A huge beach of bare pebbles - Looking at everything without veils - And as keen as the lens of an eye, the unglazed horizon.
B. Pasternak
12.1. Structure of the lens
The lens is part of the light-conducting and light-refracting system of the eye. This is a transparent, biconvex biological lens that provides dynamic optics of the eye thanks to the mechanism of accommodation.
In the process of embryonic development, the lens is formed in the 3rd-4th week of the embryo’s life from the
toderma covering the wall of the optic cup. The ectoderm is drawn into the cavity of the optic cup, and the rudiment of the lens in the form of a vesicle is formed from it. Lens fibers are formed from elongating epithelial cells inside the vesicle.
The lens has the shape of a biconvex lens. The anterior and posterior spherical surfaces of the lens have different radii of curvature (Fig. 12.1). Front surface
Rice. 12.1. The structure of the lens and the location of the ligament of cinnamon that supports it.
ness is flatter. Its radius of curvature (R = 10 mm) is greater than the radius of curvature of the rear surface (R = 6 mm). The centers of the anterior and posterior surfaces of the lens are called the anterior and posterior poles, respectively, and the line connecting them is the axis of the lens, the length of which is 3.5-4.5 mm. The line of transition of the front surface to the back is the equator. The diameter of the lens is 9-10 mm.
The lens is covered with a thin structureless transparent capsule. The part of the capsule lining the anterior surface of the lens is called the “anterior capsule” (“anterior bag”) of the lens. Its thickness is 11-18 microns. The inside of the anterior capsule is covered with single-layer epithelium, while the posterior capsule does not have it; it is almost 2 times thinner than the anterior one. The epithelium of the anterior capsule plays important role in the metabolism of the lens, is characterized by high activity of oxidative enzymes compared to the central part of the lens. Epithelial cells are actively multiplying. At the equator they lengthen, forming the growth zone of the lens. The elongated cells turn into lens fibers. Young ribbon-like cells push old fibers towards the center. This process continues continuously throughout life. Centrally located fibers lose their nuclei, become dehydrated and contract. Densely layered on top of each other, they form the nucleus of the lens (nucleus lentis). The size and density of the kernel increase over the years. This does not affect the degree of transparency of the lens, however, due to a decrease in overall elasticity, the volume of accommodation gradually decreases (see section 5.5). By the age of 40-45 years of life there is already a fairly dense core. This mechanism of lens growth ensures the stability of its external dimensions. The closed capsule of the lens does not allow dead cells to desquamate
come out. Like all epithelial formations, the lens grows throughout life, but its size practically does not increase.
Young fibers, constantly formed at the periphery of the lens, form an elastic substance around the core - the lens cortex (cortex lentis). The bark fibers are surrounded by a specific substance that has the same refractive index of light. It ensures their mobility during contraction and relaxation, when the lens changes shape and optical power during the process of accommodation.
The lens has a layered structure - it resembles an onion. All fibers extending from the growth zone along the circumference of the equator converge in the center and form a three-pointed star, which is visible during biomicroscopy, especially when opacities appear.
From the description of the structure of the lens, it is clear that it is an epithelial formation: it has neither nerves, nor blood or lymphatic vessels.
The vitreous artery (a. hyaloidea), which in the early embryonic period participates in the formation of the lens, is subsequently reduced. By the 7-8th month, the choroid plexus around the lens resolves.
The lens is surrounded on all sides by intraocular fluid. Nutrients pass through the capsule by diffusion and active transport. The energy needs of avascular epithelial formation are 10-20 times lower than the needs of other organs and tissues. They are satisfied through anaerobic glycolysis.
Compared to other structures of the eye, the lens contains the largest amount of proteins (35-40%). These are soluble α- and β-crystallins and insoluble albuminoid. Lens proteins are organ-specific. When immunized
an anaphylactic reaction may occur to this protein. The lens contains carbohydrates and their derivatives, reducing agents glutathione, cysteine, ascorbic acid, etc. Unlike other tissues, the lens contains little water (up to 60-65%), and its amount decreases with age. The content of protein, water, vitamins and electrolytes in the lens differs significantly from the proportions found in the intraocular fluid, vitreous body and blood plasma. The lens floats in water, but despite this, it is a dehydrated formation, which is explained by the peculiarities of water-electrolyte transport. The lens contains a high level of potassium ions and a low level of sodium ions: the concentration of potassium ions is 25 times higher than in the aqueous humor of the eye and the vitreous humor, and the concentration of amino acids is 20 times higher.
The lens capsule has the property of selective permeability, so the chemical composition of the transparent lens is maintained at a certain level. Changes in the composition of the intraocular fluid affect the transparency of the lens.
In an adult, the lens has a slight yellowish tint, the intensity of which may increase with age. This does not affect visual acuity, but may affect the perception of blue and violet colors.
The lens is located in the eye cavity in the frontal plane between the iris and the vitreous body, dividing the eyeball into anterior and posterior sections. In front, the lens serves as a support for the pupillary part of the iris. Its posterior surface is located in the recess of the vitreous body, from which the lens is separated by a narrow capillary slit, which widens when exudate accumulates in it.
The lens maintains its position in the eye with the help of fibers of the circular suspensory ligament of the ciliary body (ligament of Zinn). Thin (20-22 microns thick) arachnoid filaments extend in radial bundles from the epithelium of the ciliary processes, partially intersect and are woven into the lens capsule on the anterior and posterior surfaces, providing an impact on the lens capsule during the operation of the muscular apparatus of the ciliary (ciliary) body.
12.2. Functions of the lens
The lens performs a number of very important functions in the eye. First of all, it is the medium through which light rays pass unhindered to the retina. This light transmission function. It is provided by the main property of the lens - its transparency.
The main function of the lens is light refraction. In terms of the degree of refraction of light rays, it ranks second after the cornea. The optical power of this living biological lens is within 19.0 diopters.
Interacting with the ciliary body, the lens provides the function of accommodation. It is capable of smoothly changing optical power. The self-adjusting image focusing mechanism (see section 5.5) is possible due to the elasticity of the lens. This ensures dynamic refraction.
The lens divides the eyeball into two unequal sections - a smaller anterior one and a larger posterior one. Is this a partition or separation barrier between them. The barrier protects the delicate structures of the anterior part of the eye from the pressure of the large mass of the vitreous body. When the eye loses its lens, the vitreous body moves anteriorly. The anatomical relationships change, and after them the functions. Difficulty
conditions for the hydrodynamics of the eye are created due to narrowing (compression) of the angle of the anterior chamber of the eye and blockade of the pupil area. Conditions arise for the development of secondary glaucoma. When the lens is removed along with the capsule, changes also occur in the posterior part of the eye due to the vacuum effect. The vitreous body, which has received some freedom of movement, moves away from the posterior pole and hits the walls of the eye when the eyeball moves. This is the cause of severe retinal pathology, such as edema, detachment, hemorrhages, and ruptures.
The lens is a barrier to the penetration of microbes from the anterior chamber into the vitreous cavity - protective barrier.
12.3. Anomalies of lens development
Malformations of the lens can have different manifestations. Any changes in the shape, size and location of the lens cause pronounced disturbances in its function.
Congenital aphakia - absence of the lens - is rare and, as a rule, is combined with other malformations of the eye.
Microphakia - small lens. Usually this pathology is combined
occurs with a change in the shape of the lens - spherophakia (spherical lens) or a violation of the hydrodynamics of the eye. Clinically, this manifests itself as high myopia with incomplete vision correction. The small round lens, suspended on long, weak threads of the circular ligament, has significantly greater mobility than normal. It can be inserted into the lumen of the pupil and cause pupillary block with a sharp increase in intraocular pressure and pain. To free the lens, you need by medication dilate the pupil.
Microphakia in combination with lens subluxation is one of the manifestations Marfan syndrome, hereditary malformation of the entire connective tissue. Ectopia of the lens and changes in its shape are caused by hypoplasia of the ligaments that support it. With age, the separation of the ligament of cinnamon increases. At this point, the vitreous body protrudes in the form of a hernia. The equator of the lens becomes visible in the pupil area. Complete dislocation of the lens is also possible. In addition to ocular pathology, Marfan syndrome is characterized by damage to the musculoskeletal system and internal organs(Fig. 12.2).
Rice. 12.2. Marfan syndrome.
a - the equator of the lens is visible in the pupil area; b - hands with Marfan syndrome.
It is impossible not to pay attention to the peculiarities of the patient’s appearance: tall stature, disproportionately long limbs, thin, long fingers (arachnodactyly), poorly developed muscles and subcutaneous fatty tissue, curvature of the spine. Long and thin ribs form an unusually shaped chest. In addition, developmental defects are identified cardiovascular system, autonomic-vascular disorders, dysfunction of the adrenal cortex, disruption of the circadian rhythm of glucocorticoid excretion in the urine.
Microspherophakia with subluxation or complete dislocation of the lens is also observed with Marchesani syndrome- systemic hereditary damage to mesenchymal tissue. Patients with this syndrome, unlike patients with Marfan syndrome, have a completely different appearance: short stature, short arms with which it is difficult for them to grasp their own head, short and thick fingers (brachydactyly), hypertrophied muscles, asymmetrical compressed skull.
Coloboma of the lens- defect of lens tissue in the midline in the lower part. This pathology is observed extremely rarely and is usually combined with coloboma of the iris, ciliary body and choroid. Such defects are formed due to incomplete closure of the embryonic fissure during the formation of the secondary optic cup.
Lenticonus- cone-shaped protrusion of one of the surfaces of the lens. Another type of lens surface pathology is lentiglobus: the front or back surface of the lens has a spherical shape. Each of these developmental anomalies is usually observed in one eye and can be combined with opacities in the lens. Clinically, lenticonus and lentiglobus are manifested by increased
refraction of the eye, i.e. the development of high myopia and difficult-to-correct astigmatism.
For abnormalities in the development of the lens that are not accompanied by glaucoma or cataracts, no special treatment is required. In cases where, due to congenital pathology of the lens, a refractive error that cannot be corrected by glasses occurs, the altered lens is removed and replaced with an artificial one (see section 12.4).
12.4. Lens pathology
Features of the structure and functions of the lens, the absence of nerves, blood and lymphatic vessels determine the uniqueness of its pathology. There are no inflammatory or tumor processes in the lens. The main manifestations of lens pathology are a violation of its transparency and loss of its correct location in the eye.
12.4.1. Cataract
Any clouding of the lens is called a cataract.
Depending on the number and location of opacities in the lens, polar (anterior and posterior), fusiform, zonular (layered), nuclear, cortical and complete cataracts are distinguished (Fig. 12.3). The characteristic pattern of the location of opacities in the lens may be evidence of congenital or acquired cataracts.
12.4.1.1. Congenital cataract
Congenital lens opacities occur when exposed to toxic substances during its formation. Most often this viral diseases mothers during pregnancy, such as
Rice. 12.3. Localization of opacities in various types of cataracts.
influenza, measles, rubella, and toxoplasmosis. Endocrine disorders in women during pregnancy and insufficiency of the parathyroid glands, leading to hypocalcemia and impaired fetal development, are of great importance.
Congenital cataracts can be hereditary with a dominant type of transmission. In such cases, the disease is most often bilateral, often combined with malformations of the eye or other organs.
When examining the lens, you can identify certain signs that characterize congenital cataracts, most often polar or layered opacities, which have either smooth round outlines or a symmetrical pattern, sometimes it can be like a snowflake or a picture of the starry sky.
Small congenital opacities in the peripheral parts of the lens and on the posterior capsule can be
found in healthy eyes. These are traces of the attachment of vascular loops of the embryonic vitreous artery. Such opacities do not progress and do not interfere with vision.
Anterior polar cataract-
This is a clouding of the lens in the form of a round spot of white or gray color, which is located under the capsule at the anterior pole. It is formed as a result of disruption of the process of embryonic development of the epithelium (Fig. 12.4).
Posterior polar cataract in shape and color it is very similar to the anterior polar cataract, but is located at the posterior pole of the lens under the capsule. The area of opacity may be fused with the capsule. The posterior polar cataract is a remnant of the reduced embryonic vitreous artery.
One eye may have opacities at both the anterior and posterior poles. In this case they talk about anteroposterior polar cataract. Congenital polar cataracts are characterized by regular rounded outlines. The sizes of such cataracts are small (1-2 mm). Foreign
Rice. 12.4. Congenital anterior polar cataract with remnants of the embryonic pupil membrane.
where polar cataracts have a thin radiant rim. In transmitted light, polar cataracts are visible as black spot on a pink background.
Fusiform cataract occupies the very center of the lens. The opacification is located strictly along the anteroposterior axis in the form of a thin gray ribbon, shaped like a spindle. It consists of three links, three thickenings. This is a chain of interconnected pinpoint opacities under the anterior and posterior capsules of the lens, as well as in the region of its nucleus.
Polar and fusiform cataracts usually do not progress. From early childhood, patients adapt to look through the transparent areas of the lens and often have complete or fairly high vision. No treatment is required for this pathology.
Layered(zonular) cataracts are more common than other congenital cataracts. Opacities are located strictly in one or several layers around the nucleus of the lens. Transparent and cloudy layers alternate. Usually the first cloudy layer is located at the border of the embryonic and “adult” nuclei. This is clearly visible in the light section of biomicroscopy. In transmitted light, such a cataract is visible as a dark disk with smooth edges against the background of a pink reflex. With a wide pupil, in some cases local opacities are also detected in the form of short spokes, which are located in more surface layers relative to the cloudy disk and have a radial direction. They seem to be sitting astride the equator of a cloudy disk, which is why they are called “riders.” Only in 5% of cases are layered cataracts unilateral.
Bilateral damage to the lenses, clear boundaries of transparent and turbid layers around the nucleus, symmetrical arrangement of peripheral spoke-shaped opacities with
The relative orderliness of the pattern indicates a congenital pathology. Layered cataracts can also develop in the postnatal period in children with congenital or acquired insufficiency of the parathyroid glands. Children with symptoms of tetany are usually diagnosed with layered cataracts.
The degree of vision loss is determined by the density of opacities in the center of the lens. The decision on surgical treatment depends mainly on visual acuity.
Total Cataracts are rare and are always bilateral. The entire substance of the lens turns into a cloudy soft mass due to gross violation embryonic development of the lens. Such cataracts gradually resolve, leaving behind wrinkled, cloudy capsules fused to each other. Complete resorption of the lens substance can occur even before the baby is born. Total cataracts lead to significant vision loss. Such cataracts require surgical treatment in the first months of life, since blindness in both eyes at an early age is a threat to the development of deep, irreversible amblyopia - atrophy of the visual analyzer due to its inactivity.
12.4.1.2. Acquired cataract
Cataracts are the most commonly observed eye disease. This pathology occurs mainly in older people, although it can develop at any age due to various reasons. Clouding of the lens is a typical response of its avascular substance to the influence of any unfavorable factor, as well as to changes in the composition of the intraocular fluid surrounding the lens.
A microscopic examination of the cloudy lens reveals swelling and disintegration of the fibers, which lose connection with the capsule and contract; vacuoles and cracks filled with protein liquid are formed between them. Epithelial cells swell, lose their correct shape, and their ability to perceive dyes is impaired. The cell nuclei become denser and intensely stained. The lens capsule changes slightly, which allows the capsular bag to be preserved during surgery and used to fix the artificial lens.
Depending on the etiological factor, several types of cataracts are distinguished. To simplify the presentation of the material, we will divide them into two groups: age-related and complicated. Age-related cataracts can be considered as a manifestation of age-related involution processes. Complicated cataracts occur when exposed to unfavorable factors of the internal or external environment. Play a certain role in the development of cataracts immune factors(see chapter 24).
Age-related cataract. They used to call her senile. It is known that age-related changes V different organs and tissues do not occur in the same way for everyone. Age-related (senile) cataracts can be found not only in old people, but also in older people and even people of active middle age. It is usually bilateral, but opacities do not always appear simultaneously in both eyes.
Depending on the location of the opacities, cortical and nuclear cataracts are distinguished. Cortical cataracts are almost 10 times more common than nuclear cataracts. Let us first consider the development cortical form.
During the development process, any cataract goes through four stages of maturation: initial, immature, mature and overripe.
Early signs initial cortical Cataracts can be caused by vacuoles located subcapsularly and water gaps formed in the cortex of the lens. In the light section of a slit lamp they are visible as optical voids. When areas of turbidity appear, these cracks are filled with fiber decay products and merge with the general background of turbidity. Typically, the first foci of opacification occur in the peripheral areas of the lens cortex and patients do not notice developing cataracts until opacities occur in the center, causing decreased vision.
Changes gradually increase in both the anterior and posterior cortical layers. The transparent and cloudy parts of the lens refract light differently, and therefore patients may complain of diplopia or polyopia: instead of one object, they see 2-3 or more. Other complaints are also possible. In the initial stage of cataract development, in the presence of limited small opacities in the center of the lens cortex, patients are bothered by the appearance of flying flies that move in the direction where the patient is looking. The duration of the initial cataract can vary - from 1-2 to 10 years or more.
Stage immature cataract characterized by watering of the lens substance, progression of opacities, and a gradual decrease in visual acuity. The biomicroscopic picture is represented by lens opacities of varying intensity, interspersed with transparent areas. During a normal external examination, the pupil may still be black or slightly grayish due to the fact that the superficial subcapsular layers are still transparent. With side lighting, a semi-lunar “shadow” is formed from the iris on the side from which the light falls (Fig. 12.5, a).
Rice. 12.5.Cataract. a - immature; b - mature.
Swelling of the lens can lead to a serious complication - phacogenic glaucoma, which is also called phacomorphic. Due to the increase in the volume of the lens, the angle of the anterior chamber of the eye narrows, the outflow of intraocular fluid becomes more difficult, and intraocular pressure increases. In this case, it is necessary to remove the swollen lens against the background of antihypertensive therapy. The operation normalizes intraocular pressure and restores visual acuity.
Mature Cataract is characterized by complete clouding and slight compaction of the lens substance. During biomicroscopy, the nucleus and posterior cortical layers are not visible. Upon external examination, the pupil is bright gray or milky white. The lens appears to be inserted into the lumen of the pupil. There is no “shadow” from the iris (Fig. 12.5, b).
With complete opacification of the lens cortex, objective vision is lost, but light perception and the ability to determine the location of a light source are preserved (if the retina is preserved). The patient can distinguish colors. These important indicators are the basis for a favorable prognosis regarding the return of full vision after cataract removal.
You. If an eye with cataracts does not distinguish between light and dark, then this is evidence of complete blindness caused by gross pathology in the visual-nervous system. In this case, cataract removal will not restore vision.
Overripe Cataracts are extremely rare. It is also called milk or Morgagni cataract after the scientist who first described this phase of cataract development (G. V. Morgagni). It is characterized by complete disintegration and liquefaction of the cloudy cortex of the lens. The core loses its support and falls down. The lens capsule becomes like a sac with a cloudy liquid, at the bottom of which lies the nucleus. In the literature you can find a description of further changes in the clinical condition of the lens in the event that the operation was not performed. After the cloudy liquid is absorbed, vision improves for a certain period of time, and then the core softens, dissolves, and only the wrinkled bag of the lens remains. In this case, the patient goes through many years of blindness.
With overripe cataracts, there is a risk of developing severe complications. When a large amount of protein masses is absorbed, pronounced phagocytic
naya reaction. Macrophages and protein molecules clog the natural fluid outflow pathways, resulting in the development of phacogenic (phacolytic) glaucoma.
Overripe milk cataracts can be complicated by rupture of the lens capsule and the release of protein detritus into the eye cavity. Following this, phacolytic iridocyclitis develops.
If the noted complications of overripe cataract develop, it is necessary to urgently remove the lens.
Nuclear cataract is rare: it accounts for no more than 8-10% of total number age-related cataracts. The opacification appears in the inner part of the embryonic nucleus and slowly spreads throughout the nucleus. Initially, it is homogeneous and non-intense, so it is regarded as age-related thickening or sclerosis of the lens. The kernel can acquire a yellowish, brown and even black color. The intensity of opacities and coloration of the nucleus increases slowly, and vision gradually decreases. Immature nuclear cataracts do not swell and the thin cortical layers remain transparent (Fig. 12.6). The compacted large core refracts light rays more strongly, which cli-
Rice. 12.6. Nuclear cataract. Light section of the lens during biomicroscopy.
Clinically manifested by the development of myopia, which can reach 8.0-9.0 and even 12.0 diopters. When reading, patients stop using presbyopic glasses. In myopic eyes, cataracts usually develop according to the nuclear type, and in these cases there is also an increase in refraction, i.e., an increase in the degree of myopia. Nuclear cataract remains immature for several years and even decades. In rare cases, when it fully matures, we can talk about a mixed type cataract - nuclear-cortical.
Complicated cataract occurs when exposed to various unfavorable factors of the internal and external environment.
In contrast to cortical and nuclear age-related cataracts, complicated ones are characterized by the development of opacities under the posterior capsule of the lens and in the peripheral parts of the posterior cortex. The predominant location of opacities in the posterior part of the lens can be explained by worse conditions for nutrition and metabolism. With complicated cataracts, opacities first appear at the posterior pole in the form of a barely noticeable cloud, the intensity and size of which slowly increase until the opacities occupy the entire surface of the posterior capsule. These cataracts are called posterior cup cataracts. The core and most of the cortex of the lens remain transparent, however, despite this, visual acuity is significantly reduced due to the high density of the thin layer of opacities.
Complicated cataract caused by the influence of unfavorable internal factors. Very vulnerable metabolic processes in the lens can be negatively affected by changes occurring in other tissues of the eye, or by general pathology of the body. Severe recurrent inflammation
Body diseases of the eye, as well as degenerative processes, are accompanied by changes in the composition of the intraocular fluid, which in turn leads to disruption of metabolic processes in the lens and the development of opacities. As a complication of the underlying eye disease, cataracts develop with recurrent iridocyclitis and chorioretinitis of various etiologies, dysfunction of the iris and ciliary body (Fuchs syndrome), advanced and terminal glaucoma, detachment and pigmentary degeneration of the retina.
An example of a combination of cataracts with a general pathology of the body is cachectic cataract, which occurs in connection with the general deep exhaustion of the body during fasting, after suffering infectious diseases(typhoid, malaria, smallpox, etc.), as a result chronic anemia. Cataracts can occur due to endocrine pathology (tetany, myotonic dystrophy, adiposogenital dystrophy), Down's disease and some skin diseases (eczema, scleroderma, neurodermatitis, atrophic poikiloderma).
In modern clinical practice Diabetic cataracts are the most common type to be observed. It develops in severe cases of the disease at any age, is more often bilateral and is characterized by unusual initial manifestations. Subcapsularly in the anterior and posterior sections of the lens, opacities form in the form of small, evenly spaced flakes, between which vacuoles and thin water slits are visible in places. The unusualness of the initial diabetic cataract lies not only in the localization of the opacities, but also mainly in the ability to reverse development with adequate treatment of diabetes. In elderly people with severe sclerosis of the lens nucleus, diabetic
Chinese posterior capsular opacities can be combined with age-related nuclear cataracts.
The initial manifestations of complicated cataracts, which occur when metabolic processes in the body are disrupted due to endocrine, skin and other diseases, are also characterized by the ability to resolve when rational treatment general disease.
Complicated cataract caused by exposure to external factors. The lens is very sensitive to everything unfavorable factors external environment, be it mechanical, chemical, thermal or radiation exposure (Fig. 12.7, a). It can change even in cases where there is no direct damage. It is enough that the neighboring parts of the eye are affected, since this always affects the quality of products and the rate of exchange of intraocular fluid.
Post-traumatic changes in the lens can manifest not only as clouding, but also as displacement of the lens (dislocation or subluxation) as a result of complete or partial separation of the ligament of cinnamon (Fig. 12.7, b). After blunt trauma, a round pigment imprint of the pupillary edge of the iris may remain on the lens - the so-called cataract, or Vossius ring. The pigment dissolves within a few weeks. Completely different consequences are observed if, after a contusion, a true clouding of the lens substance occurs, for example, rosette, or radiant, cataract. Over time, the cloudiness in the center of the rosette intensifies and vision steadily decreases.
When the capsule ruptures, aqueous humor containing proteolytic enzymes permeates the substance of the lens, causing it to swell and become cloudy. Gradually disintegration and resorption occur
Rice. 12.7. Post-traumatic changes in the lens.
a - foreign body under the capsule of the clouded lens; b - post-traumatic dislocation of the transparent lens.
lens fibers, after which a wrinkled lens bag remains.
The consequences of burns and penetrating wounds of the lens, as well as emergency measures, are described in Chapter 23.
Radiation cataract. The lens is capable of absorbing rays with a very short wavelength in the invisible, infrared, part of the spectrum. It is when exposed to these rays that there is a risk of developing cataracts. X-rays and radium rays, as well as protons, neutrons and other nuclear fission elements leave traces in the lens. Exposure of the eye to ultrasound and microwave current can also lead to
development of cataracts. Rays of the visible spectrum (wavelength from 300 to 700 nm) pass through the lens without damaging it.
Occupational radiation cataracts can develop in hot shop workers. Work experience, duration of continuous contact with radiation and compliance with safety regulations are of great importance.
Care must be taken when administering radiation therapy to the head, especially when irradiating the orbit. Special devices are used to protect the eyes. After the explosion of the atomic bomb, characteristic radiation cataracts were detected in residents of the Japanese cities of Hiroshima and Nagasaki. Of all the tissues of the eye, the lens turned out to be the most susceptible to hard ionizing radiation. In children and young people it is more sensitive than in elderly and senile people. Objective data indicate that the cataractogenic effect of neutron radiation is tens of times stronger than other types of radiation.
The biomicroscopic picture of radiation cataracts, as well as other complicated cataracts, is characterized by disk-shaped opacities irregular shape located under the posterior capsule of the lens. The initial period of cataract development can be long, sometimes several months or even years, depending on the radiation dose and individual sensitivity. Reversal of radiation cataracts does not occur.
Cataracts due to poisoning. The literature describes severe cases of ergot poisoning with mental disorders, convulsions and severe eye pathology - mydriasis, impaired oculomotor function and complicated cataracts, which were discovered several months later.
Naphthalene, thallium, dinitrophenol, trinitrotoluene and nitro dyes have a toxic effect on the lens. They can enter the body in different ways - through the respiratory tract, stomach and skin. Experimental cataracts in animals are obtained by adding naphthalene or thallium to the feed.
Complicated cataracts can be caused not only by toxic substances, but also by excess of certain medications, such as sulfonamides, and common food ingredients. Thus, cataracts can develop when animals are fed galactose, lactose and xylose. Lens opacities found in patients with galactosemia and galactosuria are not an accident, but a consequence of the fact that galactose is not absorbed and accumulates in the body. There is no strong evidence of the role of vitamin deficiency in the occurrence of complicated cataracts.
Toxic cataracts in the initial period of development can resolve if the flow of the active substance into the body has stopped. Long-term exposure to cataractogenic agents causes irreversible opacities. In these cases, surgical treatment is required.
12.4.1.3. Cataract treatment
In the initial stage of cataract development, conservative treatment to prevent rapid clouding of the entire lens substance. For this purpose, instillation of drugs that improve metabolic processes is prescribed. These drugs contain cysteine, ascorbic acid, glutamine and other ingredients (see section 25.4). Treatment results are not always convincing. Rare forms of initial cataracts can resolve if timely treatment is performed. rational therapy that illness
vania, which caused the formation of opacities in the lens.
Surgical removal of the cloudy lens is called cataract extraction.
Cataract surgery was performed as early as 2500 BC, as evidenced by the monuments of Egypt and Assyria. Then they used the technique of “pressing” or “reclinating” the lens into the vitreous cavity: they pierced the cornea with a needle, pushed the lens with a jerk, tore off the ligaments of Zinn and tipped it into the vitreous. Only in half of the patients the operations were successful; in the rest, blindness occurred due to the development of inflammation and other complications.
The first operation to remove the lens for cataracts was performed by the French physician J. Daviel in 1745. Since then, the surgical technique has been constantly changing and improving.
The indication for surgery is a decrease in visual acuity, leading to limited ability to work and discomfort in everyday life. The degree of cataract maturity does not matter when determining the indications for its removal. For example, with a cup-shaped cataract, the nucleus and cortical masses can be completely transparent, but a thin layer of dense opacities localized under the posterior capsule in the central section sharply reduces visual acuity. For bilateral cataracts, the eye with poorer vision is operated on first.
Before surgery, both eyes must be examined and assessed general condition body. It is always important for the doctor and the patient to predict the results of the operation in terms of preventing possible complications, as well as regarding the function of the eye after surgery. For
In order to get an idea of the safety of the visual-nervous analyzer of the eye, its ability to localize the direction of light (light projection) is determined, the field of view and bioelectric potentials are examined. Cataract removal surgery is also performed in cases of identified disorders, with the hope of restoring at least residual vision. Surgical treatment is absolutely futile only in cases of complete blindness, when the eye does not sense light. If signs of inflammation are found in the anterior and posterior segments of the eye, as well as in its appendages, anti-inflammatory therapy must be carried out before surgery.
During the examination, previously undiagnosed glaucoma may be identified. This requires special attention from the doctor, since when removing cataracts from a glaucomatous eye, the risk of developing the most severe complication - expulsive hemorrhage, which can result in irreversible blindness, increases significantly. In case of glaucoma, the doctor decides to perform a preliminary antiglaucomatous operation or a combined intervention of cataract extraction and antiglaucomatous surgery. Cataract extraction for operated, compensated glaucoma is safer, since sudden sharp changes in intraocular pressure are less likely during the operation.
When determining the tactics of surgical treatment, the doctor takes into account any other features of the eye identified during the examination.
A general examination of the patient aims to identify possible foci of infection, primarily in organs and tissues located near the eye. Before surgery, foci of inflammation of any location must be sanitized. Particular attention should be paid to the condition
teeth, nasopharynx and paranasal sinuses.
Blood and urine tests, ECG and X-ray examination lungs help to identify diseases that require emergency or planned treatment.
In a clinically calm state of the eye and its appendages, the microflora of the contents of the conjunctival sac is not examined.
IN modern conditions The immediate preoperative preparation of the patient is significantly simplified, due to the fact that all microsurgical manipulations are low-traumatic, their implementation ensures reliable sealing of the eye cavity and patients do not need strict bed rest after surgery. The operation can be performed on an outpatient basis.
Cataract extraction is performed using microsurgical techniques. This means that the surgeon carries out all manipulations under a microscope, uses the finest microsurgical instruments and suture material, and is provided with a comfortable chair. The mobility of the patient's head is limited by a special headboard of the operating table, which has the shape of a semicircular table on which the instruments lie, and the surgeon's hands rest on it. The combination of these conditions allows the surgeon to perform precise manipulations without tremor of the fingers and random deviations of the patient’s head.
In the 60-70s of the last century, the lens was removed from the eye entirely in a bag - intracapsular cataract extraction (IEC). The most popular method was cryoextraction, proposed in 1961 by the Polish scientist Krvavic (Fig. 12.8). The surgical approach was performed from above through an arcuate corneoscleral incision along the limbus. The cut is large - a little
Rice. 12.8. Intracapsular cataract extraction.
a - the cornea is raised upward, the edge of the iris is pulled downwards by the iris retractor to expose the lens, the cryoextractor touches the surface of the lens, there is a white ring of freezing the lens around the tip; b - the cloudy lens is removed from the eye.
less than the semicircle of the cornea. It corresponded to the diameter of the lens being removed (9-10 mm). With a special tool - an iris retractor - they captured top edge pupil and exposed the lens. The cooled tip of the cryoextractor was applied to the front surface of the lens, frozen, and easily removed from the eye. To seal the wound, 8-10 interrupted sutures or one continuous suture were applied. Currently, this simple method is used extremely rarely due to the fact that in the postoperative period, even in the long term, severe complications can occur in the posterior part of the eye. This is explained by the fact that after intracapsular cataract extraction, the entire vitreous mass moves anteriorly and takes the place of the removed lens. The soft, pliable iris cannot contain the movement of the vitreous, resulting in hyperemia of the retinal vessels ex vacuo (vacuum effect).
Following this, hemorrhages in the retina, swelling of its central part, and areas of retinal detachment may occur.
Later, in the 80-90s of the last century, the main method of removing a cloudy lens became extracapsular cataract extraction (EEC). The essence of the operation is as follows: the anterior capsule of the lens is opened, the nucleus and cortical masses are removed, and the posterior capsule, together with the narrow rim of the anterior capsule, remains in place and performs its usual function - separating the anterior part of the eye from the posterior one. They serve as a barrier to the anterior movement of the vitreous. In this regard, after extracapsular cataract extraction, significantly fewer complications occur in the posterior part of the eye. The eye can more easily withstand various loads when running, pushing, or lifting heavy objects. In addition, the preserved lens bag is an ideal place for artificial optics.
There are different options for performing extracapsular cataract extraction. They can be divided into two groups - manual and energy cataract surgery.
With the manual EEC technique, the surgical approach is almost half as long as with the intracapsular one, since it is focused only on removing the nucleus of the lens, the diameter of which in an elderly person is 5-6 mm.
You can reduce the surgical incision to 3-4 mm to make the operation safer. In this case, it is necessary to cut the lens nucleus in half in the eye cavity with two hooks moving from opposite points of the equator towards each other. Both halves of the kernel are removed alternately.
Currently, manual cataract surgery has already been replaced by modern methods using ultrasound, water or laser energy to destroy the lens in the eye cavity. This is the so-called energy surgery, or small incision surgery. It attracts surgeons due to a significant reduction in the incidence of complications during surgery, as well as the absence of postoperative astigmatism. Wide surgical incisions have given way to punctures in the limbal area, which do not require sutures.
Ultrasonic technology phacoemulsification cataract (FEC) was proposed in 1967 by the American scientist C. D. Kelman. Widespread use of this method began in the 80-90s.
Special devices have been created to perform ultrasonic FEC. Through a puncture at the limbus 1.8-2.2 mm long, a tip of the appropriate diameter carrying ultrasonic energy is inserted into the eye. Using special techniques, they divide the core into four fragments and destroy them one by one. Through the same
Rice. 12.9. Energy methods of cataract extraction.
a - ultrasonic phacoemulsification of soft cataracts; b - laser extraction of hard cataracts, independent splitting
kernels.
The tip delivers a balanced salt solution of BSS into the eye. The lens masses are washed out through the aspiration channel (Fig. 12.9, a).
In the early 80s N. E. Temirov proposed hydromonitor phacofragmentation of soft cataracts by transmitting high-speed pulsed streams of heated isotonic sodium chloride solution through a special tip.
In 1994, a group of domestic ophthalmologists (V.G. Kopaeva, Yu.V. Andreev) under the leadership of Academician S.N. Fedorov developed the technology for the first time in the world destruction and evacuation of cataracts any degree of hardness using laser energy and an original vacuum installation. Known other laser systems can effectively destroy only soft cataracts. The operation is performed bimanually through two punctures at the limbus. At the first stage, the pupil is dilated and the anterior capsule of the lens is opened in the form of a circle with a diameter of 5-7 mm. Then a laser (0.7 mm in diameter) and a separate irrigation-aspiration (1.7 mm) tips are inserted into the eye (Fig. 12.9, b). They barely touch the surface of the lens in the center. The surgeon observes how, within a few seconds, the core of the lens “melts” and a deep cup is formed, the walls of which disintegrate into fragments. When they are destroyed, the energy level decreases. Soft cortical masses are aspirated without the use of a laser. The destruction of soft and medium-hard cataracts occurs in a short period of time - from a few seconds to 2-3 minutes; removal of dense and very dense lenses requires from 4 to 6-7 minutes.
Laser cataract extraction (LEC) expands age-related indications, since during the operation there is no pressure on the lens and there is no need for mechanical fragmentation of the nucleus. The laser tip does not heat up during operation, so there is no need to inject a large amount of balanced saline solution. In patients under 40 years of age, laser energy is often not required, since the powerful vacuum system of the device copes with the suction of the soft substance of the lens. Folding soft in-
Traocular lenses are inserted using an injector.
Cataract extraction is called the crown jewel of eye surgery. This is the most common eye surgery. It brings deep satisfaction to the surgeon and the patient. Often patients come to the doctor by touch, and after the operation they immediately become sighted. The operation allows you to return the visual acuity that was in the given eye before the development of cataracts.
12.4.2. Dislocation and subluxation of the lens
A dislocation is the complete separation of the lens from the suspensory ligament and its displacement into the anterior or posterior chamber of the eye. This happens sharp decline visual acuity, since a lens with a power of 19.0 diopters fell out of the optical system of the eye. The dislocated lens must be removed.
Subluxation of the lens is a partial tear of the ligament of Zinn, which can have a different extent around the circumference (see Fig. 12.7, b).
Congenital dislocations and subluxations of the lens are described above. Acquired displacement of the biological lens occurs as a result of blunt trauma or severe shock. Clinical manifestations Lens subluxation depends on the size of the resulting defect. Minimal damage may go unnoticed if the anterior limiting membrane of the vitreous body is not damaged and the lens remains transparent.
The main symptom of lens subluxation is iris trembling (iridodonesis). The delicate tissue of the iris rests on the lens at the anterior pole, so the vibration of the subluxated lens is transmitted
iris. Sometimes this symptom can be seen without using special methods research. In other cases, you have to carefully observe the iris under side lighting or in the light of a slit lamp in order to catch a slight wave of movement with small displacements of the eyeball. With sharp eye movements to the right and left, slight fluctuations of the iris cannot be detected. It should be noted that iridodonesis is not always present even with noticeable lens subluxation. This occurs in cases where, along with a tear of the ligament of Zinn in the same sector, a defect appears in the anterior limiting membrane of the vitreous body. In this case, a strangulated hernia of the vitreous body occurs, which plugs the resulting hole, props up the lens and reduces its mobility. In such cases, lens subluxation can be recognized by two other symptoms detected by biomicroscopy: this is the uneven depth of the anterior and posterior chambers of the eye due to more pronounced pressure or anterior movement of the vitreous in the area of weakening of the lens support. When a vitreous hernia is strangulated and fixed by adhesions, the posterior chamber in this sector increases and at the same time the depth of the anterior chamber of the eye changes, most often it becomes smaller. Under normal conditions, the posterior chamber is inaccessible for inspection, therefore the depth of its peripheral sections is judged by indirect sign- different distances from the edge of the pupil to the lens on the right and left or above and below.
The exact topographic position of the vitreous body, lens and supporting ligament behind the iris can only be seen with ultrasound biomicroscopy (UBM).
With uncomplicated subluxation of the lens, visual acuity is
does not decrease significantly and treatment is not required, but complications may develop over time. A subluxated lens may become cloudy or cause secondary glaucoma. In such cases, the question of its removal arises. Timely diagnosis of lens subluxation allows you to choose the right surgical tactics, assess the possibility of strengthening the capsule and placing an artificial lens in it.
12.4.3. Aphakia and pseudophakia
Afakia- this is the absence of a lens. An eye without a lens is called aphakic.
Congenital aphakia is rare. Typically, the lens is surgically removed due to clouding or dislocation. There are known cases of lens loss due to penetrating wounds.
When examining an aphakic eye, attention is drawn to the deep anterior chamber and iris tremors (iridodonesis). If the posterior capsule of the lens is preserved in the eye, then it restrains the shocks of the vitreous body during eye movements and the trembling of the iris is less pronounced. With biomicroscopy, a light section reveals the location of the capsule, as well as the degree of its transparency. In the absence of a lens bag, the vitreous body, held only by the anterior limiting membrane, is pressed against the iris and slightly protrudes into the pupillary area. This condition is called a vitreous hernia. When the membrane ruptures, vitreous fibers emerge into the anterior chamber. This is a complicated hernia.
Correction of aphakia. After removal of the lens, the refraction of the eye changes dramatically. High degree of hypermetropia occurs.
The refractive power of the lost lens must be compensated by optical means- glasses, contact lens or artificial lens.
Spectacle and contact correction of aphakia is currently rarely used. When correcting aphakia of an emmetropic eye for distance, a spectacle glass with a power of +10.0 diopters will be required, which is significantly less than the refractive power of the removed lens, which on average
it is equal to 19.0 diopters. This difference is explained primarily by the fact that spectacle lens occupies a different place in the complex optical system of the eye. In addition, the glass lens is surrounded by air, while the lens is surrounded by liquid, with which it has almost the same refractive index of light. For a hypermetrope, the power of the glass must be increased by the corresponding number of diopters; for a myope, on the contrary, it must be reduced. If before the opera-
Rice. 12.10. Designs of various IOL models and the place of their fixation in the eye.
If myopia was close to 19.0 diopters, then after the operation the too strong optics of the myopic eyes are completely neutralized by removing the lens and the patient will do without distance glasses.
An aphakic eye is incapable of accommodation, so for work at close range, glasses 3.0 diopters stronger than for distance are prescribed. Spectacle correction cannot be used for monocular aphakia. +10.0D lens is strong magnifying glass. If it is placed in front of one eye, then in this case the images in the two eyes will be too different in size, they will not merge into a single image. For monocular aphakia, contact (see section 5.9) or intraocular correction is possible.
Intraocular correction of aphakia - This surgery, the essence of which is that the clouded or dislocated natural lens is replaced with an artificial lens of the required strength (Fig. 12.11, a). The diopter power of new eye optics is calculated by a doctor using special tables, nomograms or computer program. The calculation requires the following parameters: refractive power of the cornea, depth of the anterior chamber of the eye, thickness of the lens and length of the eyeball. The overall refraction of the eye is planned taking into account the wishes of the patients. For those who drive a car and lead an active life, emmetropia is most often planned. Low myopic refraction can be planned if the second eye is myopic, as well as for those patients who spend most of the working day working desk, want to write and read or perform other precise work without glasses.
In recent years, bifocal, multifocal, accommodating, refractive-diffraction intraocular lenses have appeared.
lenses (IOLs) that allow you to see objects at different distances without additional glasses correction.
The presence of an artificial lens in the eye is referred to as “pseudophakia.” An eye with an artificial lens is called pseudophakic.
Intraocular correction of aphakia has a number of advantages over spectacle correction. It is more physiological, eliminates patients’ dependence on glasses, does not cause narrowing of the field of vision, peripheral scotomas, or distortion of objects. An image of normal size is formed on the retina.
Currently, there are many IOL designs (Fig. 12.10). Based on the principle of attachment in the eye, there are three main types of artificial lenses:
Anterior chamber lenses are placed in the corner of the anterior chamber or attached to the iris (Fig. 12.11, b). They come into contact with very sensitive tissues of the eye - the iris and cornea, so they are rarely used nowadays;
Pupillary lenses (pupillary) are also called irisclip lenses (ICL) (Fig. 12.11, c). They are inserted into the pupil according to the clip principle; these lenses are held in place by front and rear supporting (haptic) elements. The first lens of this type - the Fedorov-Zakharov lens - has 3 posterior arms and 3 anterior antennas. In the 60-70s of the 20th century, when intracapsular cataract extraction was mainly performed, the Fedorov-Zakharov lens was widely used all over the world. Its main disadvantage is the possibility of dislocation of the supporting elements or the entire lens;
Posterior chamber lenses (PCLs) are placed in the lens capsule after removal of the nucleus and
Rice. 12.11. Artificial and natural lens of the eye.
a - cloudy lens, removed from the eye entirely in a capsule, next to it is an artificial lens; b - pseudophakia: the anterior chamber IOL is fixed on the iris in two places; c- pseudophakia: the iris-clip lens is located in the pupil; d - pseudophakia: the posterior chamber IOL is located in the lens capsule, a light section of the anterior and posterior surfaces of the IOL is visible.
cortical masses during extracapsular cataract extraction (Fig. 12.11, d). They take the place of a natural lens in the overall complex optical system of the eye, and therefore provide the highest quality of vision. ZKLs are better than others at strengthening the separation barrier between the front and posterior sections eyes, prevent the development of many severe postoperative complications, such as secondary glaucoma, retinal detachment, etc. They come into contact only with the lens capsule, which does not have nerves and vessels and is not capable of an inflammatory reaction. This type of lens is currently preferred.
IOLs are made from hard (polymethyl methacrylate, leucosapphire, etc.) and soft (silicone, hydrogel, acrylate, collagen copolymer, etc.) material. They can be monofocal or multifocal, spherical, aspherical or toric (for astigmatism correction).
Two artificial lenses can be inserted into one eye. If for some reason the optics of the pseudophakic eye turn out to be incompatible with the optics of the other eye, then it is supplemented with another artificial lens of the required optical power.
IOL manufacturing technology is constantly being improved, and lens designs are changing, as required by modern cataract surgery.
Correction of aphakia can also be performed by others surgical methods, based on enhancing the refractive power of the cornea (see Chapter 5).
12.4.4. Secondary, membranous cataract and fibrosis of the posterior capsule of the lens
Secondary cataract occurs in an aphakic eye after extracapsular cataract extraction. This is the proliferation of the subcapsular epithelium of the lens remaining in the equatorial zone of the lens bag.
In the absence of the lens nucleus, the epithelial cells are not constrained, so they grow freely and do not stretch. They swell in the form of small transparent balls of different sizes and line the posterior capsule. With biomicroscopy, these cells look like soap bubbles or grains of caviar in the lumen of the pupil (Fig. 12.12a). They are called Adamyuk-Elschnig balls after the names of the scientists who first described secondary cataracts. In the initial stage of development of secondary cataracts,
you have no subjective symptoms. Visual acuity decreases when epithelial growths reach the central zone.
Secondary cataracts are subject to surgical treatment: epithelial growths are washed out or discision (dissection) of the posterior capsule of the lens is performed, on which Adamyuk-Elschnig balls are placed. Discision is performed with a linear incision within the pupillary zone. The operation can also be performed using a laser beam. In this case, the secondary cataract is also destroyed within the pupil. A round hole with a diameter of 2-2.5 mm is formed in the posterior capsule of the lens. If this is not enough to ensure high visual acuity, the hole can be enlarged (Fig. 12.12, b). In pseudophakic eyes, secondary cataracts develop less frequently than in aphakic eyes.
Membranous cataract is formed as a result of spontaneous resorption of the lens after injury, leaving only the fused anterior and posterior capsules of the lens in the form of a thick cloudy film (Fig. 12.13).
Rice. 12.12. Secondary cataract and its dissection.
a - transparent corneal graft, aphakia, secondary cataract; b - the same eye after laser discision of a secondary cataract.
Rice. 12.13. Membranous cataract. Large iris defect after penetrating eye injury. A membranous cataract is visible through it. The pupil is displaced downwards.
Membranous cataracts are dissected in the central zone with a laser beam or a special knife. In the resulting hole, if indicated, an artificial lens of a special design can be strengthened.
Fibrosis of the posterior capsule of the lens is usually defined as thickening and opacification of the posterior capsule after extracapsular cataract extraction.
In rare cases, posterior capsule opacification may be detected on the operating table after removal of the lens nucleus. Most often, clouding develops 1-2 months after surgery due to the fact that the posterior capsule was not sufficiently cleaned and invisible thin areas of transparent masses of the lens remained, which subsequently become cloudy. Such fibrosis of the posterior capsule is considered a complication of cataract extraction. After surgery, the posterior capsule always contracts and thickens as a manifestation of physiological fibrosis, but it remains transparent.
Dissection of the clouded capsule is performed in cases where visual acuity is sharply reduced. Sometimes sufficiently high vision is maintained even in the presence of significant opacities on the posterior capsule of the lens. It all depends on the location of these opacities. If there is at least a small gap left in the very center, this may be enough for the passage of light rays. In this regard, the surgeon decides on dissecting the capsule only after assessing the function of the eye.
Questions for self-control
Having become acquainted with the structural features of a living biological lens, which has self-regulating mechanism By focusing the image, you can establish a number of surprising and somewhat mysterious properties of the lens.
The riddle will not be difficult for you when you have already read the answer.
1. The lens does not have blood vessels or nerves, but is constantly growing. Why?
2. The lens grows throughout life, but its size practically does not change. Why?
3. There are no tumors or inflammatory processes in the lens. Why?
4. The lens is surrounded by water on all sides, but the amount of water in the lens substance gradually decreases over the years. Why?
5. The lens does not have blood or lymphatic vessels, but it can become cloudy with galactosemia, diabetes, malaria, typhoid and other general diseases of the body. Why?
6. You can choose glasses for two aphakic eyes, but not for one if the other eye is phakic. Why?
7. After removal of cloudy lenses with an optical power of 19.0 diopters, spectacle correction for distance it is not +19.0 diopters, but only +10.0 diopters. Why?
The human eye is a complex optical system whose task is to transmit the correct image to the optic nerve. The components of the organ of vision are the fibrous, vascular, retinal membranes and internal structures.
The fibrous membrane is the cornea and sclera. Through the cornea, refracted particles enter the organ of vision. The opaque sclera acts as a framework and has protective functions.
Through the choroid, the eyes are supplied with blood, which contains nutrients and oxygen.
Below the cornea is the iris, which provides the color to a person's eyes. In its center is a pupil that can change size depending on the lighting. Between the cornea is the intraocular fluid, which protects the cornea from microbes.
The next part of the choroid is called due to which the production of intraocular fluid occurs. The choroid is in direct contact with the retina and provides it with energy.
The retina is made up of several layers of nerve cells. Thanks to this organ, light perception and image formation are ensured. After this, information is transferred via optic nerve into the brain.
The internal part of the organ of vision consists of the anterior and posterior chambers filled with transparent intraocular fluid, the lens and the vitreous body. has a jelly-like appearance.
An important component of the human visual system is the lens. The functions of the lens are to ensure the dynamism of eye optics. It helps you see various items equally good. Already at the 4th week of embryo development, the lens begins to form. We will consider its structure and functions, as well as the principle of operation and possible diseases in this article.
Structure
This organ is similar to a biconvex lens, the front and back surfaces of which have different curvatures. The central part of each of them is the poles, which are connected by an axis. The axis length is approximately 3.5-4.5 mm. Both surfaces are connected along a contour called the equator. An adult has a size of 9-10 mm in the optical lens of the eye; it is covered on top by a transparent capsule (anterior bursa), inside of which there is a layer of epithelium. The posterior capsule is located on the opposite side; it does not have such a layer.
The ability to grow the eye lens is provided by epithelial cells that constantly multiply. Nerve endings, blood vessels, lymphoid tissue the lens is absent; it is entirely an epithelial formation. The transparency of this organ is affected by the chemical composition of the intraocular fluid; if this composition changes, clouding of the lens may occur.
Lens composition
The composition of this organ is as follows - 65% water, 30% protein, 5% lipids, vitamins, various inorganic substances and their compounds, as well as enzymes. The main protein is crystallin.
Operating principle
The lens of the eye is the anatomical structure of the anterior segment of the eye; normally it should be perfectly transparent. The principle of operation of the lens is to focus light rays reflected from an object into the macular zone of the retina. For the image on the retina to be clear, it must be transparent. When light hits the retina, an electrical impulse is generated that travels through the optic nerve to the visual center of the brain. The brain's job is to interpret what the eyes see.
The role of the lens in the functioning of the human vision system is very important. First of all, it has a light-conducting function, that is, it ensures the passage of light to the retina. The light-conducting functions of the lens are ensured by its transparency.
In addition, this organ takes an active part in the refraction of light flux and has an optical power of about 19 diopters. Thanks to the lens, the functioning of the accommodative mechanism is ensured, with the help of which the focusing of the visible image is spontaneously adjusted.
This organ helps us easily move our gaze from distant objects to those that are nearby, which is ensured by a change in the refractive power of the eyeball. When the fibers of the muscle that surrounds the lens contract, the tension of the capsule decreases and the shape of this optical lens of the eye changes. It becomes more convex, due to which nearby objects are clearly visible. When the muscle relaxes, the lens becomes flat, allowing you to see distant objects.
In addition, the lens is a partition that divides the eye into two sections, thereby protecting the anterior sections of the eyeball from excessive pressure of the vitreous body. This is also an obstacle to the passage of microorganisms that do not enter the vitreous body. This demonstrates the protective functions of the lens.
Diseases
The causes of diseases of the optical lens of the eye can be very diverse. These include violations of its formation and development, and changes in location and color that occur with age or as a result of injuries. There is also abnormal development of the lens, which affects its shape and color.
Pathologies such as cataracts or clouding of the lens often occur. Depending on the location of the opacification zone, there are anterior, layered, nuclear, posterior and other forms of the disease. Cataracts can be either congenital or acquired during life as a result of injuries, age-related changes and a number of other reasons.
Sometimes injuries and rupture of the filaments that ensure the correct position of the lens can lead to its displacement. When the threads are completely broken, the lens dislocates, partial rupture leads to subluxation.
Symptoms of lens damage
As a person ages, a person's visual acuity decreases and it becomes much more difficult to read at close range. A slowdown in metabolism leads to changes in the optical properties of the lens, which becomes denser and less transparent. The human eye begins to see objects with less contrast, and the image often loses color. When more severe opacities develop, visual acuity decreases significantly and cataracts occur. The location of the clouding affects the degree and speed of vision loss.
Age-related clouding takes a long time to develop, up to several years. Because of this, impaired vision in one eye may go unnoticed for a long time. But even at home you can determine the presence of cataracts. To do this, you need to look at a blank sheet of paper with one eye, then with the other. If the disease is present, the leaf will appear dull and have a yellowish tint. People with this pathology need bright lighting in which they can see well.
Cloudiness of the lens can be caused by an inflammatory process (iridocyclitis) or long-term use medicines that contain steroid hormones. Various studies confirmed that with glaucoma, clouding of the optical lens of the eye occurs faster.
Diagnostics
Diagnostics consists of testing visual acuity and examination with a special optical device. The ophthalmologist evaluates the size and structure of the lens, determines the degree of its transparency, the presence and location of opacities, which lead to a decrease in visual acuity. When examining the lens, the method of lateral focal illumination is used, in which its anterior surface, located within the pupil, is examined. If there is no opacification, the lens is not visible. In addition, there are other research methods - examination in transmitted light, examination using a slit lamp (biomicroscopy).
How to treat?
Treatment is mainly surgical. Pharmacy chains offer various drops, but they are not able to restore the transparency of the lens, and also do not guarantee the cessation of the development of the disease. Surgery is the only procedure that ensures complete recovery. To remove cataracts, extracapsular extraction with suturing of the cornea can be used. There is another method - phacoemulsification with minimal self-sealing incisions. The removal method is chosen depending on the density of the opacities and the condition of the ligamentous apparatus. The experience of the doctor is no less important.
Since the eye lens plays an important role in the functioning of the human vision system, various injuries and disruptions to its functioning often lead to irreparable consequences. The slightest signs of visual impairment or discomfort in the eye area are a reason to immediately consult a doctor, who will make a diagnosis and prescribe the necessary treatment.
Lens – important element optical system of the eye, the average refractive power of which is 20-22 diopters.It is located in the posterior chamber of the eye and has an average size of 4-5 mm in thickness and 8-9 mm in height. The thickness of the lens normally increases very slowly but steadily with age. It is presented in the form of a biconvex lens, the front surface of which is flatter and the back more convex.
The lens is transparent, thanks to the function of special proteins crystallins, it has a thin, also transparent capsule or lens sac, to which fibers of the zonules of the ciliary body are attached along the circumference, which fix its position and can change the curvature of its surface. The ligamentous apparatus of the lens ensures the immobility of its position exactly on the visual axis, which is necessary for clear vision. The lens consists of a nucleus and cortical layers around this nucleus - the cortex. At a young age, it has a rather soft, gelatinous consistency, so it easily succumbs to the tension of the ciliary body ligaments during the process of accommodation.
For some congenital diseases The lens may have an abnormal position in the eye due to weakness and imperfect development of the ligamentous apparatus, and may also have congenital opacities in the nucleus or cortex, which can reduce vision.
Symptoms of the lesion
With age, the structure of the nucleus and cortex of the lens becomes denser and responds less well to the tension of the ligamentous apparatus and weakly changes the curvature of its surface. Therefore, upon reaching 40 years of age, a person who has always seen well in the distance becomes more difficult to read at close range.Age decline metabolism in the body, and consequently its decrease in the intraocular structures, leads to a change in the structure and optical properties of the lens. In addition to its compaction, it begins to lose its transparency. At the same time, the image that a person sees may become yellower, less bright in colors, and duller. There is a feeling that you are looking “as if through a cellophane film,” which does not go away even when using glasses. With more severe opacities, visual acuity may decrease significantly down to light perception. This condition of the lens is called cataract.
Cataract opacities can be located in the nucleus of the lens, in the cortex, directly under its capsule and, depending on this, will reduce visual acuity more or less, faster or slower. All age-related lens opacities occur rather slowly over several months or even years. Therefore, people often do not notice for a long time that the vision of one eye has become worse. When looking at the clean white sheet paper, it may appear more yellowish and dull with one eye than the other. Halos may appear when looking at a light source. You may notice that you can only see in very good lighting.
Often, lens opacities are caused not by age-related metabolic disorders, but by long-term inflammatory diseases of the eye, such as chronic iridocyclitis, as well as long-term use of tablets or drops containing steroid hormones. Many studies have reliably confirmed that in the presence of glaucoma, the lens in the eye becomes cloudy faster and more often.
Blunt trauma to the eye can also cause progression of opacities in the lens and/or disruption of its ligamentous apparatus.
Diagnosis of the condition of the lens
Diagnosis of the condition and functions of the lens and its ligamentous apparatus is based on testing visual acuity and biomicroscopy of the anterior segment. An ophthalmologist can use the device to evaluate the size and structure of your lens, the degree of its transparency, and determine in detail the presence and location of opacities in it that reduce visual acuity. For a more detailed examination of the lens and its ligamentous apparatus, pupil dilation may be required. Moreover, with a certain location of the opacities, after dilation of the pupil, vision may improve, since the diaphragm will begin to transmit light through the transparent areas of the lens.
Sometimes a lens that is relatively thick in diameter or long in height can fit so closely to the iris or ciliary body that it can narrow the angle of the anterior chamber of the eye, through which the main outflow of intraocular fluid occurs. This mechanism is the main one in the occurrence of narrow-angle or closed-angle glaucoma. Ultrasound biomicroscopy or anterior segment optical coherence tomography may be required to assess the relationship of the lens with the ciliary body and iris.
Treatment of lens diseases
Treatment of lens diseases is usually surgical.There are many drops available that are designed to stop age-related clouding of the lens, but they cannot return you to its original clarity or guarantee that it will stop further clouding. Today, the operation of removing a cataract - a clouded lens - with replacement with an intraocular lens is an operation with full recovery.
Techniques for cataract removal vary from extracapsular extraction with suturing of the cornea to phacoemulsification with minimal self-sealing incisions. The choice of removal method depends on the degree and density of lens opacities, the strength of its ligamentous apparatus, and also, importantly, on the qualifications of the ophthalmic surgeon.
At the present stage, it is known that the lens develops from the ectodermal placode, which forms the lens vesicle during invaginationat the 3rd week of embryogenesis. According to some researchers, the placode invaginates due to contraction of cytoplasmic filaments, which have a diameter of 3.5 - 4.5 nm and are located parallel to the tops of the cells.
At the initial stage of lens development, there is a thickening of the ectoderm upon contact with the optic vesicle - the lens placode. At subsequent stages of development (days 22-23), the cells of the lens placode invaginate posteriorly, forming a concave fossa. This invagination continues in the future, and this group of cells, detaching from the surface ectoderm, turns into a lens vesicle. During this period, a delicate basement membrane, initially associated with the superficial ectoderm, covers the lens vesicle, in which the cells are extended inward. The basement membrane, or lens capsule, is so thin that in the early stages of development it is not visible under light microscopy.
The ectoderm preserved above the lens vesicle closes as the vesicle sinks and subsequently differentiates into the anterior epithelium of the cornea. After invagination, the lens vesicle separates from the ectoderm, plunging into the optic cup. After immersion, the forming lens acquires a rounded shape. Initially, cell division is observed throughout the lens vesicle; subsequently, mitoses are found only in its proximal wall. At this time, the cells of the inner wall stop premitotic DNA synthesis and, accordingly, do not absorb labeled thymidine.
At this stage of development, certain differences in the structure of the anterior and posterior walls of the lens vesicle are also revealed. The anterior wall remains single-layered and consists of cuboid cells. The cells of the posterior wall gradually elongate and form ribbon-like fibers. The lumen of the vesicle decreases in volume and takes on a crescent shape as a result of the growth of fibers. This lumen is soon obliterated by the fibers, and a solid lens is fully formed by the end of the 4th week of embryonic development.
The lens capsule is a true basement membrane and is formed as a result of the activity of epithelial cells. It occurs in the 5th week of embryonic development.
At the end of the 6th week, the cells of the posterior surface of the vesicle begin to elongate, turning into primary fibers. The bases of these fibers are adjacent to the posterior half of the capsule formed on the outer surface of the lens vesicle by its cells, and the apices quickly reach the epithelial cells of the anterior half of the vesicle and by 6.5 weeks its entire cavity is filled with them. These fibers are elongated differentiated cells, the nuclei of which are gradually resorbed, mitochondria gradually disappear. A capsular membrane is formed.
The sutures of the lens begin to form in the 2nd month of embryonic development, directly during the formation of the primary nucleus of the lens. During the formation of the primary nucleus of the lens, the lens fibers spread from the anterior to the posterior pole, which is the reason for its sphericity. Further growth is manifested by uneven elongation of the lens fibers, so that they join at the anterior and posterior poles to form a Y-shaped suture.
Initially, there are two similar seams - anterior and posterior. The main role of seams is that they allow fiber joints to be connected linearly. This determines the ellipsoidal shape of the lens. On late stages During pregnancy and at birth, the growth of sutures is uneven. Instead of a simple Y-shaped seam, the formation of a complex dendritic pattern is observed.
By the 9th week, the rudiment of the embryonic nucleus of the lens is formed. Compaction of primary fibers leads to a decrease in the volume of the lens substance and, as a rule, to a weakening of the tension of its capsule, which is compensated by the formation of new fibers, called secondary. Thus, already at the beginning of the embryonic development of the lens, the mechanism of its physiological regeneration is activated, which then functions throughout life. The formation of secondary fibers begins at the 9-10th week of embryonic development and then continues with a gradually fading intensity during postnatal ontogenesis, practically stopping only in old age.
It is generally accepted that the source of the formation of these fibers are the epithelial cells of the anterior capsule. In the embryonic and postembryonic periods of development, these cubic cells multiply under the entire anterior capsule, but most intensively near the equator. Cells located in the equator region of the lens stop multiplying and begin to differentiate, moving their bases along the posterior capsule towards the posterior pole. At the same time, they lengthen in such a way that the bases of the developing secondary cells - fibers - end up at the posterior capsule, and the apices - under its epithelium at the anterior one. The ends of the fibers grow towards the outer and inner poles of the lens. The fibers retain nuclei for some time, located in their middle part, a little closer to the top, and, superimposed in concentric layers on the underlying primary fibers, they push the latter inside the lens. New layers of differentiating fibers push the previously formed ones away from the capsule, as a result of which the bases and apices of the latter “tear off” from the bag, forming at the end of the 10th week, respectively, the posterior and anterior lens sutures, or stars. The posterior star of the lens appears first, and after 2 weeks the anterior one appears. These stars consist of a cementing substance located between the fibers of the lens and are not located superficially, but penetrate to the core, by which they are separated from each other. At first, the seams have 3-4 shoulders, and then their number increases. The nuclei of primary and secondary fibers that find themselves deep in the lens gradually lose DNA and degenerate. The structure of the lens formed in this way does not undergo fundamental changes until the end of intrauterine development, but secondary fiber formation leads to an increase in its size and weight parallel to the growth of the eyeball, which increases 11-12 times during this period.
After the final formation of the embryonic nucleus, further formation of new fibers occurs only in the equatorial region. The new fibers are arranged concentrically around the old fibers along the equator. It is in this area that numerous mitoses are visible. The growth of fibers in the equatorial region continues throughout a person’s life. At the same time, the lens constantly increases in size and weight. The growth rate decreases markedly with age.
The increase in the mass of the lens and the eye as a whole in the prenatal period occurs in such a way that their share in relation to the mass of the fetus decreases. Thus, the mass of the lens at the 10th week of development is 0.02% of body weight, at birth - 0.04%, and in an adult - only 0.0006%. It should be noted that in the embryonic period around the lens bag it is formed from the surrounding mesenchyme choroid, performing a trophic function in relation to it. It receives blood supply through the vitreous artery, as well as from the branches of the pupillary membrane and is most developed from the 2nd to the 6th month of embryogenesis. By the time of birth it is reduced. Only in 23.3% of newborns does the resorption of its remnants continue.
If these temporary structures are preserved, visual functions may be impaired, which require surgical correction. There is an opinion that some types of pathology of the eye and lens, in particular, may be associated with the inclusion of embryonic development mechanisms during endogenous damage to their structures.
As the lens fibers differentiate and move towards the central regions of the lens, the cells lose their nuclei, intracytoplasmic organelles, and then the cytoplasmic membrane.
A progressive increase in the number of lens fibers in the equator region leads to the appearance of zones that characterize different periods of lens development. This division into zones is a consequence of the presence of optical differences between the old, more sclerotic zone of the center of the lens, and the new, more transparent zone. In adults, the following zones are found:
- embryonic nucleus - transparent primary lens fibers formed between the 1st and 3rd months of embryonic development;
- fetal nucleus - secondary fibers formed at 3-8 months of embryonic development;
- infantile nucleus - formed during last weeks embryonic development to the prepubertal period;
- adult core - formed after the end of the prepubertal period;
- cortex - superficial fibers lying under the epithelium - in front and under the capsule - behind.
Lens shape and size
The lens is a transparent, biconvex, disc-shaped, semi-solid formation located between the iris and the vitreous body.
- The lens is unique in that it is the only “organ” in the body of humans and most animals that consists of a single type of cell throughout all stages of embryonic development and postnatal life until death.
- Its significant difference is the absence of blood vessels and nerves.
- It is also unique in its metabolic characteristics (anaerobic oxidation predominates),
- chemical composition (presence of specific crystallin proteins),
- lack of tolerance of the body to its proteins.
Most of these features are associated with the nature of its embryonic development.
The anterior and posterior surfaces of the lens are connected 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 using the ciliary band (zonules of Zinn). Due to the relaxation of the ciliary girdle during contraction of the ciliary muscle, deformation of the lens occurs. At the same time, its main function is performed - a change in refraction, allowing a clear image to be obtained on the retina, regardless of the distance to the object. To fulfill this role, the lens must be transparent and elastic, which it is.
The lens grows continuously throughout a person's life, thickening by approximately 29 microns per year. Starting from the 6-7th week of intrauterine life (18 mm embryo), it increases in anteroposterior size as a result of the growth of primary lens fibers. At the stage of development, when the length of the embryo reaches 18-26 mm, the lens has an approximately spherical shape. With the appearance of secondary fibers (the size of the embryo is 26 mm), the lens flattens and its diameter increases.
The ciliary band apparatus, which appears when the embryo is 65 mm long, does not affect the increase in the diameter of the lens. Subsequently, the lens quickly 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. A factor contributing to the increase in diameter is the compaction of the core. The tension of the ciliary band causes a change in the shape of the lens.
The diameter of the adult lens measured along the equator is 9-10 mm. In the center, its thickness at the time of birth is approximately 3.5-4 mm, at 40 years old - 4 mm, and by old age it slowly increases to 4.75-5 mm. The thickness of the lens depends on the state of the accommodative ability of the eye.
| Age-related features of the diameter, mass and volume of the human lens | |
| Age, years | Sagittal diameter (thickness), mm |
| Newborn | 3,5 |
| 10 | 3,9 |
| 20-50 | 4,0-4,14 |
| 60-70 | 4,77 |
| 80-90 | 5,0 |
| Equatorial diameter, mm | |
| Newborn | 6,5 |
| after 15 years | 9,0 |
| Weight, mg | |
| Newborn | 65 |
| First year of life | 130 |
| 20-30 | 174 |
| 40-50 | 204 |
| 90 | 250 |
| Volume, ml | |
| 30-40 | 0,163 |
| 80-90 | 0,244 |
| Capsule thickness, microns | |
| Anterior pole | 8-14 |
| Equator | 7-17 |
| Posterior pole | 2-4 |
| Lens fibers, microns | |
| Length (mm) | 8-12 |
| Thickness (µm) | 4,6 |
| Quantity | 2100-2300 |
In contrast to thickness, the equatorial diameter of the lens changes less with age. At birth it is 6.5 mm, and in the 2nd decade of life it is 9-10 mm, and subsequently remains unchanged.
The anterior surface of the lens is less convex than the posterior one. It is a part of a sphere with a radius of curvature equal to an average of 10 mm (8-14 mm). The anterior surface borders the anterior chamber of the eye through the pupil, and along the periphery - with 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, through the ciliary girdle, is attached to the processes of the ciliary body.
The center of the anterior surface of the lens is called the anterior pole. It is located approximately 3 mm behind the posterior surface of the cornea.
The posterior surface of the lens has a large curvature - the radius of curvature is 6 mm (4.5-7.5 mm). It is usually considered in conjunction with the vitreous membrane of the anterior surface of the vitreous body. However, between these structures there is a gap-like space filled with liquid. This space behind the lens was described by E. Berger in 1882. It can be observed with anterior biomicroscopy.
The equator of the lens 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 the ciliary band is attached to this area. The folds disappear during accommodation, that is, when the tension of the ligament ceases.
The refractive index of the lens is 1.39, that is, slightly greater than the refractive index of the anterior chamber. 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. The accommodative power, equal to 15-16 diopters at birth, decreases by half by the age of 25, and at the age of 50 it is only 2 diopters.
With a biomicroscopic examination of the lens with a dilated pupil, features of its structural organization can be detected. Firstly, its multi-layered nature is visible. The following layers are distinguished, counting from front to center:
- capsule;
- subcapsular light zone (cortical zone);
- light narrow zone of inhomogeneous scattering;
- translucent zone of the cortex.
The listed zones make up the superficial cortex of the lens. There are also two deeper zones of the cortex. They are also called perinuclear. These zones are characterized by the presence of green autofluorescence when the lens is illuminated with blue light.
The nucleus is considered to be the prenatal part of the lens. It also has layering. In the center is located clear zone, called the germinal (embryonic) nucleus. Examination of the lens with a slit lamp can also reveal lens sutures. Mirror microscopy at high magnification allows you to see epithelial cells and lens fibers.
Lens capsule
The lens is covered on all sides by a capsule. The capsule is nothing more than the basement membrane of epithelial cells. It is the thickest basement membrane of the human body. The capsule is thicker in front (up to 15.5 microns) than in the back. The thickening along the periphery of the anterior capsule is more pronounced, since the bulk of the ciliary girdle is attached to this place. With age, the thickness of the capsule increases, especially in the front. This is due to the fact that the epithelium, which is the source of the basement membrane, is located anteriorly and is involved in the remodeling of the capsule noted as the lens grows.
The capsule is a fairly powerful barrier to bacteria and inflammatory cells, but is freely passable for molecules whose size is comparable to the size of hemoglobin. Although the capsule does not contain elastic fibers, it is extremely elastic and is constantly under the influence of external forces, that is, in a stretched state. For this reason, 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 YAG capsulotomy.
In a light microscope, the capsule appears transparent and homogeneous. In polarized light, its lamellar fibrous structure is revealed. In this case, the fibers are located parallel to the surface of the lens. The capsule also stains positively during the PAS reaction, which indicates the presence of a large number of proteoglycans in its composition.
Ultrastructurally, the capsule has a relatively amorphous structure. A slight lamellarity is observed due to the scattering of electrons by thread-like elements folded into plates.
About 40 plates are revealed, the thickness of each of which is approximately 40 nm. At higher microscope magnification, delicate fibrils with a diameter of 2.5 nm are revealed. The plates are located strictly parallel to the surface of the capsule.
In the postnatal period, some thickening of the posterior capsule is noted, which indicates the possibility of secretion of basal material by the posterior cortical fibers.
R. F. Fisher (1969) found that 90% of the loss of elasticity of the lens occurs as a result of changes in the elasticity of the capsule. This assumption has been questioned by R. A. Weale (1982).
In the equatorial zone of the anterior capsule of the lens, electron-dense inclusions appear with age, 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 frequency of striations of which is 110 nm.
The attachment points of the ciliary band to the capsule are called Berger's plates. Their other name is pericapsular membrane. This is a superficially located layer of the capsule with a thickness of 0.6 to 0.9 microns. It is less dense and contains more glycosaminoglycans than the rest of the capsule. The pericapsular membrane contains fibronectin, vitreonectin and other matrix proteins, which play a role in attaching the girdle to the capsule. The fibers of this fibrogranular layer are only 1-3 nm thick, while the thickness of the fibrils of the ciliary band is 10 nm.
Like other basement membranes, the lens capsule is rich in type IV collagen. It also contains types I, III and V collagen. In addition, many other extracellular matrix components are found in it - lamilin, fibronectin, heparan sulfate and entactin.
The permeability of the human lens capsule has been studied by many researchers. The capsule freely allows water, ions and other small molecules to pass through. It is a barrier to the path of protein molecules with the size of albumin (Mr 70 kDa; molecular diameter 74 A) and hemoglobin (Mr 66.7 kDa; molecular radius 64 A). Differences in bandwidth the capsule was normal and no cataracts were found.
The lens epithelium consists of a single layer of cells that lie beneath the anterior capsule of the lens and extend to the equator. Cells in transverse sections are cuboid-shaped, and in planar preparations they are polygonal. Their number approaches 500,000 in adulthood. The density of epithelial cells in the central zone is 5009 cells per 1 mm2 in men and 5781 in women. Density increases towards the periphery of the lens. As a person ages, cell density decreases.
Aerobic oxidation (Krebs cycle) accounts for only 3% of the metabolic volume of the entire lens. Moreover, this type of respiration is observed only in epithelial cells and outer lens fibers. However, this oxidation pathway provides up to 20% of the energy requirement of the lens. This energy is used to provide active transport and synthetic processes necessary for lens growth, synthesis of membranes, crystallins, cytoskeletal proteins and nucleoproteins.
The pentose phosphate shunt, which is involved in the synthesis of nucleoproteins, also functions. The epithelium and surface fibers of the lens cortex are involved in the removal of sodium from it, thanks to the activity of the Na+-, K+- pump. This uses the energy of ATPase. In the posterior part of the lens, sodium ions diffuse passively into the posterior chamber aqueous
Depending on the structural features and function, several zones of the epithelial lining are distinguished.
- Central zone
consists of a relatively constant number of cells that slowly decreases with age. They are polygonal in shape. The width of the cells is 11-17 µm, and the height is 5-8 µm. With their apical surface they are adjacent to the most superficially located lens fibers. The nuclei are shifted to the apical surface of the cells, are large in size and have numerous nuclear pores. They usually have two nucleoli. The cytoplasm contains a moderate amount of ribosomes, polysomes, smooth and rough endoplasmic reticulum, and small mitochondria. The lamellar complex (Golgi apparatus) is pronounced. Lysosomes, dense bodies and glycogen particles are also found. Visible are cylindrical microtubules with a diameter of 24 nm, intermediate-type microfilaments (10 nm), and alpha-actinin filaments.
The so-called matrix proteins - actin, vinmetin, spectrin, alpha-actinin and myosin - were identified in the cytoplasm of epithelial cells. These proteins provide rigidity to the cell cytoplasm. Alpha-crystallin is also present in the epithelium. Beta and gamma crystallins are absent. The cells are attached to the lens capsule using hemidesmosomes.In the centrally located zone, mitoses are rare. At different pathological conditions, primarily after injury, they are more numerous. - Intermediate zone located closer to the periphery of the lens. The cells of thisthe zones are cylindrical with a centrally located core. The basement membrane has a folded appearance.
- Germinal zone adjacent to the preequatorial zone. It is characterized by pronounced proliferative activity of cells (66 mitoses per 100,000 cells). The cells of this zone migrate posteriorly as they divide and subsequently turn into lens fibers. Some of them also shift anteriorly, into the intermediate zone.The cytoplasm of epithelial cells contains few organelles. There are short profiles of rough endoplasmic reticulum, ribosomes, small mitochondria and a lamellar complex. The number of organelles increases in the equatorial region as the levels of the structural elements of the cytoskeleton, actin, vinmetin, microtubule protein, superctrin, alpha-actinin and myosin increase. Entire actin network-like structures can be distinguished, especially in the apical and basal parts of the cells.
The process of formation of lens fibers
After final cell division, one or both daughter cells move into an adjacent transition zone, in which they are organized into meridionally oriented rows. Subsequently, these cells differentiate into secondary fibers of the lens, turning 180° and lengthening anteriorly and posteriorly. New lens fibers maintain polarity such that the posterior (basal) part of the fiber is in contact with the capsule (basal lamina), while the anterior (apical) part is separated from it by the epithelium. These transitional forms of cells are rich in ribosomes (polysomes) and contain a large number of multivesicular bodies. Microtubules are also numerous. With further differentiation, the cells take on a pyramidal shape with numerous “tubercles” directed towards the capsule.
The premitotic state of epithelial cells is preceded by the synthesis DNA while the differentiation of cells into lens fibers is accompanied by increased RNA synthesis, since at this stage the synthesis of structural and membrane specific proteins is noted. During the process of terminal differentiation of lens fibers, the nuclei become pyknotized and then disappear. Organelles also disappear. It was found that the loss of mitochondrial nuclei occurs suddenly and in one cell generation. Intensity mitotic divisions decreases with age. Young rats form approximately five new fibers per day, while old rats form one.
The nucleoli of differentiating cells increase, 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.
The germinal zone, unlike the central zone, is protected by the iris from the adverse effects of light energy, especially ultraviolet (300-400 nm).
Features of epithelial cell membranes
With the exception of the basement membrane of epithelial cells, which connects the cell with the lens capsule, the cytoplasmic membranes of neighboring epithelial cells form a certain 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 corresponding lens fibers. The basal part of the cells is attached to the anterior capsule using hemidesmosomes, and the lateral surfaces of the cells are connected by desmosomes.
Gap junctions are found on the lateral surfaces of the membranes of adjacent cells, through which small molecules can penetrate. Tight junctionsAlthough they are found between epithelial cells, they are rare. Structural organization of crunch membranesfacial fibers and the nature of intercellular contacts indicate the possible presence on the surface of cells of receptors that control the processes of endocytosis.
Endocytosis, in turn, plays an important role 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 identified. It is believed that these formations ensure the movement of nutrients and metabolites between cells.
Lens fibers
The transition from epithelial cells of the germinal zone to the lens fiber is accompanied by the disappearance of “finger impressions” between the cells and the beginning of elongation of the basal and apical parts of the cell. As lens fibers gradually accumulate, the nucleus of the lens is formed. This displacement of cells leads to the formation of an S- or C-like arc, directed forward and consisting of a chain of cell nuclei.
The deeper located fibers of the lens are 150 microns thick. When they lose their cores, the nuclear arc disappears. In the equator region, the width of the zone of nuclear cells is about 300-500 µm.
The lens fibers have a spindle- or belt-like shape, arranged in an arc in the form of concentric layers. In a cross section at the equator they are hexagonal in shape. As they move toward the center of the lens, their uniformity in size and shape is gradually disrupted. In the equator region in an adult, the width of the lens fiber ranges from 10 to 12 µm, and the thickness - from 1.5 to 2 µm.
IN rear parts The fibers of the lens are thinner, which is explained by the asymmetrical shape of the lens and the greater thickness of the anterior cortex. The ends of the fibers meet at a specific point and form seams.
In the fetal nucleus There is an anterior vertical Y-shaped seam and a posterior inverted Y-shaped seam. After birth, many branches are added to the existing sutures. As a result, the seams take on a star-like appearance. The main importance of seams is that, thanks to this complex system contact between cells, the shape of the lens is maintained throughout life.
Features of lens fiber membranes
Button-loop contacts. Membranes of adjacent hru steel fibers are connected using a variety of specialized formations that change their structure as the fiber moves from the surface deep into the lens. In the superficial 8-10 anterior layers of the cortex, the fibers are connected using “button-loop” (“ball and socket”) formations distributed evenly along the length of the fiber. Contacts of this type exist only between cells of the same layer, that is, cells of the same generation, and are absent between cells of different generations. This allows the fibers to move relative to each other as they grow.
In deeper fibers, button-loop contact is found somewhat less frequently and is distributed unevenly and involuntarily along the fiber. They are also visible between cells of different generations.
In the deepest layers of the cortex and nucleus, except for the indicated contacts complex interdigitations appear in the form of ridges, depressions and grooves. Desmosomes were also found, but only between differentiating and not mature lens fibers.
It is believed that contacts between lens fibers are necessary to maintain the rigidity of the structure throughout life, which helps maintain the transparency of the lens.
Another type of intercellular contacts has been found in the human lens. This is a gap contact. Such contacts are thought to serve two roles.
- Firstly, since they connect the lens fibers over a long distance, the architecture 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).
Two types of gap contacts have been identified: crystalline (with high ohmic resistance) and non-crystalline (low). In some tissues (liver), these types of gap contacts can be transformed into one another when the ionic composition of the environment changes. In the lens fiber they are not capable of such transformation.
- The first type of gap junctions is found at the junction of fibers with epithelial cells, and the second - only between fibers
- The second type of gap contacts (low-resistance) have intramembrane particles that do not allow neighboring membranes to approach each other by more than 2 nm. Due to this, the levels of ions and molecules in the deep layers of the lens are low. The latter spread quite easily between the lens fibers and their concentration normalizes quite quickly.
There are also species differences in the number of gap junctions. Yes, in the lensthey occupy the following area of the fiber surface: in humans - 5%, in frogs - 15%, in rats - 30%, and in chickens - 60%. There are no gap contacts in the seam area.
High refractive power The lens is achieved by a high concentration of protein filaments, and transparency is ensured by their strict organization, uniformity of fiber structure within each generation and a small volume of intercellular space (less than 1% of the lens volume). Transparency is also facilitated by a small number of intracytoplasmic organelles, as well as the absence of nuclei in the lens fibers. All of these factors minimize light scattering between fibers.
There are other factors that affect refractive power. One of them is an increase in protein concentration as it approaches the lens nucleus. This is why there is no chromatic aberration. Of no less importance in the structural integrity and transparency of the lens is the regulation of the ionic content and degree of hydration of the fibers.
At birth, the lens is transparent. With age, as it grows, the core acquires a yellowish tint, which is probably due to the influence on it ultraviolet radiation(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 destructive effects of short-wave light energy. Pigments accumulate in the nucleus with age, and in some individuals they are involved in the formation of pigmentary cataracts. In old age and, especially, with nuclear cataracts, the amount of insoluble proteins in the lens nucleus, which are crystallins with “cross-linked molecules,” increases in the nucleus of the lens.
Metabolic activity in the central areas of the lens is insignificant. There is no protein metabolism. Exactly according to Therefore, they are long-lived proteins and are easily damaged by oxidizing agents, which lead to the conformation of the protein molecule and form sulfhydryl groups. The development of cataracts is characterized by an increase in light scattering zones. This may be caused by a violation of the regularity of the arrangement of lens fibers, changes in the structure of membranes and scattering associated with the transformation of protein molecules. Swelling of the lens fibers and their destruction lead to disruption of water-salt metabolism.
Eyelash girdle device
The zonular apparatus of the lens consists of fibers extending from the ciliary body to the equator of the lens. They quite rigidly fix the lens in a certain position and allow the ciliary muscle to perform its main function, namely, through contractions, lead to deformation of the lens. In this case, naturally, its refractive ability changes. The ciliary band forms a ring that looks like a triangle on a meridional section. The base of this triangle is concave and opposes the equator of the lens, the apex is directed towards the processes of the ciliary body, its flat part and the dentate line.
The fibers of the ciliary girdle apparatus consist of a glycoprotein of non-collagenous origin, linked via O- and N-bonds to oligosaccharides. The presence of these bonds explains their positive histochemical staining during the PHIK reaction.
The fibers of the ciliary girdle apparatus have a tubular structure and resemble elastic fibers both in their chemical composition and in their relationship to proteolytic enzymes (resistance to collagenase and trypsin). This feature was used in intracapsular cataract extraction using alpha-chemotrypsin, which lyses the ciliary girdle apparatus, but does not act on the lens capsule.
It has recently been established that the fibrils of the ciliary girdle apparatus are rich in cysteine and are similar to the microfibrillar component of elastic tissue. These fibers are called fibrillin and are stained with the appropriate monoclonal antibodies. In other tissues, fibrillin is a matrix for the formation of elastic fibers. Fibrillin, unlike oxytalan (a microfibrillar component of elastic tissue), never turns into elastic fibers.
The gene that controls fibrillin synthesis is located on chromosome 15q21.1. Marfan syndrome, which causes lens dislocation and various diseases of the cardiovascular system, is associated with mutations in the gene that controls fibrillin synthesis.
As stated above, the ciliary band consists of fibers with a diameter of 10 nm (from 8 to 12 nm), having a tubular structure in cross section. In cases where the filaments are folded into a bundle, a periodicity of 40-55 μm appears. Fine-grained and fibrous material is found between the fibers.
The ciliary girdle apparatus originates from the outer layer of the lens capsule in the equatorial region. Moreover, in front, the ligaments are attached to the capsule along 2.5 mm, and behind - for 1 mm. In this case, fibrils emanating from the anterior part of the equatorial surface of the lens are directed posteriorly and attached to the ciliary processes (anterior ligaments), and fibrils emanating from the posterior surface of the capsule are directed to the flat part of the ciliary body and the dentate line (posterior ligaments).
The equatorial ligaments extend from the ciliary processes directly to the equator. There are also hyaloid ligaments that extend from the flat part of the ciliary body to the edge of the lens at the point of its contact with the vitreous body. Here they are woven into the “hyaloidocapsular ligaments” (corresponding to the annular fibers of the ligament of Wegener).
Due to the fact that the ligaments from the lens are directed to various departments ciliary body, potential spaces are formed between them. This Hanover channels (between the conditionally anterior and posterior selected ligaments) and Petit channel (between the posterior ligaments and the anterior surface of the vitreous). The use of scanning electron microscopy has contributed to a greater understanding of the structural features of the ligaments and their attachment to the lens.
The vast majority of fibers arise from the pars plana of the ciliary body anteriorly at a distance of 1.5 mm from the dentate line. Here they intertwine with the internal limiting membrane of epithelial cells or with the fibers of the anterior vitreous. Most fibers fold into bundles consisting of 2-5 fibrils. Some fibrils sometimes penetrate between epithelial cells. Fibrils are also found between pigmented epithelial cells of the ciliated epithelium and are woven into their basement membrane and Bruch's membrane.
The anterior ligaments extend until they reach the posterior edge of the appendicular part. Here they form a zonular plexus, which is located between the ciliary processes, and are attached to their lateral walls. Fibrils of the zonular sple The tenia are tightly attached at the base of the ciliary ridges, stabilizing the entire ligament system. Somewhat anterior to the process of the ciliary body, the zonular plexus is divided into three bundles of fibers heading to the anterior, equatorial and posterior capsule of the lens.
The nature of the preequatorial, equatorial and transequatorial attachment of the ciliary girdle is different.
- Preequatorial ligaments relatively dense. They are all attached at the same distance from the equator (1.5 mm) in the form of a double row of ligaments 5-10 µm wide. When attached, the ligaments narrow and flatten in the plane of the lens capsule, thereby forming ciliary plates.The anterior ligaments at the site of attachment give off thin fibrils (from 0.07 to 0.5 µm) to a depth of 0.6-1.6 µm into the capsule. The ciliary plate thickens from 1 to 1.7 microns.It is indicated that the number of anterior ligaments decreases with age. In this case, the insertions of the anterior ligaments are shifted to the center of the capsule.
- Equatorial fibers less. They, like the anterior and posterior ones, split in a brush-like manner when attached to the capsule. Fibrils are usually 10 to 15 µm wide, but can reach 60 µm.The posterior fibers are attached in two or three layers in a zone 0.4 to 0.5 mm wide. Anteriorly they are attached to the posterior edge of the equator of the lens, posteriorly they extend to approximately 1.25 mm from the edge of the equator. The fibers of the ciliary band are immersed into the lens capsule by approximately 2 microns.
- Post-equatorial fibers , at first glance, seem less developed than the front ones. This opinion is erroneous because they are attached to the capsule at different levels, including intertwining with the fibers of the anterior surface of the vitreous. The vitreous ligaments are a separate layer of fibers that connect the anterior part of the vitreous body with the flat and process parts of the ciliary body.
Streeten suggests that the mucus-like nature of the ciliary band acts as a barrier to the spread of substances between the posterior chamber of the eye and the vitreous.
Age-related changes in the ciliary girdle
During the embryonic period, its fibers are more delicate and less interconnected. They also contain more proteoglycans. In old age, the number of fibers decreases, especially meridional ones, and they break more easily. In the first two decades of life, the ciliary girdle inserts in the lens capsule are quite narrow. Over time, they expand and move towards the center of the lens capsule. At the same time, the ligament-free surface of the anterior lens capsule decreases from 8 mm at the age of 20 years to 6.5 mm in the 8th decade of life. Sometimes it is reduced to 5.5 mm, which significantly complicates capsulotomy during extracapsular cataract extraction.
During intracapsular cataract extraction, most of the ligamentous complex is removed from the capsule. Only the tips of the anterior zonular inserts and a certain amount of meridional fibers are preserved. The ciliary band is weakened by pseudoexfoliation of the lens capsule, which can cause its rupture during cataract removal.
Vision is one of the ways of understanding the world. The ability to see is largely controlled by the lens of the eye, which, having a simple structure, carries important functions. It allows you to quickly refocus from a close object to a distant one.
The structure of the eye can be compared to the optical system of a camera. And if the analogue of photographic film here is the retina, then instead of a professional lens system - the cornea and lens.
When light enters the eye, it first encounters and passes through the cornea. It is dome-shaped and characterized by a complete absence of blood vessels. Coming out of it, the light enters the so-called anterior chamber of the eye. Only after this stage comes the turn of the lens.
The structure of the “eye lens”
The lens is a lens that refracts light. Its optical power is 18 - 20 diopters, which is comparatively less than that of the cornea. Along the entire circumference there are ligaments, similar to knots of threads, that connect to the muscles of the walls of the eye.
These muscles have the ability to contract and relax, because of this the curvature of the lens changes and a person can see near and far.
The structure of the lens is somewhat reminiscent of a grape with one seed. It contains a capsular bag (or simply a shell), a core (having a high density) and lens masses (the density is much lower than that of the core), which are compared to grape pulp. As a person ages, the core becomes increasingly dense, which makes it difficult to see well up close.
Around the nucleus is the ciliary body, which is a continuation of the vessels. It has projections that produce fluid inside the eye. They penetrate through the pupil into and then into venous system.
What functions does the lens perform?
As mentioned above, this lens plays a significant role in the functioning of vision, therefore all functions of the lens are important:
- ensures the passage of light to the retina, which directly depends on the transparency of the lens;
- takes part in the refraction of the flow of light;
- activates an opportunistic mechanism that allows you to see either near or far;
- “works” as a partition that divides the eye into two sections of different sizes.
Lens diseases
This important part of the eye, like the entire body as a whole, is susceptible to various diseases. They can be caused by various reasons (deviations in development, changes in color or location, etc.). There are cases when the eye is injured, which carries the threat of rupture of the knitting threads, which requires urgent treatment.
There is a disease that requires replacing the lens with an artificial one - cataracts. With this disease, the lens becomes cloudy and the person stops seeing objects clearly. The causes of cataracts can vary, but most often age-related changes are to blame. The structure of the lens allows it to be replaced with an artificial one without affecting the rest of the eye, which guarantees minimal risks during the operation.
How the lens is replaced with an artificial one
Every person experiences fear when hearing the word “operation”. However, changing the lens lasts approximately 15 minutes and is carried out under local anesthesia. Immediately after it, the patient is observed in the hospital for a day, and then sent home, where he can watch TV and read the newspaper. The only restriction is that you are not allowed to carry weights weighing more than two kilograms for two weeks.
After instilling anesthetic drops (this is local anesthesia), the eye is fixed with a speculum. An ophthalmologist surgeon removes the cloudy lens through an incision in the cornea and puts an artificial one in its place. The operation is quite complex and requires jewelry work, but is still considered safe, since the lens does not come into contact with the rest of the eye.
Summarizing the lens
It consists of epithelial cells and has no blood vessels. Throughout life, transformations in its shape, size and transparency are observed. This change in the lens, which leads to clouding and blurred vision, is called a cataract and can be treated surgically.
The functions of the lens are compared to the optical lens in a camera and allow us to clearly see objects at different distances. At a young age, the lens is softer and more elastic, which allows you to see well. With age, it becomes more dense, which can lead to the development of cataracts. To protect yourself eye diseases, visit an ophthalmologist once every six months for preventive purposes.
