CORNEA

From Big Medical Encyclopedia

CORNEA [cornea (PNA, JNA, BN A); synonym cornea] — forefront of a fibrous cover of an eyeglobe. River — avascular, highly sensitive, transparent, optically a homogeneous cover with a smooth, specular, brilliant surface; has spherical shape. The river occupies about 1/6 parts of the space of a fibrous cover. The translucent zone of transition of R. to an opaque sclera carries the name of a limb (limbus). To a limb, width to-rogo makes 1 m, there corresponds the deep circular furrow of a sclera (sulcus sclerae) serving as conditional border between R. and a sclera.

Except the protective and basic functions inherent to a fibrous cover of an eyeglobe, R. is the main refracting environment of its optical system. The refracting R.'s force makes apprx. 40,0 dptr. Force of refraction in a vertical meridian is slightly more, than in horizontal that causes so-called fiziol. an astigmatism (see. Astigmatism of an eye ). Horizontal diameter of R. on average 11 mm, vertical — 10 mm, thickness of the central part it apprx. 0,9 mm, on the periphery apprx. 1,2 mm. R.'s curvature surpasses curvature scleras (see): radius of curvature of a front surface of R. is equal to 7,7 mm, scleras — 11 mm.

Fig. 1. The diagrammatic representation of a histologic structure of a cornea is normal (cross section): I \front epithelium; II \boumenova cover; III \stroma of a cornea; IV \destsemetova cover; V \endothelium of a cornea; 1 — fibroblasts of a cornea; 2 — the main substance of a stroma; 3 — a nerve of a cornea.

The river consists of five layers located outside inside in the following sequence (fig. 1): front epithelium (epithelium ant.), boumenova cover (front boundary plate, T., lamina limitans ant.), stroma of a cornea (own substance, T., substantia propria corneae), destsemetova cover (back boundary plate, T., lamina limitans post.), an endothelium of a cornea — endothelium sog-neale (a back epithelium, T., epithelium post.).

The cornea is formed after an otshnu-rovyvaniye of a rudiment of a crystalline lens from a cover epithelium. R.'s stroma arises from a mesoderm, and the boumenova a cover is formed of its front layers. Destsemetova a cover is derivative an endothelium, and an endothelium of a cornea, as well as an epithelium, has an ectodermal origin.

A front epithelium — the surface layer of R. which is continuation conjunctivas (see), has thickness of 40 — 50 microns. Cells of an epithelium are located in five — seven layers and closely adjoin to each other, connecting tonofibrils and intercellular bridges. Superficial epithelial cells form microfolds and microvillis, spaces between to-ry-mi are filled with the precorneal film consisting of the lacrimal liquid, a secret of meibomian glands (glands of a cartilage a century, T.) and slime. The deepest layer of a front epithelium consisting of cylindrical cells is called basal. Reproduction of its cells provides regeneration of a front epithelium. In intervals between basal cells separate leukocytes, quantity meet to-rykh can sharply increase at inflammatory processes in the River.

The front epithelium performs protective function and is the important regulator of a metabolism in the River. It is protected from influence of the environment by a precorneal film, cover disturbance of integrity, napr, at a dry keratoconjunctivitis, precedes it patol. to changes, as nitkovidny opacifications of Shirmer.

Fig. 2. The diagrammatic representation of structure of layers of a cornea is normal: and — front layers of a cornea: I \a front epithelium, II — a boumenova a cover, III — superficial plates of a stroma of a cornea, 1 — a surface of epithelial cells from mikrovorsina, 2 — the nerve of a cornea getting into an epithelium through a boumenova a cover, 3 — a schwannian cover of a nerve of a cornea, 4 — a lymphocyte, 5 — cells of a basal layer of a front epithelium, 6 — fibroblasts, 7 — collagenic fibers (plates); — elements of a stroma of a cornea: 1 — fibroblasts of a cornea (their relative positioning is below shown on the right), 2 — collagenic plates; in — back layers of a cornea, I — deep plates of a stroma of a cornea, II — a destsemetova a cover, III — an endothelium of a cornea, 1 — a surface of endothelial cells, 2 — a kernel of an endothelial cell, 3 — collagenic fibers (plates).

Boumenova the cover represents an unstructured homogeneous membrane, rykhlo connected with a basal layer of a front epithelium (fig. 2, a). According to a submicroscopy, a boumenov the cover consists of collagen, the main substance and does not contain cells. Thickness it in the center P. makes 8 — 14 microns. Long thin collagenic fibrilla, diameter to-rykh apprx. 25 nanometers, are randomly disseminated in a boumenovy cover and in its deep departments mix up with thicker collagenic arcadian plates of a stroma of R. Boumenov the cover throughout is penetrated by nervous branches, lengthways to-rykh at patol. processes leukocytes and edematous liquid get into R. from a stroma in a front epithelium. Feature of a boumenovy cover is good resilience at injuries and weak at infectious processes.

The stroma of a cornea is its main component — it borrows apprx. 9/10 all thickness of R. (thickness of a stroma in the center P. reaches 500 microns). R.'s stroma is located under a boumenovy cover and merges with it without the expressed border; consists of collagenic plates, fibroblasts (keratotsit; a synonym corneal little bodies) and the main substance, and R.'s fibroblasts make apprx. 5% of its volume. Collagenic plates are located in parallel each other and surfaces of a cornea. In different layers of a stroma collagenic plates go under various corners. Between them there are R.'s fibroblasts contacting with each other long shoots (fig. 2, b). The parallel arrangement, identical index of refraction of components of a stroma substantially provides the most important property of a cornea — its transparency.

Destsemetova a cover — a homogeneous, transparent, elastic membrane. It is not connected densely with a stroma and at many patol. states easily exfoliates from it, forming folds. In the field of a limb of a destsemetov the cover razvoloknyatsya and takes part in formation of a skeleton of trabeculas of an iridokornealny corner — angulus iridocornealis (trabecular network of an angle of iris, T.). Destsemetova a cover consists of very thin collagenic fibrilla, to-rye are distributed quite evenly and connect with each other in nodules (fig. 2, c). Thickness of a descemete cover increases with age. At newborns it makes 3 — 4 microns, at adults, according to Hougen (M. of Hogan, 1973), increases to 10 — 12 microns. Destsemetova a cover is much steadier against infectious processes, than against injuries, has ability to regeneration. On the tinktorialny properties has looking alike the capsule crystalline lens (see) and vitreous membrane.

R.'s endothelium covers its back surface in the form of one layer of cells and is washed by intraocular liquid. It consists of 500 thousand polygonal form of cells 5 microns high and 18 — 20 microns wide. Kernels of cells of an endothelium are located is central, have the oval form, diameter of their 7 microns (fig. 2, c). Cytoplasm of cells of an endothelium contains a large number of mitochondrions and free ribosomes. To covers of endothelial cells prilezhat numerous pinotsitozny bubbles. On a back surface of an endothelium of R. cellular membranes form from 20 to 30 cellular microshoots 0,5 — 0,6 microns high and 0,1 — 0,2 wide mkmvystupayushchy in an anterior chamber of an eye. Cells of an endothelium of R. fasten to the basal membrane (a descemete cover) by means of the cytoplasmatic shoots entering the cracks which are available in it.

Safety of an endothelium of R. has big fiziol. value for normal functioning R. Endotely represents «the pump» double action, providing a nasasyvaniye and pumping out of substances from R., and is some kind of barrier. Disturbance of its barrier function is caused by injuries, operative measures, a viral infection, etc. and often leads to development of an edematous violent keratopathy with the subsequent opacification R. Endotely R. has ability to regeneration, edges is carried out thanks to expansion next with damaged and wandering cells.

The river does not contain blood vessels, only surface layers of a limb have regional vascular network (a perilimbalny circulatory texture), for the account a cut generally and food of River is carried out. A certain value in maintenance of viability of R. belongs to nutrients, pronikagoshchshm in it from moisture of an anterior chamber of an eyeglobe and the lacrimal liquid.

The river is very rich with nerve terminations. Along with the sensitive innervation which is carried out by a trifacial (see. Corneal reflex ), in R. there is a sympathetic innervation performing trophic function.

At newborn R. has the smaller vertical and horizontal sizes, than at the adult. By 11 — 12 years the sizes P., its curvature and optical power reach average parameters of an eye of the adult (see. Eye ). At advanced age in R. the amount of moisture and vitamins decreases, globulinovy fractions of proteins prevail, salts of calcium and lipids are laid. First of all the area of a limb changes. The lack of hepatoflavin causes growing of vessels into R. on the periphery, its refracting force and sensitivity decreases. In comparison with persons of young age also decrease more than by 1,5 times of quantity of cells of an endothelium of R. is noted that creates premises for more frequent development of endothelial and epithelial edematous dystrophies.

The pathological anatomy

Pathology of a cornea is shown first of all in its opacification representing the infiltrate accompanying the active process and cicatricial changes which are the result of various processes in R.

Infiltrat represents a zone of damage of R. and consists usually of cells of type lymphoid and a small number of mast cells with eosinophilic granularity in cytoplasm. The size of infiltrates, their form and depth of an arrangement in R. are various. To infiltrate often there corresponds defect of a front epithelium and R.'s stroma, however at nek-ry forms of an inflammation defect is absent. At purulent inflammatory processes in R. to the infiltrate getting a yellowish shade R.'s fibroblasts and cells like polinuklear-ny leukocytes with piknotichny kernels in the form of purulent little bodies migrating from regional vascular network of a cornea are added.

At virus keratitis (see) a pathoanatomical picture of changes in R. has the features. At primary herpetic keratitis inflammatory reactions with dominance of altera-tivno-necrotic and exudative changes develop. At a recurrent keratitis proliferative changes prevail, the cellular basis to-rykh is made by lymphocytes and plasmocytes. In addition to cellular infiltration, at a research of the disks P. removed in time keratoplasty (see), find the phenomena of hypostasis, a necrosis and neogenic vessels. The inflammatory phenomena are followed by the expressed disturbance of trophic processes of River. Dystrophic changes in R. are shown in the form of destruction of the main substance of a stroma and its collagenic structures.

Cicatricial opacifications consist of the fibrous cicatricial fabric replacing cellular infiltrate or arising in the course of substitution of defect of fabric P. On the density cicatricial opacifications have various appearance and are classified as a cataract (leucoma), a spot (macula) or a cloudlet (nubecula). Depending on an arrangement and expressiveness they can reduce visual acuity in various degree (see. Cataract ).

Methods of inspection

Methods of inspection include R.'s survey at side lighting with use of two magnifying glasses, biomicroscopy (see. Biomicroscopy of an eye ), a keratoskopiya (see. Ophthalmometry ), keratografiya (see), mirror contact and contactless microscopy of an endothelium by means of a mirror endothelial microscope, definition of permeability of R. by means of a local electrophoresis with flyuorestseiny (keratopenetroflyuorimetriya), and also definition of sensitivity of R. various algezimetra (metal hairs of different diameter).

A specular brilliant and wet surface — one of properties of the normal River. Thanks to it receiving a mirror image of various objects placed before an eye is possible. Use of various devices (keratoskop, fotokeratosko, oftalmometr is based on this principle, etc.), with the help to-rykh define a form, curvature and a refraction of a cornea. Fotokeratoskop — the device created on the basis of Placido's keratoskop, allowing to photograph rings of a keratoskop on a film and in the subsequent to carry out the mathematical analysis of the received image.

See also Inspection of the patient, ophthalmologic .

Pathology

R.'s Pathology is very different. Allocate malformations, damages, inflammatory diseases, dystrophies, tumors. At advanced age the so-called senile arch — arcus senilis having an appearance of ring-shaped opacification of the grayish color which is usually separated from a limb by a narrow transparent zone meets. Pathology is connected with adjournment in fabric P. of lipids, first of all cholesterol. Intensive opacification, similar to a senile arch, in a peripheral zone P. (arcus juvenilis) is observed also at young people.

The ectasia (stretching) of R., partial or full, develops at various anomalies and patol. processes — hydrophthalmia (see), secondary posttraumatic to glaucoma (see) etc.

Malformations. Carry cases of increase (megalo-cornea) and reduction (microcornea) of size P to them. These changes can sometimes have the acquired character (e.g., at a hydrophthalmia, an atrophy of an eyeglobe). The combination of increase and change of a form P. is observed at a keratoglobus and a keratoconus.

The keratoglobus (keratoglobus) — the anomaly of development of R. which is characterized by spherical shape is noted by protrusion and R. Neredko's thinning at the same time increase in diameter R. Keratoglobus usually is combined with short-sightedness (see) and an astigmatism, is followed by decrease visual acuities (see). Defeat is bilateral, comes to light at children's age and has tendency to slow progressing. The keratoglobus is korrigirut by means of contact lenses, an effective method of its treatment — R.'s change (see. Keratoplasty ). At timely diagnosis and adequate treatment the forecast favorable.

Fig. 3. Outward of an eye at a keratoconus: the arrow specified a cornea, shaped a cone.

The keratoconus (keratoconus) — a cone-shaped cornea, meets more often than a keratoglobus. The etiology of a keratoconus is not found out. A certain significance in development it is attached to heredity and an allergic factor. The disease is shown usually at young and youthful age, affects both eyes, has the progressing character. The river at this disease gets a form of a cone, the top to-rogo not always matches its center (fig. 3). The keratoconus develops gradually, without the inflammatory phenomena. The first symptom of a disease is decrease in visual acuity owing to development of an astigmatism, degree and the direction of axes to-rogo periodically change. The anterior chamber of an eyeglobe goes deep, at biomicroscopy thinning and striate opacifications of R., tending to slow progressing are defined. Perhaps, however, development of the acute keratoconus («an edema of a cornea») coming owing to a sudden rupture of a descemete cover and penetration of moisture from an anterior chamber of an eye in a stroma of River. The differential diagnosis at an acute keratoconus, first of all, is carried out with discal keratitis (see).

In initial stages of a disease high visual acuity can be reached by means of contact lenses. At an acute keratoconus are effective introduction of an autoblood to an anterior chamber of an eye and bandage strengthening of a cornea a silicone lens. In the developed stages of the progressing keratoconus when contact lenses do not increase visual acuity, the good effect can be gained by means of a through keratoplasty.

Congenital anomaly of R. is so-called embryotoxon (embryotoxon) — the ring-shaped opacification of edge of R. resembling a senile arch; is not subject to treatment. Also anomalies of pigmentation of P are described. (melanosis corneae) — an issue of treatment in these cases is resolved individually. R.'s dermoid — the high-quality education which is localized generally on the periphery in the field of a limb in the form of a semi-spherical dense tumor belongs to rare anomalies. Treatment operational.

Damages of a cornea can be caused by action of mechanical, thermal, volumetric and other disturbing factors (see. Eye ; Foreign bodys, eyes ).

Diseases. Inflammatory diseases of a cornea — the most frequent type of pathology which is differing in a big variety a wedge, forms and being one of the main reasons for decrease in visual acuity and a blindness (see. Keratitis ). Among inflammatory diseases a virus keratitis, in particular herpetic is most widespread (treelike, metaherpetic, discal). A herpetic keratitis differs in a heavy, recurrent current. Their treatment is based on use of antiviral means, carrying out microsurgical interventions and an early medical keratoplasty. For the prevention of a recurrence of a herpetic keratitis with success use a domestic antiherpetic polyvalent vaccine.

The combined defeat of a cornea and conjunctiva — a keratoconjunctivitis is often observed. The viral infection is the most frequent reason of a keratoconjunctivitis (adenoviruses, picornaviruses, a virus of herpes, an atypical virus of trachoma p other). The adenoviral keratoconjunctivitis is most widespread among Sander's follicular diseases. Also bacterial and allergic keratoconjunctivites are frequent (see. Conjunctivitis ).

Dystrophies (degeneration) divide into two groups: primary, to-rykh local and general frustration of a metabolism with adjournment in R. are the cornerstone of products patol. exchange (nodular, spotty, trellised, speckled, endothelial and epithelial, limy, fatty, tape-like, regional, etc.); secondary, developing after the postponed keratitis, injuries, eye operations (a violent edematous keratopathy). Medicinal treatment of dystrophies of a cornea is a little effective. At their progressing which is followed by considerable decrease in visual acuity, the operational method of treatment is of great importance.

Tumors of a cornea meet seldom. Carry to high-quality new growths of R. epithelial a hyperplasia and papilloma, to malignant — planocellular cancer, is more rare the sarcoma developing in areas of a limb. Treatment of the high-quality progressing new growths operational with the help diathermocoagulations (see), keratoplasty (see), malignant new growths — operational (keratoplasty) in combination with radiation therapy (see).

Operations on a cornea widely apply as to treatment of the diseases and their outcomes which are followed by R.'s opacification (see. Keratoplasty , Keratoprotezirovaniye ), and for correction of anomalies of a refraction (see. Keratomilez , Keratofakiya ). Actively the refraction keratoplasty develops (a radial keratotomy, an inter lamellar change of biological lenses, etc.).



Bibliography: Arkhangelsk V. N. Eye diseases, M., 1969; Belyaev V. S. To cornea operation and sclera, M., 1981; Eye diseases, under the editorship of T. I. Eroshevsky and A. A. Bochkaryova, M., 1977; Kowalewski E. I. Eye diseases, M., 1980; Krasnov M. M., etc. Mirror microscopy of a back epithelium of a cornea, Vestn. oftalm., No. 1, page 31, 1982, bibliogr.; The multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 1, page 206, M., 1962; N. A. Bunches Skye, etc. Bases of change of a cornea, Kiev, 1971, bibliogr.; Hogan M. J., Alvarado J. And. a, Weddell J. S. Histology of the human eye, an atlas and textbook, Philadelphia, 1971; Kaye G. I., Sibley R. G. a. H o e f 1 e F. B. Recent studies on the nature and function of the corneal endothelial barrier, Exp. Eye Res., v. 15, p. 585, 1973, bibliogr.; Sugar A. Clinical specular microscopy, Surv. Ophthal., v. 24, p. 21, 1979, bibliogr.


A. A. Kasparov.

Яндекс.Метрика