Cornea is front convex, transparent, avascular plate of the eyeball, which is direct continuation of the sclera. Human cornea is approximately 1/6 of the outer layer of the eye. It looks like a convex-concave lens, in place of transition into the sclera (limbus) it has the form of translucent ring up to 1 mm. Its because the deeper layers of the cornea posteriorly extend slightly further than the front.
The diameter of the cornea is almost absolute constant at 10 ± 0,56 mm, however, the vertical size is usually 0.5-1 mm smaller than the horizontal. Its thickness in the center is about 450-600 microns, and at the periphery ≈650-750 microns. This index also correlates with age: in 20-30 years old corneal thickness is 0.534 and 0.707 mm, and in 71-80 years - 0.518 and 0.618 mm.
The distinctive properties of the cornea:
- Sphericity (radius of curvature of the anterior surface ~ 7.7 mm and 6.8 mm of the posterior)
- Shiny mirrored
- Has high tactile and pain sensitivity but low temperature sensitivity
- Refracts light rays with a power of 40-43 diopters
Cornea is the optic structure, its refractive power in infants is on average 45D (diopters), and to 7 years as an adults - about 40D. Corneal refractive power in vertical meridian is slightly more than the horizontal (physiological astigmatism).
- Horizontal diameter in adults - 11 mm, neonatal - 9 mm
- Vertical diameter - 10 mm, neonatal - 8 mm.
- Center thickness - 0.4-0.6 mm, peripheral - 0,8-1,2 mm.
- The radius of curvature of the anterior surface of the cornea in adults - 7.5 mm, neonatal - 7 mm.
Growth of the cornea is carried out by thinning and stretching the tissue.
Composition of the cornea
The composition includes water, mesenchymal origin collagen, mucopolysaccharidoses, proteins (albumin, globulin), lipids, vitamins. Corneal transparency depends on the validity of the arrangement of elements and their indices of refraction as well as on its water content (normally up to 75%, increasing of the water content in excess of 86% leads to corneal opacities).
Corneal changes in advanced age
- decreases the amount of moisture and vitamins
- globulin protein fractions prevail over the albumin,
- deposite calcium salts and lipids
Therefore primarily change the transition area of cornea to sclera - limbus: the superficial layers of the sclera are looming on the cornea and the inner layers are slightly behind; cornea becomes like glass, inserted in the rim of watches. Due to metabolic disorders, formed a so-called "arcus senilis", which reduced corneal sensitivity.
The structure of the cornea
- Corneal epithelium: an exceedingly thin multicellular epithelial tissue layer (non-keratinized stratified squamous epithelium) of fast-growing and easily regenerated cells, kept moist with tears. Epithelial thickness is 0.04 mm. Epithelium has a protective function and is a regulator of the water content of the cornea. The corneal epithelium, in turn, is protected from the external environment so-called liquid or basal layer.
- Bowman's layer (also known as the anterior limiting membrane): is a tough layer composed of collagen (mainly type I collagen fibrils), laminin, nidogen, perlecan and other HSPGs that protects the corneal stroma. Can be described as an acellular, condensed region of the apical stroma, composed primarily of randomly organized yet tightly woven collagen fibrils. These fibrils interact with and attach onto each other. Due to its low level of metabolism it's not capable to regenerate, so where will be turbidity after any injury.
Center thickness is about 20 micrometres, while at the periphery is eight to 14 micrometres thick.
- Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly arranged collagen fibers along with sparsely distributed interconnected keratocytes, which are the cells for general repair and maintenance, also there are solitary wandering cells - fibroblasts and lymphoid elements, which perform a protective function. They are parallel and are superimposed like book pages. For a better connection of the layers between them there is a mucoprotein.The corneal stroma consists of approximately 200 layers of mainly type I collagen fibrils. Each layer is 1.5-2.5 μm. Up to 90% of the corneal thickness is composed of stroma.
- Descemet's membrane (also posterior limiting membrane): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium, from which the cells are derived. This layer is composed mainly of collagen type IV fibrils, less rigid than collagen type I fibrils, and is around 5-20 μm thick, depending on the subject's age. Just anterior to Descemet's membare, a very thin and strong layer, the Dua's Layer, 15 microns thick and able to withstand 1.5 to 2 bars of pressure, may exist according to one study.
- Corneal endothelium: a simple squamous or low cuboidal monolayer, approx 5 μm thick, of mitochondria-rich cells. These cells are responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. (The term endothelium is a misnomer here. The corneal endothelium is bathed by aqueous humor, not by blood or lymph, and has a very different origin, function, and appearance from vascular endothelia.) Unlike the corneal epithelium the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells which reduces the overall cell density of the endothelium, which has an impact on fluid regulation. If the endothelium can no longer maintain a proper fluid balance, stromal swelling due to excess fluids and subsequent loss of transparency will occur and this may cause corneal edema and interference with the transparency of the cornea and thus impairing the image formed. Iris pigment cells deposited on the corneal endothelium can sometimes be washed into a distinct vertical pattern by the aqueous currents - this is known as Krukenberg's Spindle.
Physiology of the cornea
Corneal temperature is about 10 ° C below body temperature, which is caused by direct contact with moist surface of the cornea and external environment, as well as the lack of blood vessels. When the eyelids are closed corneal temperature nearby the limbus is about 35,4 ° C and 35,1 °C in the center (with eyelids open ~ 30 ° C).
Therefore it is possible the growth of mold fungus which caused a specific keratitis.
Cause of absence of lymph and blood vessels, metabolism and nutrition of the cornea occurs by diffusion and osmosis (due lacrimal fluid of the anterior chamber and moisture from pericorneal blood vessels).
Lack of blood vessels in the cornea is compensated by abundant innervation, which is represented by trophic, sensory and autonomic nerve fibers. Metabolic processes in the cornea are regulated by trophic nerves extending from the trigeminal and facial nerves.
High corneal sensitivity provided by the system of long ciliary nerves (from the ophthalmic branch of the trigeminal nerve) forming perilimbal plexus around the cornea. Entering into the cornea, they lose the myelin sheath and become invisible. In cornea formed three tiers of nerve plexus - in the stroma, beneath the Bowman's layer and subepithelial. The closer to the surface of the cornea, nerve endings become thinner and their intertwining becomes denser. Almost each cell of the corneal epithelium provided a separate nerve ending. This explains high tactile sensitivity of the cornea and sharply expressed pain in outcrop sensory endings (epithelial erosion).
The high sensitivity of the cornea underlies its protective function. Even with a light touch to the surface of the cornea occurs unconditional corneal reflex - eyelids close, eyeball rotates upward, removing the cornea from the danger, appears lacrimal fluid washes away the dust particles.
Afferent part of corneal reflex arc carries the trigeminal nerve, the efferent - facial nerve. Loss of corneal reflex occurs in severe brain lesions (shock, coma). Disappearance of corneal reflex is an indicator of depth anesthesia. Reflex disappears in some lesions of the cornea and the upper cervical part of spinal cord.
Fast forward reaction of edge looped vessels network to any irritation of the cornea is due to the fibers of the sympathetic and parasympathetic nerves present in perilimbal plexus. They are divided into two closure, one of which extends to the walls of the vessel and the other penetrates the cornea and is contacted with an extensive network of the trigeminal nerve. They are divided into two endings, one of which extends to the walls of the vessel and the other penetrates the cornea and contactes with an extensive network of the trigeminal nerve.