Cornea
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{{Infobox Anatomy |
Name = Cornea |
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Image = Schematic diagram of the human eye en.svg |
Caption = Schematic diagram of the human eye. (Cornea labeled at center top.) |
Image2 = Gray871.png |
Caption2 = Vertical section of human cornea from near the margin. (Waldeyer.) Magnified. <BR>1. [[Epithelium]]. <BR>2. [[Bowmang's membrane|Anterior elastic lamina]]. <BR>3. [[substantia propria]]. <BR>4. [[Descemet's membrane|Posterior elastic lamina]]. <BR>5. [[Endothelium]] of the [[anterior chamber]]. <BR>a. Oblique fibers in the anterior layer of the [[substantia propria]]. <BR>b. Lamellæ the fibers of which are cut across, producing a dotted appearance. <BR>c. [[Corneal]] corpuscles appearing [[fusiform]] in section. <BR>d. Lamellæ the fibers of which are cut longitudinally. <BR>e. Transition to the [[sclera]], with ass more distinct fibrillation, and surmounted by a thicker [[epithelium]]. <BR>f. Small bloodvessels cut across near the margin of the cornea. |
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DorlandsPre = c_55 |
DorlandsSuf = 12259559 |
}}
The '''cornea''' is the [[transparency (optics)|transparent]] front part of the [[eye]] that covers the [[Iris (anatomy)|iris]], [[pupil]], and [[anterior chamber]]. Together with the [[Lens (anatomy)|lens]], the cornea [[refraction|refracts]] light, and as a result helps the eye to [[focus (optics)|focus]], accounting for approximately 80% of the eye's [[optical power]].<ref name="Cassin">Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainsville, Florida: Triad Publishing Company, 1990.</ref><ref name="Goldstein">Goldstein, E. Bruce. ''Sensation & Perception''. 7th Edition. Canada: Thompson Wadsworth, 2007.</ref> In humans, the refractive power of the cornea is approximately 43 [[dioptre]]s.<ref>Najjar, Dany "[http://www.eyeweb.org/optics.htm Clinical optics and refraction]"</ref> While the cornea contributes most of the eye's focusing power, its focus is fixed. The [[curvature]] of the lens, on the other hand, can be adjusted to "tune" the focus depending upon the object's distance. Medical terms related to the cornea often start with the prefix "''[[wikt:kerat-|kerat-]]''".
==Structure==
The cornea has [[myelinated|unmyelinated]] [[nerve]] endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary [[reflex action|reflex]] to close the [[eyelid]]. Because transparency is of prime importance the cornea does not have [[blood vessel]]s; it receives nutrients via [[diffusion]] from the tear fluid at the outside and the [[aqueous humour]] at the inside and also from [[neurotrophins]] supplied by nerve fibres that innervate it. In [[human]]s, the cornea has a diameter of about 11.5 [[millimetre|mm]] and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, and [[immune privilege|immunologic privilege]] makes the cornea a very special tissue. The cornea is the only part of a human body that has no blood supply; it gets oxygen directly through the air.
[[Image:Limbus.png|thumb|right|200px|[[Limbus]] is the border between cornea and sclera]]
It borders with the [[sclera]] by the [[corneal limbus]].
===Layers===
The human cornea, like that of other [[primate]]s, has five layers. The corneas of cats, dogs, and other carnivores have only four.<ref name="Merindano">Merindano MD; Costa J; Canals M; Potau JM, and Ruano D. [http://www.med.ub.es/sae/EJA/EJA_V6_N3_02.pdf#search=%22%22mammalian%20cornea%22%2C%20layers%22 "A comparative study of Bowman's layer in some mammals: Relationships with other constituent corneal structures."] ''European Journal of Anatomy.'' Volume 6, Number 3, December 2002.</ref> From the anterior to posterior they are:
#'''[[Corneal epithelium]]''': a thin [[epithelium|epithelial]] multicellular tissue layer (stratified squamous epithelium) of fast-growing and easily-regenerated [[biological cell|cells]], kept moist with [[tears]]. Irregularity or edema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity.
#'''[[Bowman's layer]]''' (also erroneously known as the ''anterior limiting membrane'', when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of irregularly-arranged [[collagen]] fibers. This layer is absent in carnivores.<ref name="Merindano"/>
#'''[[Corneal stroma]]''' (also ''substantia propria''): a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely populated keratocytes. The corneal stroma consists of approximately 200 layers of type I [[collagen]] fibrils. There are 2 theories of how transparency in the cornea comes about:
## The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils.(Maurice)
## The spacing of the neighbouring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek)
#'''[[Descemet's membrane]]''' (also ''posterior limiting membrane''): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium.
#'''[[Corneal endothelium]]''': a simple [[squamous]] or low [[cuboid]]al monolayer of mitochondria-rich cells 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 humour]], not by [[blood]] or [[lymph]], and has a very different origin, function, and appearance from [[endothelium|vascular endothelia]].)
===Innervation===
The cornea is one of the most sensitive tissues of the body, it is densely innervated with sensory nerve fibres via the [[ophthalmic division]] of the [[trigeminal nerve]] by way of 70 - 80 [[long ciliary nerves]]; and [[short ciliary nerves]] derived from the [[oculomotor nerve]].
The nerves enter the cornea via three levels, ''scleral, episcleral and conjunctival''. Most of the bundles give rise by subdivision to a network in the stroma, from which fibres supply the different regions. The three networks are ''midstromal, subepithelial/Bowman's layer, and epithelium''. The receptive fields of each nerve ending are very large, and may overlap.
Corneal nerves of the subepithelial layer converge and terminate near the apex of the cornea in a [[logarithmic spiral]] pattern.<ref name="Yu">Yu CQ, Rosenblatt MI. Transgenic corneal neurofluorescence in mice: a new model for in vivo investigation of nerve structure and regeneration.
Invest Ophthalmol Vis Sci. 2007 Apr;48(4):1535-42. </ref>
==Diseases and disorders==
{{main|List of eye diseases and disorders}}
===Treatment and management===
[[image:Cornea.jpg|thumb|[[Slit lamp]] image of the cornea, iris and lens]]
====Surgical procedures====
Various [[refractive surgery|refractive eye surgery]] techniques change the shape of the cornea in order to reduce the need for corrective lenses or otherwise improve the refractive state of the eye. In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer [[laser]].
If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be [[Cornea transplant|transplanted]]. Because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea.
There are also synthetic corneas (keratoprostheses) in development. Most are merely plastic inserts, but there are also composed of biocompatible synthetic materials that encourage tissue ingrowth into the synthetic cornea, thereby promoting biointegration.
==== Non-surgical procedures====
[[Orthokeratology]] is a method using specialized hard or rigid gas-permeable [[contact lens]]es to transiently reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses.
==References==
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*Daxer A, Misof K, Grabner B, Ettl A, Fratzl P. "Collagen fibrils in the human corneal stroma: structure and aging." Invest ''Ophthalmol Vis Sci.'' 1998 Mar;39(3):644-8. PMID 9501878.
*Daxer A, Fratzl P. "Collagen fibril orientation in the human corneal stroma and its implication in keratoconus." ''Invest Ophthalmol Vis Sci.'' 1997 Jan;38(1):121-9. PMID 9008637.
*Fratzl P, Daxer A. [http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1262438&blobtype=pdf "Structural transformation of collagen fibrils in corneal stroma during drying. An x-ray scattering study."] ''Biophys J.'' 1993 Apr;64(4):1210-4. PMID 8494978.
== See also ==
* [[Astigmatism (eye)]]
* [[Keratitis]]
==External links==
* {{UMichAtlas|eye_1}} - "Sagittal Section Through the Eyeball"
*[http://www.nei.nih.gov/health/cornealdisease/ Facts About The Cornea and Corneal Disease] National Eye Institute (NEI)
*[http://www.KCGlobal.org/ Keratoconus (Irregular Astigmatism), Patient Support]
*[[http://keratos.free.fr/], Keratos, Patient Support]
{{Eye}}
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