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Cornea
Schematic diagram of the human eye. (Cornea labeled at center top. )
Vertical section of human cornea from near the margin. (Waldeyer. ) Magnified.
1. Epithelium. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body
2. Anterior elastic lamina.
3. substantia propria. The substantia propria (or stroma of Cornea) is fibrous tough unyielding and perfectly transparent
4. Posterior elastic lamina. Descemet's membrane is the Basement membrane that lies between the corneal proper substance also called stroma and the endothelial layer of the Cornea
5. Endothelium of the anterior chamber. The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the The anterior chamber is the fluid-filled space inside the Eye between the iris and the Cornea 's innermost surface the Endothelium.
a. Oblique fibers in the anterior layer of the substantia propria. The substantia propria (or stroma of Cornea) is fibrous tough unyielding and perfectly transparent
b. Lamellæ the fibers of which are cut across, producing a dotted appearance.
c. Corneal corpuscles appearing fusiform in section. The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber.
d. Lamellæ the fibers of which are cut longitudinally.
e. Transition to the sclera, with ass more distinct fibrillation, and surmounted by a thicker epithelium. The sclera, called the white or white of the eye, is the opaque (usually white though certain animals such as horses and lizards can have black sclera fibrous In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body
f. Small bloodvessels cut across near the margin of the cornea.
Gray's subject #225 1006
Dorlands/Elsevier c_55/12259559

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. the Peripheral organs of the Special senses the organs of Taste ( Peripheral gustatory or Elsevier, the world's largest Publisher of Medical and Scientific literature, forms part of the Reed Elsevier group In Optics, transparency (also called pellucidity) is the Material property of allowing Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain The iris consists of Pigmented Fibrovascular tissue known as a stroma. The pupil is the hole that is located in the center of the iris of the eye and that controls the amount of light that enters the Eye. The anterior chamber is the fluid-filled space inside the Eye between the iris and the Cornea 's innermost surface the Endothelium. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of the eye's optical power. The lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused Refraction is the change in direction of a Wave due to a change in its Speed. In Geometrical optics, a focus, also called an image point, is the point where Light rays originating from a point on the object converge. Optical power ( dioptric power or refractive power) is the degree to which a lens or Mirror converges or diverges light [1][2] In humans, the refractive power of the cornea is approximately 43 dioptres. A dioptre, or diopter, is a Unit of measurement of the Optical power of a lens or curved Mirror, which is equal to the reciprocal [3] 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. In Mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry Medical terms related to the cornea often start with the prefix "kerat-".

Contents

Structure

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. Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth A nerve is an enclosed cable-like bundle of peripheral Axons (the long slender projections of Neurons. A reflex action, also known as a reflex, is an involuntary and almost instant movement in response to stimulus. An eyelid is a thin fold of skin that covers and protects an Eye. 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. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement The aqueous humor is a thick watery substance that is between the lens and the cornea Neurotrophins are a family of proteins that induce the survival development and function of Neurons. In humans, the cornea has a diameter of about 11. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus 5 mm and a thickness of 0. The Millimetre ( American spelling: millimeter, symbol mm) is a unit of Length in the Metric system, equal to 5–0. 6 mm in the center and 0. 6–0. 8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. Immune privilege is a term used to describe certain sites in the body which are able to tolerate the introduction of Antigen without eliciting an inflammatory immune The cornea is the only part of a human body that has no blood supply; it gets oxygen directly through the air.


Libus is the border between cornea and sclera
Libus is the border between cornea and sclera

It borders with the sclera by the corneal limbus. The sclera, called the white or white of the eye, is the opaque (usually white though certain animals such as horses and lizards can have black sclera fibrous The corneal limbus is the border of the Cornea and the Sclera.

Layers

The human cornea, like that of other primates, has five layers. A primate is a member of the biological order Primates ( Latin: "prime first rank" the group that contains Lemurs the Aye-aye The corneas of cats, dogs, and other carnivores have only four. [4] From the anterior to posterior they are:

  1. Corneal epithelium: a thin epithelial multicellular tissue layer (stratified squamous epithelium) of fast-growing and easily-regenerated cells, kept moist with tears. The corneal epithelium ( epithelium corneæ anterior layer) is made up of epithelial tissue and covers the front of the Cornea. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called Tears are the liquid product of a process of lacrimation to clean and lubricate the Eyes The word lacrimation may also be used in a medical or literary sense 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.
  2. 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. The Bowman's membrane ( Bowman's layer, anterior limiting lamina, anterior elastic lamina) is a smooth layer in the Eye. Collagen is the main Protein of Connective tissue in Animals and the most abundant protein in Mammals making up about 50% of the whole-body protein This layer is absent in carnivores. [4]
  3. Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely populated keratocytes. The substantia propria (or stroma of Cornea) is fibrous tough unyielding and perfectly transparent The corneal stroma consists of approximately 200 layers of type I collagen fibrils. Collagen is the main Protein of Connective tissue in Animals and the most abundant protein in Mammals making up about 50% of the whole-body protein 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)
  4. Descemet's membrane (also posterior limiting membrane): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium. Descemet's membrane is the Basement membrane that lies between the corneal proper substance also called stroma and the endothelial layer of the Cornea
  5. 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 corneal endothelium is a monolayer of specialized flattened mitochondria-rich cells that lines the posterior surface of the Cornea and faces the Anterior In Anatomy, squamous epithelium (from Latin squama, "scale" is an Epithelium characterised by its most superficial layer consisting In anatomy the Cuboid bone is a bone in the foot See also Hyperrectangle Oblong (The term endothelium is a misnomer here. A misnomer is a term which suggests an interpretation that is known to be untrue 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 aqueous humor is a thick watery substance that is between the lens and the cornea Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells such as nutrients and oxygen—and transports Waste products The lymphatic system in Vertebrates is a network of conduits that carry a clear fluid called Lymph. The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the )

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 ophthalmic nerve is one of the three branches of the Trigeminal nerve, the fifth Cranial nerve. The trigeminal nerve (the fifth Cranial nerve, also called the fifth nerve or simply V) is responsible for sensation in the face The long ciliary nerves, two or three in number are given off from the Nasociliary, as it crosses the Optic nerve. The branches of the Ciliary ganglion are the 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. Definition In Polar coordinates ( r, θ the curve can be written as r = ae^{b\theta}\ or \theta [5]

Diseases and disorders

Main article: List of eye diseases and disorders

Treatment and management

Slit lamp image of the cornea, iris and lens
Slit lamp image of the cornea, iris and lens

Surgical procedures

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. This is a partial list of human Eye diseases and disorders. The World Health Organization publishes a classification of known diseases and injuries called the The slit lamp is an instrument consisting of a high-intensity Light source that can be focused to shine as a slit Refractive eye surgery is any Eye surgery used to improve the refractive state of the Eye and decrease or eliminate dependency on Glasses or Contact In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer laser. A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission.

If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted. Corneal transplantation, also known as corneal grafting or penetrating keratoplasty, is a Surgical procedure where a damaged or diseased Cornea 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 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. Orthokeratology ( Ortho-K, also known as "dream lens" and "dream vision" is the use of rigid gas-permeable Contact lenses normally worn only at A contact lens (also known simply as a contact) is a corrective, cosmetic, or therapeutic lens usually placed on the Cornea

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. ^ Najjar, Dany "Clinical optics and refraction"
  4. ^ a b 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.

See also

External links

Astigmatism is an optical defect whereby vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina Keratitis is a condition in which the Eye 's Cornea, the front part of the eye becomes inflamed The University of Michigan Ann Arbor ( U of M, U-M, UM or simply Michigan) is a top-ranked Coeducational public research

Dictionary

cornea

-noun

  1. (anatomy) The transparent layer forming the front of the eye.
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