General description of the cornea
The cornea is a critical to the functioning of the vision eye component: it is the first structure that meets the light entering the eye. Its main role is to converge the incident light rays who then head through the anterior Chamber of the eye to the lens, before meeting the retina and initiate the Visual casecade. The cornea is a vascularized and transparent, curved geometry fabric. It is similar to a hemispherical Dome, and is often compared to a window located at the front of the eye, in direct contact with the air. The cornea is covered with the tear film and consists mainly of collagen fibers intertwined, which constitute 90% of the corneal thickness: this collagen tissue is the corneal stroma: it provides a large part of the transparency and the cornea's biomechanical resistance properties. It is covered with a sheet called epithelium, and lined on its inner face of a monolayer of cells called endothelial cells. These allow the cornea to maintain a constant, below 80%/mg, corneal hydration rate to avoid a reduction of corneal transparency which would then be caused by edema of the stroma.
The vertical diameter of the cornea is between 10 and 11 mm and the horizontal diameter between 11 and 12 mm. Its central average curvature RADIUS is 7.8 mm; towards the edges, this RADIUS increases (the cornea is aspherical). The Central average thickness of the cornea in the caucaseienne population is close to 530 microns (there are inter-racial variations, the corneal tissue being slightly thinner in the population of North Africa). The cornea is thicker on the outskirts, towards its edge that makes junction with scleral tissue and is called sclero-corneal limbe. It is central that the thickness of the cornea is minimal.
The histological description of the cornea assigns three main components to this fabric: theepithelium (covered in the tear film in contact air), the stroma, and theendothelium (in contact with the aqueous that bathes the anterior Chamber of the eye).
The thickness of the tear film (of the order of a micron) is negligible with respect to that of the epithelium (at the Center about 50 microns). The stroma is about 90% of the corneal wall thickness.
The cornea is a remarkable natural fabric in many ways: transparency, biomechanical resistance, Optical power… These properties are at the heart of the technical challenges of corneal refractive surgery, based on the interaction of laser light and the corneal tissue. Two laser are used:
The excimer laser allows to photoablater, i.e. to remove a thin layer of fabric (half a micron by impact), without thermal effects.
The femtosecond laser causes a "disruption" of the corneal tissue and cut with precision. These tools offer the ability to make specific changes to form: in fact, the study of corneal geometry and mathematical modelling of some properties are essential steps for the development of diagnostic techniques)corneal topography) and effective and innovative surgical correction (surgery laser, grafts, inlays).
The "in vivo" histological study of the cornea, made possible thanks to the confocal microscopyoffers a privileged point of view and a single micronique wide window: transparency of it allows you to visit its structures at the level of the cell, in order to better understand, diagnose and treat many corneal pathologies.
Thus, the degree of accuracy achieved today by the investigative techniques and surgical tools available to surgeons ophthalmologists is of the order of a micron, and in return stimulates a need for rigour in the study of the properties of the cornea.
Functional description of the cornea
The cornea provides 2/3 of the optical power of the eye structures (the 1/3 remaining being vested in the lens) - see the calculation of the power of the cornea paraxial optics. This important optical power stems from two characteristics: the cornea has a curvature greater than that of the lens (not pineapple) and above all, she is in contact with the air, offering the biggest difference of refractive index to the incident light rays (index of the cornea; 1,376, index of air: 1, equal to 0.376 difference: comparison index between the aqueous and the lens is about 10 times less). This strong gradient of index is specific to land animals, which have a cornea that is more arched than animals in the aquatic environment. The eyes of marine animals possess a cornea which is a resistant transparent membrane, but whose refractive power is very low, the water having a refractive index close of that of the corneal tissue: it's the lens that is given the role of bringing light to the retinal tissue (the lens of some fish is almost spherical, while it is at least for his earlier, less curved and elliptical face in humans). In the water, our eyes become farsighted, because the power of the cornea is neutralized due to the Equalization of the indices of refractions; that's why we see blurred at all distances when you open your eyes underwater (unless you are severely nearsighted!).
To allow the light information to be transmitted to the retina without loss in quality and quantity too prejudicial, the cornea must remain transparent, and maintain a certain regularity of curvature. Only 4% to 6% of the incident light is reflected from the corneal surface; This property of reflection is used for the conduct of the review of corneal topography specular. The image of reflection by the anterior surface of the cornea of a set by the eye point light source is called corneal reflection or image of Purkinje.
The light reflected by the cornea is "lost" to the retinal photoreceptors, and in the animal Kingdom, may consist of an undesirable phenomenon, because that can facilitate detection by a predator.
The cornea has the same Embryological origin as the skin tissue: However, in contrast to the skin, it is remarkably transparent. This property is the result of several features: the tear film that covers the cornea represents a particularly smooth thin, because constantly renewed and distributed on the surface of the cornea by eyelid during blinking. Corneal epithelium, which is the most superficial histological structure of the cornea is a layer of compact cells, whose refractive index is homogeneous. The corneal stroma, which is the main component in terms of thickness, consists of a regular arrangement of collagen lamellae (one would expect about 200 to 250 strips of roughly rectangular section from the surface to the depth, each being about 2 microns thick), between which are arranged in flattened fibroblastic cells (keratocytes). The collagen fibers consist of fibrils of collagen, whose diameter is between 32 and 36 nm, and 25 to 50 nm spacing. Corneal transparency is partly conditioned by the regularity of the provision of these elements, smaller than the wavelengths of the visible spectrum.
The endothelium is a monolayer of contiguous cells, which are responsible for maintaining the cornea in a State of relative dehydration; in case of failure of these celllules (acquired or congenital, as in the Fuchs dystrophy), appears an edema which one of the consequences is to reduce the transparency of the cornea (corneal architecture disorganization).
Innervation of the cornea
The density in sensory nerve endings is another remarkable feature of the cornea: this innervation is the densest of the tissues of the body. The corneal tissue is 100 times more sensitive than the conjunctiva. This explains the difficulty to tolerate any "dust" on the surface of the cornea, sometimes complicating the adaptation of some lenses. Fortunately, the anesthesia of these endings is possible thanks to simple drops of anesthetic eye drops.
Corneal nerve fibers come from the ciliary nerves (anterior Branch), of which 60 to 80 trunks penetrate the cornea in a radiaire direction at the level of the lamina in the stroma deep and then become more superficial to form a "network" or "plexus" under the corneal epithelium. On the other hand, the endothelium and Descemet membrane are devoid of innervation. The neurotransmitters involved in corneal sensitivity include: acethylcholine, catecholamines, substance P, neuropeptide Y, intestinal peptide, etc.
Anatomy of the cornea
The cornea is composed of several layers of tissues of different nature, which should ensure optical functions of refraction and transparency, while satisfying the metabolic requirements of living tissue.
Corneal epithelium plays an important role, both optical and protection (barrier). It is in contact with the tear film, which is the first interface encountered by incident light. The epithelium is stratified (several layers of cells stacked) and composed of squamous non keratinized cells. Its central thickness is approximately 50 microns; This thickness increases towards the edges of the cornea. Corneal epithelial cells are joined, in order to avoid that the water of tears (tear film) enters in the corneal stroma. This epithelium is in constant renewal: the renewal is assured by a contingent of "stem cells", called limbal stem cells as located at the junction of the cornea and the conjunctiva. These cells proliferate and differentiate into epithelial cells, migrating from the edges toward the Center, then height, to the surface of the epithelium. During these processes, the epithelial cells and their core flatten out gradually, and when they occupy the most superficial part of the epithelium, cover of micrivollosites (in contact with the tear film). The microvilli allow to increase the contact surface between the superficial epithelium and tears.
Surface cells then sloughed in the tear film. It takes between one and two weeks to a stem cell to perform a complete migration cycle. The cells in the basal layer of the epithelium (in contact with the layer of Bowman) secrete a basal membrane thickness is close to 50 nanometers, which is made up of collagen type IV, laminin and other protein components.
During the technique of laser correction PKR (to sight refractive procedures), the epithelium is removed on an area of about 8 to 9 mm in diameter. Epithelial growth requires a duration of 4 to 5 days.
Layer of Bowman
This layer has a thickness of 8 to 14 microns. It consists of fibilles entralacees in a mucoproteinique matrix collagens. These fibrils are 20 to 25 nm in diameter, without preferred orientation. The layer of Bowman is not a membrane individualized, but a transition to the underlying stroma area; its boundary with the stroma is not clearly identifiable. The role of Bowman layer remains uncertain. Partial disappearance after PKR, its circular section device (flap of) LASIK) does not seem to induce particular anomaly.
The stroma is the main layer of the cornea, which it occupies about 90% of the thickness. It is composed of cells (keratocytes) and extra cellular matrix that contains of many macro molecules, whose harmonious arrangement is the guarantor of maintaining the transparency of the corneal tissue. The dimensions of the keratocytes vary within the corneal stroma. Similarly, their density decreases (stroma) corneal surface to deeper layers. These keratocytes digest and produce collagen constitutive of the extra matrix cell tissue.
The most superficial stroma is formed by the posterior part of the Bowman layer, which is composed of an extracellular matrix made of collagen and proteoglycans. Collagens involved in the formation of the acellular layer are fibrils of types I and V, where intermesh type VI collagen filaments. The main Proteoglycans are the decorin and the lumican (respectively associated with the dermatan sulfate and keratan sulfate).
The collagen fibrils are between 25 and 35 nm in diameter, and are organized into flattened parallel beams known as 'strips' or 'fibre '. The corneal stroma is composed of a stack of 200 to 300 strips of collagens interlaced; the deepest lamellae thickness is of the order of 2.5 microns.
The most superficial corneal strips are short, and intertwined, interconnected by bridges, so that in the posterior stroma, these strips are long, thick, and traverse the entirety of the corneal diameter. Recent works show that there are arched fibers that connect the layer of Bowman and the deeper fibers. The fibers are connected. : form a network, which encapsulates the anterior cornea and contributes to the rigidity of it.
This structural aspect is certainly an evolutionary trait that participates in the form of hemispherical dome of the cornea. Marine animals (ex: white shark), where the corneas are little arched (lesser refractive role due to the low difference in index of refraction between the water and the stroma), ultra structural studies reveal a more simple structure: corneal stroma lamellae are parallel, and connected by vertical fibers that anchor the lamellar layers.
The affected corneas of Keratoconus study that previous fibers seem to cater to a simpler, less interlaced model. The biomechanical rigidity of the cornea is altered, which makes it more prone to responsible for deformations of topographic irregularity.
The keratocytes are the family of the fibroblast cells: these cells are flat, located between the slats. They synthesize collagen and the extra cellular matrix of the corneal tissue.
The extra cellular matrix filled the space between the fibrils, lamellae (fiber) and the keratocytes. It contains the Proteoglycans are macromolecules as the glycosaminoglycane.
The elasticity of the corneal tissue is low: considered that the strain linked to intra ocular pressure is only 0.25% in physiological conditions.
Corneal hydration is an important parameter: its rate is close to 78%. This rate is largely conditioned by the integrity of the endothelial and epithelial barriers. At the level of the endothelium occur of the phenomena of active pumping, through mechanisms of ionic pumps dependent on enzymes of type ATPase.
The transparency of the corneal stroma is the result of arranging regular and harmonious of the collagen fibrils, whose spacing is a few tens of NM, which allows to generate destructive light interference in the lateral directions to the main direction of propagation of light waves (reduction of light broadcasting).
The stroma is the layer that is cut and carved in corneal refractive surgery. Its thickness determines in part the possibilities offered by LASIK (see:) LASIK and corneal thickness).
It corresponds to the basal membrane of the endothelium, which is the deepest cell layer of the cornea. It is produced continuously during the existence, which contributes to its thickening over time. In children, its thickness is of 5 microns, while it can reach 15 microns after 60 years. It consists of two layers: the blade earlier (3 Micron), which is formed during Embryological Development made up of collagen, and the rear blade, which is the layer secreted by the endothelial cells.
Descemet membrane is devoid of elastic fibers, but yet it has elastic properties that make that detached from the stroma, the memrbane of Descemet rolls up on itself. This is commonly observed in endothelial transplant surgery (DMEK).
The endothelium is made up of a mosaic of contiguous cells with hexagonal contour. This particular distribution is related to an optimization of the paving of the posterior face of the cornea: the hexagonal perimeter ensures a maximum density of cells in contact, for a minium of perimeter of cell membranes. We find this type of arrangement at the level of internal articles of retinal photoreceptors, and in nature within the nests of beehives. This provision ensures maximum contact between the surface of the cells and the above stroma. The hexagonal cells of contour percentage can be estimated during the measurements by specular microscopy of the endothelium. Its decrease is observed in some endothelial pathologies. Endothelial cells have a relatively uniform size, but case of endothelial disease, they can introduce variations of surface, estimated by the calculation of a coefficient of variation (CV).
Endothelial cells are fitted with a cellular pump system which allows you to reject the water molecules that can stream passively in the cornea to the aqueous; This allows to control the State of hydration of the cornea, and so its transparency.
Unlike the epithelial cells, endothelial cells do not proliferate in vivo, and though there might be a contingent of endothelial stem cells, their density declines during the existence. More than 6000 cells by mm2 at birth, endothelial density declines up to 2000 to 3000 cells per mm2 (loss estimated at 0.5% per year). This decline is characterized by an enlargement of the cells, in order to cover the posterior corneal surface. The density of endothelial cells is more important in the periphery to the center of the cornea.
Maintaining a sufficient quota of endothelial cells is necessary for maintaining an optimal rate of hydration of the cornea. Some dystrophy as the Fuchs dystrophy are characterized by the presence of corneal edema because there is a significant loss in endothelial cells (ex: cornea also). This edema is the origin of the Visual symptoms of the disease.
Descemet membrane is the basal membrane of the corneal endothelium, which secretes her; its thickness is about 12 microns in adults. Over time, the membrane material can accumulate in some places. On the periphery of the cornea endothelial deposits can be observed in adults: they are called "body of Hassall - Henle. The realization of repositories (aspects of "drops") makes the diagnosis of cornea also. In as endothelial dystrophy case the cornea also / Fuchs dystrophythe cornea thickens as stromal edema is a result of the epithelial malfunction.
Recently, a new anatomical entity, the "layer of Dua', has been identified within the corneal tissue: she received the name of the author of the publication scientific principles (Professor Harminder Dua). This layer would be between stroma and Descemet membrane; She may have an interest in deep corneal transplant surgery.