Cornea

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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 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 12 mm and a thickness of 0.5 mm - 0.7 mm in the center and 1.0 mm - 1.2 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 45 dioptres, roughly three-fourths of the eye's total power.

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

Layers of the cornea

The cornea consists of five layers. From the outside to the inside they are:

Corneal epithelium: a thin epithelial layer of fast-growing and easily-regenerated cells, kept moist with tears

Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers

Corneal stroma (also substantia propria): a thick, transparent middle layer responsible for most of the focusing that the cornea performs, consisting of regularly-arranged collagen fibers along with (few) fibroblasts (If the stroma is damaged, for example by injury or infection, it can lose its transparency, causing vision problems. The corneal stroma consists of approximately 200 layers of type I collagen fibrils. The ordering of the fibrils is responsible for the transparency of the tissue.)

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 epithelium, an inner lining acting as a barrier to prevent water inside the eyeball from moving into and hydrating the cornea, which would lead to blurred vision (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 composed mostly of dense connective tissue, similar to the surrounding sclera. However, the collagen fibers are arranged in a parallel pattern, allowing light waves to constructively interfere, allowing the light to pass through relatively uninhibited. The cornea is innervated by the long posterior ciliary nerves that branch from the trigeminal nerve's ophthalmic division.

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 glasses or otherwise improve the refractive state of the eye. In the techniques used today, parts of the cornea are removed with lasers.

If the corneal stroma has developed opaque patches known as leukomas, a cornea of a deceased donor can be transplanted. Because there are few blood vessels in the cornea, there are also few problems with rejection of the new cornea.

There are also synthetic corneas in development. Most are merely plastic inserts, but there are also some made of plastics that encourage the eye tissue to grow into the synthetic cornea, making it a full replacement.

Non-surgical procedures involving the cornea

Orthokeratology is a method using specialized hard or rigid gas-permeable contact lenses to reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses.

References

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.