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|Schematic diagram of the human eye. (Cornea labeled at center top.)|
|Gray's||subject #225 1006|
|Vertical section of human cornea from near the margin. (Waldeyer.) Magnified. |
2. Anterior elastic lamina.
3. substantia propria.
4. Posterior elastic lamina.
5. Endothelium of the anterior chamber.
a. Oblique fibers in the anterior layer of the substantia propria.
b. Lamellæ the fibers of which are cut across, producing a dotted appearance.
c. Corneal corpuscles appearing fusiform in section.
d. Lamellæ the fibers of which are cut longitudinally.
e. Transition to the sclera, with more distinct fibrillation, and surmounted by a thicker epithelium.
f. Small bloodvessels cut across near the margin of the cornea.
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power . Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. The cornea contributes more to the total refraction than the lens does, but, whereas the curvature of the lens can be adjusted to "tune" the focus, the curvature of the cornea is fixed.
The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; 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 humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5 mm - 0.6 mm in the center and 0.6 mm - 0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue.
Medical terms related to the cornea often start with "kerat-".
The human cornea, like that of other primates, has five layers. The corneas of cats, dogs, and other carnivores have only four. From the anterior to posterior they are:
- Corneal epithelium: a thin epithelial multicellular layer of fast-growing and easily-regenerated 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.
- 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: 1) 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) 2) 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 cuboidal 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 vascular endothelia.)
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.
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 is very large, and may overlap.
Diseases and disorders
- Main article: List of eye diseases and disorders
Treatment and management of corneal diseases and disorders
Surgical procedures involving the cornea
Various 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 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 involving the cornea
Orthokeratology is a method using specialized hard or rigid gas-permeable contact lenses to transiently reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses.
- Merindano MD; Costa J; Canals M; Potau JM, and Ruano D. "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.
- Daxer A et al. Collagen fibrils in the human corneal stroma: structure and ageing. Invest Ophthalmol Vis Sci 1998;39:644-648.
- Daxer A and Fratzl P. Collagen fibril orientation in the human corneal stroma and its implication in keratoconus. Invest Ophthalmol Vis Sci 1997;38:121-129.
- Fratzl P and Daxer A. Structural transformation of collagen fibrils in corneal stroma during drying: An X-ray scattering study. Biophys J 1993;64:1210-1214.
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