Cornea

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For other uses, see Cornea (disambiguation). Cornea_sentence_0

Cornea_table_infobox_0

CorneaCornea_header_cell_0_0_0
DetailsCornea_header_cell_0_1_0
Part ofCornea_header_cell_0_2_0 Front of eyeCornea_cell_0_2_1
SystemCornea_header_cell_0_3_0 Visual systemCornea_cell_0_3_1
FunctionCornea_header_cell_0_4_0 Refract lightCornea_cell_0_4_1
IdentifiersCornea_header_cell_0_5_0
LatinCornea_header_cell_0_6_0 corneaCornea_cell_0_6_1
MeSHCornea_header_cell_0_7_0 Cornea_cell_0_7_1
TA98Cornea_header_cell_0_8_0 Cornea_cell_0_8_1
TA2Cornea_header_cell_0_9_0 Cornea_cell_0_9_1
FMACornea_header_cell_0_10_0 Cornea_cell_0_10_1

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Cornea_sentence_1

The cornea, with the anterior chamber and lens, refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. Cornea_sentence_2

In humans, the refractive power of the cornea is approximately 43 dioptres. Cornea_sentence_3

The cornea can be reshaped by surgical procedures such as LASIK. Cornea_sentence_4

While the cornea contributes most of the eye's focusing power, its focus is fixed. Cornea_sentence_5

Accommodation (the refocusing of light to better view near objects) is accomplished by changing the geometry of the lens. Cornea_sentence_6

Medical terms related to the cornea often start with the prefix "" from the Greek word κέρας, horn. Cornea_sentence_7

Structure Cornea_section_0

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. Cornea_sentence_8

Because transparency is of prime importance, the healthy cornea does not have or need blood vessels within it. Cornea_sentence_9

Instead, oxygen dissolves in tears and then diffuses throughout the cornea to keep it healthy. Cornea_sentence_10

Similarly, nutrients are transported via diffusion from the tear fluid through the outside surface and the aqueous humour through the inside surface. Cornea_sentence_11

Nutrients also come via neurotrophins supplied by the nerves of the cornea. Cornea_sentence_12

In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Cornea_sentence_13

Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue. Cornea_sentence_14

The most abundant soluble protein in mammalian cornea is albumin. Cornea_sentence_15

The human cornea borders with the sclera via the corneal limbus. Cornea_sentence_16

In lampreys, the cornea is solely an extension of the sclera, and is separate from the skin above it, but in more advanced vertebrates it is always fused with the skin to form a single structure, albeit one composed of multiple layers. Cornea_sentence_17

In fish, and aquatic vertebrates in general, the cornea plays no role in focusing light, since it has virtually the same refractive index as water. Cornea_sentence_18

Microanatomy Cornea_section_1

The human cornea has five layers (possibly six, if the Dua's layer is included). Cornea_sentence_19

Corneas of other primates have five known layers. Cornea_sentence_20

The corneas of cats, dogs, wolves, and other carnivores only have four. Cornea_sentence_21

From the anterior to posterior the layers of the human cornea are: Cornea_sentence_22

Cornea_ordered_list_0

  1. 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. 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. It is continuous with the conjunctival epithelium, and is composed of about 6 layers of cells which are shed constantly on the exposed layer and are regenerated by multiplication in the basal layer.Cornea_item_0_0
  2. Bowman's layer (also known as the anterior limiting membrane): when discussed in lieu of a subepithelial basement membrane, Bowman's Layer is a tough layer composed of collagen (mainly type I collagen fibrils), laminin, nidogen, perlecan and other HSPGs that protects the corneal stroma. When discussed as a separate entity from the subepithelial basement membrane, Bowman's Layer 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. This layer is eight to 14 micrometres (μm) thick and is absent or very thin in non-primates.Cornea_item_0_1
  3. 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. They are parallel and are superimposed like book pages. 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. There are 2 theories of how transparency in the cornea comes about:Cornea_item_0_2
    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.Cornea_item_0_3
    2. The spacing of the neighboring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek)Cornea_item_0_4
  4. 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 membrane, a very thin and strong layer, Dua's layer, 15 microns thick and able to withstand 1.5 to 2 bars of pressure.Cornea_item_0_5
  5. 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 affects 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.Cornea_item_0_6

Nerve supply Cornea_section_2

The cornea is one of the most sensitive tissues of the body, as it is densely innervated with sensory nerve fibres via the ophthalmic division of the trigeminal nerve by way of 70–80 long ciliary nerves. Cornea_sentence_23

Research suggests the density of pain receptors in the cornea is 300-600 times greater than skin and 20-40 times greater than dental pulp, making any injury to the structure excruciatingly painful. Cornea_sentence_24

The ciliary nerves run under the endothelium and exit the eye through holes in the sclera apart from the optic nerve (which transmits only optic signals). Cornea_sentence_25

The nerves enter the cornea via three levels; scleral, episcleral and conjunctival. Cornea_sentence_26

Most of the bundles give rise by subdivision to a network in the stroma, from which fibres supply the different regions. Cornea_sentence_27

The three networks are, midstromal, subepithelial/sub-basal, and epithelial. Cornea_sentence_28

The receptive fields of each nerve ending are very large, and may overlap. Cornea_sentence_29

Corneal nerves of the subepithelial layer terminate near the superficial epithelial layer of the cornea in a logarithmic spiral pattern. Cornea_sentence_30

The density of epithelial nerves decreases with age, especially after the seventh decade. Cornea_sentence_31

Function Cornea_section_3

Refraction Cornea_section_4

The optical component is concerned with producing a reduced inverted image on the retina. Cornea_sentence_32

The eye's optical system consists of not only two but four surfaces—two on the cornea, two on the lens. Cornea_sentence_33

Rays are refracted toward the midline. Cornea_sentence_34

Distant rays, due to their parallel nature, converge to a point on the retina. Cornea_sentence_35

The cornea admits light at the greatest angle. Cornea_sentence_36

The aqueous and vitreous humors both have a refractive index of 1.336-1.339, whereas the cornea has a refractive index of 1.376. Cornea_sentence_37

Because the change in refractive index between cornea and aqueous humor is relatively small compared to the change at the air–cornea interface, it has a negligible refractive effect, typically -6 dioptres. Cornea_sentence_38

The cornea is considered to be a positive meniscus lens. Cornea_sentence_39

In some animals, such as species of birds, chameleons and a species of fish, the cornea can also focus. Cornea_sentence_40

Transparency Cornea_section_5

Upon death or removal of an eye the cornea absorbs the aqueous humor, thickens, and becomes hazy. Cornea_sentence_41

Transparency can be restored by putting it in a warm, well-ventilated chamber at 31 °C (88 °F, the normal temperature), allowing the fluid to leave the cornea and become transparent. Cornea_sentence_42

The cornea takes in fluid from the aqueous humor and the small blood vessels of the limbus, but a pump ejects the fluid immediately upon entry. Cornea_sentence_43

When energy is deficient the pump may fail, or function too slowly to compensate, leading to swelling. Cornea_sentence_44

This arises at death, but a dead eye can be placed in a warm chamber with a reservoir of sugar and glycogen that generally keeps the cornea transparent for at least 24 hours. Cornea_sentence_45

The endothelium controls this pumping action, and as discussed above, damage thereof is more serious, and is a cause of opaqueness and swelling. Cornea_sentence_46

When damage to the cornea occurs, such as in a viral infection, the collagen used to repair the process is not regularly arranged, leading to an opaque patch (leukoma). Cornea_sentence_47

When a cornea is needed for transplant, as from an eye bank, the best procedure is to remove the cornea from the eyeball, preventing the cornea from absorbing the aqueous humor. Cornea_sentence_48

Clinical significance Cornea_section_6

The most common corneal disorders are the following: Cornea_sentence_49

Cornea_unordered_list_1

  • Corneal abrasion - a medical condition involving the loss of the surface epithelial layer of the eye's cornea as a result of trauma to the surface of the eye.Cornea_item_1_7
  • Corneal dystrophy - a condition in which one or more parts of the cornea lose their normal clarity due to a buildup of cloudy material.Cornea_item_1_8
  • Corneal ulcer - an inflammatory or infective condition of the cornea involving disruption of its epithelial layer with involvement of the corneal stroma.Cornea_item_1_9
  • Corneal neovascularization - excessive ingrowth of blood vessels from the limbal vascular plexus into the cornea, caused by deprivation of oxygen from the air.Cornea_item_1_10
  • Fuchs' dystrophy - cloudy morning vision.Cornea_item_1_11
  • Keratitis - inflammation of the cornea.Cornea_item_1_12
  • Keratoconus - a degenerative disease, the cornea thins and changes shape to be more like a cone.Cornea_item_1_13
  • Corneal Foreign body- one of the most common preventable occupational hazards.Cornea_item_1_14

Management Cornea_section_7

Surgical procedures Cornea_section_8

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. Cornea_sentence_50

In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer laser. Cornea_sentence_51

If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted. Cornea_sentence_52

Because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea. Cornea_sentence_53

There are also synthetic corneas (keratoprostheses) in development. Cornea_sentence_54

Most are merely plastic inserts, but there are also those composed of biocompatible synthetic materials that encourage tissue ingrowth into the synthetic cornea, thereby promoting biointegration. Cornea_sentence_55

Other methods, such as magnetic deformable membranes and optically coherent transcranial magnetic stimulation of the human retina are still in very early stages of research. Cornea_sentence_56

Other procedures Cornea_section_9

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. Cornea_sentence_57

In 2009, researchers at the University of Pittsburgh Medical center demonstrated that stem cell collected from human corneas can restore transparency without provoking a rejection response in mice with corneal damage. Cornea_sentence_58

For corneal epithelial diseases such as Stevens Johnson Syndrome, persistent corneal ulcer etc., the autologous contralateral (normal) suprabasal limbus derived in vitro expanded corneal limbal stem cells are found to be effective as amniotic membrane based expansion is controversial. Cornea_sentence_59

For endothelial diseases, such as bullous keratopathy, cadaver corneal endothelial precursor cells have been proven to be efficient. Cornea_sentence_60

Recently emerging tissue engineering technologies are expected to be capable of making one cadaver-donor's corneal cells be expanded and be usable in more than one patient's eye. Cornea_sentence_61

Corneal retention and permeability in topical drug delivery to the eye Cornea_section_10

The majority of ocular therapeutic agents are administered to the eye via the topical route. Cornea_sentence_62

Cornea is one of the main barriers for drug diffusion because of its highly impermeable nature. Cornea_sentence_63

Its continuous irrigation with a tear fluid also results in poor retention of the therapeutic agents on the ocular surface. Cornea_sentence_64

Poor permeability of the cornea and quick wash out of therapeutic agents from ocular surface result in very low bioavailability of the drugs administered via topical route (typically less than 5%). Cornea_sentence_65

Poor retention of formulations on ocular surfaces could potentially be improved with the use of mucoadhesive polymers. Cornea_sentence_66

Drug permeability through the cornea could be facilitated with addition of penetration enhancers into topical formulations. Cornea_sentence_67

See also Cornea_section_11

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Cornea.