The cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye.
The cornea has five layers. From front to back, these layers are:
1. The corneal epithelium: This outer layer of the cornea is five to seven cells thick and measures about 50 microns — making it slightly less than 10 percent of the thickness of the entire cornea. Epithelial cells are constantly being produced and sloughed off in the tear layer of the surface of the eye. The turnover time for the entire corneal epithelium is about one week.
2. Bowman's layer: This is a very thin (8 to 14 microns) and dense fibrous sheet of connective tissue that forms the transition between the corneal epithelium and the underlying stroma.
3. The corneal stroma: This middle layer of the cornea is approximately 500 microns thick, or about 90 percent of the thickness of the overall cornea. It is composed of strands of connective tissue called collagen fibrils. These fibrils are uniform in size and are arranged parallel to the cornea surface in 200 to 300 flat bundles called lamellae that extend across the entire cornea. The regular arrangement and uniform spacing of these lamellae is what enables the cornea to be perfectly clear.
4. Descemet's membrane: This very thin layer separates the stroma from the underlying endothelial layer of the cornea. Descemet's (pronounced "DESS-eh-mays") membrane gradually thickens throughout life — it's about 5 microns thick in children and 15 microns thick in older adults.
5. The corneal endothelium: This is the innermost layer of the cornea. The back of the endothelium is bathed in the clear aqueous humor that fills the space between the cornea and the iris and pupil. The corneal endothelium is only a single layer of cells thick and measures about 5 microns. Most of the endothelial cells are hexagonal (six-sided), but some may have five or seven sides. The regular arrangement of these cells is sometimes called the endothelial mosaic.
A number of conditions can affect the cornea. Among the more common corneal problems are:
Arcus senilis: As people get older, a white ring often develops in the periphery of the cornea. This is called arcus senilis (also called corneal arcus), and it's the most common aging change in the cornea. Arcus senilis typically is separated from the limbus by an area of clear cornea. The white ring — which is composed of cholesterol and related compounds — can be barely noticeable or very prominent.
Corneal abrasion: A scratched cornea can be very painful and can lead to an eye infection.
Pterygium: A pterygium is a fibrous growth that starts on the sclera but can grow into the peripheral cornea and cause irritation, vision problems and disfigurement of the front of the eye.
Dry eyes: Though the cause of dry eyes typically begins in the tears gland and eyelids, it can lead to damage of the corneal epithelium, which causes eye discomfort and vision disturbances.
Corneal ulcer: A corneal ulcer is a serious abscess-like infection of the cornea that can lead to significant pain, scarring and vision loss.
Corneal dystrophy: A dystrophy is a weakening or degeneration of a tissue. The most common corneal dystrophy — called Fuch's dystrophy — affects the corneal endothelium, causing corneal swelling, foggy vision, light sensitivity and other problems.
Acanthamoeba keratitis: This is a very serious and painful corneal infection that can cause significant pain and vision loss.
Fungal keratitis: This is another dangerous corneal infection that (like Acanthamoeba keratitis) tends to affect contact lens wearers more often than people who wear glasses.
Keratoconus: This is a thinning and deformation of the cornea that causes vision problems that can't be corrected with regular eyeglasses or contact lenses. In some cases, vision problems from keratoconus can be corrected with scleral contact lenses or hybrid contacts. But in severe cases, a cornea transplant may be required.
Corneal ectasia: This is thinning and deformation of the cornea that resembles keratoconus but occurs as a rare complication of LASIK or other corneal refractive surgery.
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