Information about Cornea
| Cornea | |
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| Schematic diagram of the human eye. (Cornea labeled at center top.) | |
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| Vertical section of human cornea from near the margin. (Waldeyer.) Magnified. 1. Epithelium. 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. | |
| subject #225 1006 | |
| Dorlands/Elsevier | c_55/12259559 |
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. The cornea is the only part of a human body that has no blood supply, it gets oxygen directly through the air.
In humans, the refractive power of the cornea is approximately 43 dioptres, roughly two-thirds of the eye's total refractive power.[3]
Medical terms related to the cornea often start with the prefix "".
It borders with the sclera by the corneal limbus.
Layers
The human cornea, like that of other primates, has five layers. The corneas of cats, dogs, and other carnivores have only four.[4] 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.[4]
- 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 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.)
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.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.[5]
Diseases and disorders
Treatment and management of corneal diseases and disorders
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Slit lamp image of the cornea, iris and lens
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.References
1. ^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
2. ^ Goldstein, E. Bruce. "Sensation & Perception". 7th Edition. Canada: Thompson Wadsworth, 2007.
3. ^ [1]
4. ^ 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.
5. ^ 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.
2. ^ Goldstein, E. Bruce. "Sensation & Perception". 7th Edition. Canada: Thompson Wadsworth, 2007.
3. ^ [1]
4. ^ 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.
5. ^ 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.
- 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. "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
External links
- Atlas of anatomy at UMich eye_1 - "Sagittal Section Through the Eyeball"
- Facts About The Cornea and Corneal Disease National Eye Institute (NEI)
- Keratoconus (Irregular Astigmatism), Patient Support
Sensory system - Visual system - Eye | |
|---|---|
| Fibrous tunic (outer) | Conjunctiva • Sclera • Schlemm's canal • Trabecular meshwork • Limbus Cornea (Epithelium, Bowman's, Stroma, Descemet's, Endothelium) |
| Uvea (middle) | Choroid (Ciliary processes, Choriocapillaris, Bruch's membrane) • Iris (Stroma) • Pupil • Ciliary body |
| Retina (inner) | Macula • Fovea • Optic disc |
| Anterior segment | Anterior chamber • Aqueous humour • Posterior chamber • Lens |
| Posterior segment | Vitreous humour • Zonular fibers • Zonule of Zinn |
epithelium is a tissue composed of a layer of cells. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.
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The substantia propria (or stroma of cornea) is fibrous, tough, unyielding, and perfectly transparent.
It is composed of about 200 flattened lamellæ (plates of collagen fibrils), superimposed one on another.
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It is composed of about 200 flattened lamellæ (plates of collagen fibrils), superimposed one on another.
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Descemet's membrane is the basement membrane that lies between the corneal proper substance, also called stroma, and the endothelial layer of the cornea. The endothelial layer is located at the posterior of the cornea.
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endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary.
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anterior chamber is the fluid-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium.[1] Aqueous humor is the fluid that fills the anterior chamber.
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The substantia propria (or stroma of cornea) is fibrous, tough, unyielding, and perfectly transparent.
It is composed of about 200 flattened lamellæ (plates of collagen fibrils), superimposed one on another.
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It is composed of about 200 flattened lamellæ (plates of collagen fibrils), superimposed one on another.
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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.[1] Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately
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Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends.
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- Aneurysms can be classified as saccular or fusiform
- In botany, a fusiform cell or other plant structure is spindle-like and tapers at both ends
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The sclera is the opaque (usually white), fibrous, protective layer of the eye containing collagen and elastic fibers.[1] In children, it is thinner and shows some of the underlying pigment, appearing slightly blue.
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epithelium is a tissue composed of a layer of cells. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.
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Elsevier, the world's largest publisher of medical and scientific literature, forms part of the Reed Elsevier group. Based in Amsterdam, the company has substantial operations in the UK, USA and elsewhere.
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Transparency is the property of allowing the transmission of light through a material. It is the noun form of the word transparent (for example, glass is usually transparent.
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Eyes are organs of vision that detect light. Different kinds of light-sensitive organs are found in a variety of organisms. The simplest eyes do nothing but detect whether the surroundings are light or dark, while more complex eyes can distinguish shapes and colors.
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In anatomy, the iris (plural irises or irides) is the most visible part of the eye of vertebrates, including humans. The following describes the iris of vertebrates, not the independently evolved iris found in some cephalopods.
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The pupil (Latin pupilla "little doll" > pupa "doll") is the variable-sized, black circular or slit shaped opening in the center of the iris that regulates the amount of light that enters the eye.
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anterior chamber is the fluid-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium.[1] Aqueous humor is the fluid that fills the anterior chamber.
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For other uses, see Power (physics).
Optical power (dioptric power or refractive power) is the degree to which a lens or mirror converges or diverges light. It is equal to the reciprocal of the focal length of the device...... Click the link for more information.
The lens is a transparent, biconvex (lentil-shaped) structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. Its function is thus similar to a human-made optical lens.
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Refraction is the change in direction of a wave due to a change in its speed. This is most commonly seen when a wave passes from one medium to another. Refraction of light is the most commonly seen example, but any type of wave can refract when it interacts with a medium, for
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focus, also called an image point, is the point where light rays originating from a point on the object converge [1]. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle.
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In mathematics, curvature is any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, but this is defined in different ways depending on the context.
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Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. It is an outgrowth of glial cells: Schwann cells supply the myelin for peripheral neurons while oligodendrocytes supply it to those of the central nervous system.
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A nerve is an enclosed, cable-like bundle of axons (the long, slender projection of a neuron). Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves, and nerves also include the glial cells that
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ReFLEX is a wireless protocol developed by Motorola which is used for two-way paging. It is based on the one-way FLEX protocol and comes in two variants, ReFLEX25 and ReFLEX50. Later version 2.7 of the ReFLEX protocol was released. Devices compliant with ReFLEX 2.7.
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An eyelid is a thin fold of skin and muscle that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye.
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The blood vessels are part of the cardiovascular system and function to transport blood throughout the body. The most important types, arteries and veins, carry blood away from or towards the heart, respectively.
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- This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation).
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
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aqueous humour is a thick watery substance that is located in the eye.
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Locations
The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens...... Click the link for more information.
Neurotrophins, also called "neurotrophic factors", are a family of protein which induce the survival of neurons. They belong to a family of growth factors, secreted proteins usually found in the blood stream which are capable of signaling particular cells to survive, differentiate,
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