has been studied by many researchers. The capsule freely passes water, ions and other small molecules. It is a barrier to the path of protein molecules having the size of hemoglobin. Differences in the capacity of the capsule in the norm and in cataracts were not found by anyone.

lens epithelium(epithelium lentis) consists of a single layer of cells lying under the anterior lens capsule and extending to the equator (Fig. 3.4.4, 3.4.5, 3.4.8, 3.4.9). Cells are cuboidal in transverse sections, and polygonal in planar preparations. Their number ranges from 350,000 to 1,000,000. The density of epitheliocytes in the central zone is 5009 cells per mm2 in men and 5781 in women. Cell density slightly increases along the periphery of the lens.

It should be emphasized that in the tissues of the lens, in particular in the epithelium, anaerobic respiration. Aerobic oxidation (Krebs cycle) is observed only in epithelial cells and outer lens fibers, while this oxidation pathway provides up to 20% of the lens energy requirement. This energy is used to provide active transport and synthetic processes necessary for the growth of the lens, the synthesis of membranes, crystallins, cytoskeletal proteins and nucleoproteins. The pentose phosphate shunt also functions, providing the lens with pentoses necessary for the synthesis of nucleoproteins.

Lens epithelium and superficial fibers of the lens cortex involved in the removal of sodium from the lens, thanks to the activity of the Na -K + -pump. It uses the energy of ATP. In the posterior part of the lens, sodium ions are passively distributed into the moisture of the posterior chamber. The lens epithelium consists of several subpopulations of cells that differ primarily in their proliferative activity. Certain topographic features of the distribution of epitheliocytes of various subpopulations are revealed. Depending on the features of the structure, function and proliferative activity of cells, several zones of the epithelial lining are distinguished.

Central zone. The central zone consists of a relatively constant number of cells, the number of which slowly decreases with age. Epithelial cells of a polygonal shape (Fig. 3.4.9, 3.4.10, a),

Rice. 3.4.10. Ultrastructural organization of the epithelial cells of the lens capsule of the intermediate zone (a) and the equatorial region (b) (according to Hogan et al, 1971): 1 - lens capsule; 2 - apical surface of an adjacent epithelial cell; 3-finger in pressure into the cytoplasm of the epithelial cell of adjacent cells; 4 - epithelial cell oriented parallel to the capsule; 5 - nucleated epithelial cell located in the cortex of the lens

their width is 11-17 microns, and their height is 5-8 microns. With their apical surface, they are adjacent to the most superficially located lens fibers. The nuclei are displaced towards the apical surface of large cells and have numerous nuclear pores. In them. usually two nucleoli.

Cytoplasm of epithelial cells contains a moderate amount of ribosomes, polysomes, smooth and rough endoplasmic reticulum, small mitochondria, lysosomes, and glycogen granules. The Golgi apparatus is expressed. Cylindrical microtubules with a diameter of 24 nm, microfilaments of an intermediate type (10 nm), alpha-actinin filaments are visible.

Using the methods of immunomorphology in the cytoplasm of epitheliocytes, the presence of the so-called matrix proteins- actin, vinmetin, spectrin and myosin, which provide rigidity to the cytoplasm of the cell.

Alpha-crystallin is also present in the epithelium. Beta and gamma crystallins are absent.

Epithelial cells are attached to the lens capsule by hemidesmosome. Desmosomes and gap junctions are visible between epithelial cells, having a typical structure. The system of intercellular contacts provides not only adhesion between the epithelial cells of the lens, but also determines the ionic and metabolic connection between cells.

Despite the presence of numerous intercellular contacts between epithelial cells, there are spaces filled with structureless material of low electron density. The width of these spaces ranges from 2 to 20 nm. It is thanks to these spaces that the exchange of metabolites between the lens and intraocular fluid is carried out.

Epithelial cells of the central zone differ exclusively low mitotic activity. The mitotic index is only 0.0004% and approaches the mitotic index of epithelial cells of the equatorial zone in age-related cataract. Significantly, mitotic activity increases under various pathological conditions and, first of all, after injury. The number of mitoses increases after exposure of epithelial cells to a number of hormones in experimental uveitis.

Intermediate zone. The intermediate zone is closer to the periphery of the lens. The cells of this zone are cylindrical with a centrally located nucleus. The basement membrane has a folded appearance.

germinal zone. The germinal zone is adjacent to the preequatorial zone. It is this zone that is characterized by high cell proliferative activity (66 mitoses per 100,000 cells), which gradually decreases with age. The duration of mitosis in different animals ranges from 30 minutes to 1 hour. At the same time, diurnal fluctuations in mitotic activity were revealed.

The cells of this zone after division are displaced posteriorly and subsequently turn into lens fibers. Some of them are also displaced anteriorly, into the intermediate zone.

The cytoplasm of epithelial cells contains small organelles. There are short profiles of the rough endoplasmic reticulum, ribosomes, small mitochondria and the Golgi apparatus (Fig. 3.4.10, b). The number of organelles increases in the equatorial region as the number of structural elements of the cytoskeleton of actin, vimentin, microtubule protein, spectrin, alpha-actinin, and myosin increases. It is possible to distinguish whole actin mesh-like structures, especially visible in the apical and basal parts of the cells. In addition to actin, vimentin and tubulin were found in the cytoplasm of epithelial cells. It is assumed that the contractile microfilaments of the cytoplasm of epithelial cells contribute by their contraction to the movement of the intercellular fluid.

In recent years, it has been shown that the proliferative activity of epithelial cells of the germinal zone is regulated by numerous biologically active substances - cytokines. The significance of interleukin-1, fibroblast growth factor, transforming growth factor beta, epidermal growth factor, insulin-like growth factor, hepatocyte growth factor, keratinocyte growth factor, postaglandin E2 was revealed. Some of these growth factors stimulate proliferative activity, while others inhibit it. It should be noted that the listed growth factors are synthesized either by the structures of the eyeball, or by other tissues of the body, entering the eye through the blood.

The process of formation of lens fibers. After the final division of the cell, one or both daughter cells are displaced into the adjacent transitional zone, in which the cells are organized in meridianally oriented rows (Fig. 3.4.4, 3.4.5, 3.4.11).

Rice. 3.4.11. Features of the location of the lens fibers: a - schematic representation; b - scanning electron microscopy (according to Kuszak, 1989)

Subsequently, these cells differentiate into secondary fibers of the lens, turning 180° and elongating. The new lens fibers maintain polarity in such a way that the posterior (basal) portion of the fiber maintains contact with the capsule (basal lamina), while the anterior (apical) portion is separated from this by the epithelium. As epitheliocytes turn into lens fibers, a nuclear arc is formed (under microscopic examination, a number of nuclei of epithelial cells arranged in the form of an arc).

The premitotic state of epithelial cells is preceded by DNA synthesis, while cell differentiation into lens fibers is accompanied by an increase in RNA synthesis, since this stage is marked by the synthesis of structural and membrane specific proteins. The nucleoli of differentiating cells increase sharply, and the cytoplasm becomes more basophilic due to an increase in the number of ribosomes, which is explained by increased synthesis of membrane components, cytoskeletal proteins, and lens crystallins. These structural changes reflect increased protein synthesis.

During the formation of the lens fiber, numerous microtubules 5 nm in diameter and intermediate fibrils appear in the cytoplasm of cells, oriented along the cell and playing an important role in the morphogenesis of lens fibers.

Cells of varying degrees of differentiation in the region of the nuclear arc are arranged as if in a checkerboard pattern. Due to this, channels are formed between them, providing a strict orientation in space of newly differentiating cells. It is into these channels that the cytoplasmic processes penetrate. In this case, meridional rows of lens fibers are formed.

It is important to emphasize that the violation of the meridional orientation of the fibers is one of the causes of cataract development both in experimental animals and in humans.

The transformation of epitheliocytes into lens fibers occurs quite quickly. This has been shown in an animal experiment using isotopically labeled thymidine. In rats, the epitheliocyte turns into a lens fiber after 5 weeks.

In the process of differentiation and displacement of cells to the center of the lens in the cytoplasm of the lens fibers the number of organelles and inclusions decreases. The cytoplasm becomes homogeneous. The nuclei undergo pycnosis and then completely disappear. Soon the organelles disappear. Basnett found that the loss of nuclei and mitochondria occurs suddenly and in one generation of cells.

The number of lens fibers throughout life is constantly increasing. "Old" fibers are shifted to the center. As a result, a dense core is formed.

With age, the intensity of the formation of lens fibers decreases. So, in young rats, approximately five new fibers are formed per day, while in old rats - one.

Features of epithelial cell membranes. Cytoplasmic membranes of neighboring epithelial cells form a kind of complex of intercellular connections. If the lateral surfaces of the cells are slightly wavy, then the apical zones of the membranes form "finger impressions", plunging into the proper lens fibers. The basal part of the cells is attached to the anterior capsule by hemidesmosomes, and the lateral surfaces of the cells are connected by desmosomes.

On the lateral surfaces of the membranes of adjacent cells, slot contacts through which small molecules can be exchanged between lens fibers. In the region of gap junctions, kennesins of various molecular weights are found. Some researchers suggest that gap junctions between lens fibers differ from those in other organs and tissues.

It is exceptionally rare to see tight contacts.

The structural organization of lens fiber membranes and the nature of intercellular contacts indicate the possible presence on the surface receptor cells that control the processes of endocytosis, which is of great importance in the movement of metabolites between these cells. The existence of receptors for insulin, growth hormone and beta-adrenergic antagonists is assumed. On the apical surface of epithelial cells, orthogonal particles embedded in the membrane and having a diameter of 6-7 nm were revealed. It is assumed that these formations provide the movement of nutrients and metabolites between cells.

lens fibers(fibrcie lentis) (Fig. 3.4.5, 3.4.10-3.4.12).

Rice. 3.4.12. The nature of the arrangement of the lens fibers. Scanning electron microscopy (according to Kuszak, 1989): a-densely packed lens fibers; b - "finger impressions"

The transition from the epithelial cells of the germinal zone to the lens fiber is accompanied by the disappearance of "finger impressions" between the cells, as well as the beginning of elongation of the basal and apical parts of the cell. The gradual accumulation of lens fibers and their displacement to the center of the lens is accompanied by the formation of the lens nucleus. This displacement of cells leads to the formation of an S- or C-like arc (nuclear puff), directed forward and consisting of a "chain" of cell nuclei. In the equatorial region, the zone of nuclear cells has a width of about 300-500 microns.

The deeper fibers of the lens have a thickness of 150 microns. When they lose nuclei, the nuclear arc disappears. The lens fibers are fusiform or belt-like, located along the arc in the form of concentric layers. On a transverse section in the equatorial region, they are hexagonal in shape. As they sink towards the center of the lens, their uniformity in size and shape is gradually broken. In the equatorial region in adults, the width of the lens fiber ranges from 10 to 12 microns, and the thickness is from 1.5 to 2.0 microns. In the posterior parts of the lens, the fibers are thinner, which is explained by the asymmetric shape of the lens and the greater thickness of the anterior cortex. The length of the lens fibers, depending on the depth of location, ranges from 7 to 12 mm. And this despite the fact that the initial height of the epithelial cell is only 10 microns.

The ends of the lens fibers meet at a specific location and form sutures.

Seams of the lens(Fig. 3.4.13).

Rice. 3.4.13. The formation of seams at the junction of the fibers, which occurs at different periods of life: 1 - Y-shaped seam, formed in the embryonic period; 2 - a more developed suture system that occurs in childhood; 3 is the most developed suture system found in adults

The fetal nucleus has an anterior vertical Y-shaped and a posterior inverted Y-shaped suture. After birth, as the lens grows and the number of layers of lens fibers that form their sutures increases, the sutures spatially coalesce to form the star-like structure found in adults.

The main significance of the sutures lies in the fact that, thanks to such a complex system of contact between cells the shape of the lens is preserved almost throughout life.

Features of lens fiber membranes. Button-loop contacts (Fig. 3.4.12). The membranes of adjacent lens fibers are connected by a variety of specialized formations that change their structure as the fiber moves from the surface into the depths of the lens. In the superficial 8-10 layers of the anterior cortex, the fibers are connected using formations of the "button-loop" type ("ball and socket" by American authors), distributed evenly along the entire length of the fiber. Contacts of this type exist only between cells of the same layer, i.e., cells of the same generation, and are absent between cells of different generations. This allows the fibers to move relative to each other during their growth.

Between the more deeply located fibers, the button-loop contact is found somewhat less frequently. They are distributed in the fibers unevenly and randomly. They also appear between cells of different generations.

In the deepest layers of the cortex and nucleus, in addition to the indicated contacts (“button-loop”), complex interdigitations appear in the form of ridges, depressions and furrows. Desmosomes have also been found, but only between differentiating rather than mature lens fibers.

It is assumed that contacts between lens fibers are necessary to maintain the rigidity of the structure throughout life, contributing to the preservation of the transparency of the lens. Another type of intercellular contacts has been found in the human lens. This gap contact. Gap junctions serve two roles. First, since they connect the lens fibers over a long distance, the architectonics of the tissue is preserved, thereby ensuring the transparency of the lens. Secondly, it is due to the presence of these contacts that the distribution of nutrients between the lens fibers occurs. This is especially important for the normal functioning of structures against the background of reduced metabolic activity of cells (insufficient number of organelles).

Revealed two types of gap contacts- crystalline (with high ohmic resistance) and non-crystalline (with low ohmic resistance). In some tissues (the liver), these types of gap junctions can be transformed into one another when the ionic composition of the environment changes. In the lens fiber, they are incapable of such a transformation. The first type of gap junctions was found in the places where the fibers adjoin epithelial cells, and the second - only between the fibers.

Low-resistance gap contacts contain intramembrane particles that do not allow neighboring membranes to approach each other by more than 2 nm. Due to this, in the deep layers of the lens, ions and molecules of small size propagate quite easily between the lens fibers, and their concentration levels out fairly quickly. There are also species differences in the number of gap junctions. So, in the human lens, they occupy the surface of the fiber by area of ​​5%, in a frog - 15%, in a rat - 30%, and in a chicken - 60%. There are no gap contacts in the seam area.

It is necessary to dwell briefly on the factors that ensure transparency and high refractive power of the lens. The high refractive power of the lens is achieved high concentration of protein filaments, and transparency - their strict spatial organization, the uniformity of the fiber structure within each generation and a small amount of intercellular space (less than 1% of the lens volume). Contributes to transparency and a small amount of intracytoplasmic organelles, as well as the absence of nuclei in the lens fibers. All of these factors minimize the scattering of light between the fibers.

There are other factors that affect refractive power. One of them is increase in protein concentration as it approaches the nucleus of the lens. It is due to the increase in protein concentration that there is no chromatic aberration.

No less important in the structural integrity and transparency of the lens is reflation of the ionic content and degree of hydration of the lens fibers. At birth, the lens is transparent. As the lens grows, the nucleus becomes yellow. The appearance of yellowness is probably associated with the influence of ultraviolet light on it (wavelength 315-400 nm). At the same time, fluorescent pigments appear in the cortex. It is believed that these pigments shield the retina from the damaging effects of short-wavelength light radiation. Pigments accumulate in the nucleus with age, and in some people are involved in the formation of pigment cataracts. In the nucleus of the lens in old age and especially in nuclear cataracts, the amount of insoluble proteins increases, which are crystallins, the molecules of which are “crosslinked”.

Metabolic activity in the central regions of the lens is negligible. Virtually no protein metabolism. That is why they belong to long-lived proteins and are easily damaged by oxidizing agents, leading to a change in the conformation of the protein molecule due to the formation of sulfhydryl groups between protein molecules. The development of cataracts is characterized by an increase in light scattering zones. This can be caused by a violation of the regularity of the arrangement of the lens fibers, a change in the structure of the membranes and an increase in the scattering of light, due to a change in the secondary and tertiary structure of protein molecules. Edema of lens fibers and their destruction leads to disruption of water-salt metabolism.

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The lens of the human eye is of great importance in the visual process. With its help, accommodation occurs (the difference between objects at a distance), the process of refraction of light rays, protection from external negative factors and the transmission of an image from the external environment. Over time or from injury, the lens begins to darken. A cataract appears, which cannot be cured with medication. Therefore, surgical intervention is used to stop the development of the disease. This method allows you to completely recover from the disease.

Structure and anatomy

The lens is a convex lens that provides the visual process in the human eye apparatus. Its back part has a deflection, and in front the organ is almost flat. The refractive power of the lens is normally 20 diopters. But the optical power can vary. On the surface of the lens are small nodules that connect to muscle fibers. Depending on the tension or relaxation of the ligaments, the lens takes a certain shape. Such changes allow you to see objects at different distances.

The structure of the lens of the human eye includes the following parts:

• core;
• shell or capsular bag;
• equatorial part;
• lens masses;
• capsule;
• fibers: central, transitional, main.

Due to the growth of epithelial cells, the thickness of the lens increases, which leads to a decrease in the quality of vision.

Located in the back chamber. Its thickness is approximately 5 millimeters and its size is 9 mm. The lens diameter is 5 mm. With age, the core loses its elasticity and becomes more rigid. The lens cells increase in number over the years and this is due to the growth of the epithelium. This makes the lens thicker and the quality of vision lower. The organ has no nerve endings, blood vessels or lymph nodes. Near the nucleus is the ciliary body. It produces fluid, which is then supplied to the front of the eyeball. And also the body is a continuation of the veins in the eye. The visual lens consists of such components, which are shown in the table:

Lens functions

The role of this body in the process of vision is one of the main ones. For normal operation, it must be transparent. The pupil and lens allow light to pass into the human eye. It refracts the rays, after which they fall on the retina. Its main task is to transmit an image from the outside to the macular area. After entering this area, the light forms an image on the retina, it travels in the form of a nerve impulse to the brain, which interprets it. The images that fall on the lens are inverted. Already in the brain they turn over.

Accommodation works reflexively, which allows you to see objects at different distances without any effort.

The functions of the lens are involved in the process of accommodation. This is the ability of a person to perceive objects at different distances. Depending on the location of the object, the anatomy of the lens changes, which allows you to see the image clearly. If the ligaments are stretched, the lens takes on a convex shape. The curvature of the lens makes it possible to see an object up close. During relaxation, the eye sees objects in the distance. Such changes are regulated by the eye muscle, which is controlled by nerves. That is, accommodation works reflexively without additional human effort. In this case, the radius of curvature at rest is 10 mm, and in tension - 6 mm.

This body performs protective functions. The lens is a kind of shell from microorganisms and bacteria from the external environment.

In addition, it separates the two sections of the eye and is responsible for the integrity of the eye mechanism: so the vitreous will not put too much pressure on the anterior segments of the visual apparatus. According to the study, if the lens ceases to function, then it simply disappears, and the body moves forward. Because of this, the functions of the pupil and the anterior chamber suffer. There is a risk of developing glaucoma.

Organ diseases

The occurrence of cataracts is associated with a violation of metabolic processes in the organs of vision, due to which the lens becomes cloudy.

Due to cranial or ocular injuries, with age, the lens may become more cloudy, the nucleus changes its thickness. If the lens filaments break in the eye, and as a result, the lens is displaced. This leads to a deterioration in visual acuity. One of the most common diseases is cataract. This is lens fogging. The disease occurs after injury or appears at birth. There is age-related cataract, when the lens epithelium becomes thicker and cloudy. If the cortical layer of the lens becomes completely white, then they speak of the mature stage of a cataract. Depending on the place of occurrence of the pathology, the following types are distinguished:

• nuclear;
• layered;
• front;
• back.

Such violations lead to the fact that vision falls below normal. A person begins to distinguish objects at different distances worse. Older people complain of a decrease in contrast and a decrease in color perception. Clouding develops over several years, so people do not immediately notice changes. Against the background of the disease, inflammation occurs - iridocyclitis. According to the study, it has been proven that opacities develop faster if the patient has glaucoma.

Date: 01/09/2016

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• Features of the structure and function
• Diseases associated with the lens
  • Cataract treatment

The lens of the eye is a biconvex lens, which consists of long fibers located in an elastic capsule.

The eye lens is behind the iris and is fixed to the ciliary body with thin threads called Zinn's ligaments. It consists of a transparent mass of protein substances, which are surrounded by an outer shell - a bag. With age, the lens often becomes cloudy, which causes a cataract of the lens. Then the lack of its transparency prevents visual acuity. In this case, surgical treatment is used: the clouded lens is removed and an intraocular soft lens (IOL) is put in its place.

Features of the structure and function

The lens consists of:

The eye functions thanks to a biconvex lens. Its main functions include:

1. The ability to transmit light to the retina. The transmission of light depends on the transparency of the lens.
2. The ability to refract light.
3. Accommodative action of the lens together with the ciliary body.

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Diseases associated with the lens

You can list a number of diseases associated with a change in its structure:

• cataract;
• dislocation of the lens;
• glaucoma;
• inflammation of the vessels of the eye;

Cataract is a common eye disease. It occurs due to complete or partial clouding of the lens responsible for the visual function of the body. A cloudy lens accompanies progressive visual impairment and can lead to complete blindness.

Cataract is classified into:

1. , occurs in childhood, adolescence - occurs due to metabolic disorders of the body.
2. Acquired cataract. Primary cataract associated with age (senile cataract), secondary cataract - repeated, occurs, for example, due to the remnants of the lens masses after implantation of the IOL.

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Cataract treatment

The best known way to treat the eye lens is to replace your own with an artificial lens (IOL). This operation is used for cataracts caused by aging of the body or other disorders in the metabolism of the lens apparatus.

Causes of cataracts:

Currently, the most commonly used cataract surgery is phacoemulsification. This method is the crushing of the lens using ultrasonic waves with further washing out of the lens masses with special irrigation-aspiration cannulas. After washing out the crushed lens, the surgeon implants a soft intraocular lens (IOL) in its place. The diopter power of the IOL is selected individually for each patient. It is calculated based on the diagnostic results: , eye length - also taking into account the type of implanted IOL.

This operation is seamless, i.e. only 2-3 mm incisions are made to allow penetration of the cannulas and needle of the ultrasonic handpiece. After the operation, the patient undergoes a rehabilitation period of a week. Special drops are dripped into his eyes to prevent inflammatory processes in the body.

A week after surgery, and in some people even on the second day after surgery, there is a high visual acuity.

The pharmaceutical market offers a large number of drugs that prevent the progression of human lens opacity at the initial stages of development, but their effective action has not yet been proven. Therefore, the most commonly used method of treatment for its turbidity is surgery.