   #copyright

Cornea

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

          Cornea is also a commune in Caraş-Severin County, Romania, see
          Cornea, Caraş-Severin.

   Cornea
   Schematic diagram of the human eye. (Cornea labeled at centre top.)
   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.
   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, 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 centre and 0.6 mm - 0.8 mm at the
   periphery. Transparency, avascularity, and immunologic privilege makes
   the cornea a very special tissue.

   In humans, the refractive power of the cornea is approximately 43
   dioptres, roughly three-fourths of the eye's total refractive power.

   Medical terms related to the cornea often start with the prefix
   "kerat-".

Layers

   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.)

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.

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.
   Retrieved from " http://en.wikipedia.org/wiki/Cornea"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
