cornea and corneal disease The Cornea and Corneal Disease The cornea is the outermost layer of your eye. It is the clear, dome-shaped surface that covers the front of your eye. Structure of the Cornea The cornea is clear and seems to lack substance.  However, it is actually a highly organized group of cells and proteins.  Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. The cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it.  The cornea must remain transparent to refract light properly, and the presence of even the smallest blood vessels can interfere with the process. To see well, all layers of your cornea must be free of any opaque or cloudy materials. The cornea's tissue is arranged in five layers, each having important functions. The five layers of the cornea are: Epithelium The epithelium is the cornea's outermost region, comprising about 10 percent of it's thickness. The epithelium serves to: Block the passage of foreign material, such as dust, water, and bacteria, into the eye and other layers of the cornea Provide a smooth surface that absorbs oxygen and cell nutrients from tears, then distributes these nutrients to the rest of the cornea. The epithelium is filled with thousands of tiny nerve endings that make the cornea sensitive to pain when rubbed or scratched. The part of the epithelium that serves as the foundation on which the epithelial cells anchor and organize themselves is called the basement membrane. Bowman's Layer Lying directly below the basement membrane of the epithelium is a transparent sheet of tissue known as Bowman's layer. It is composed of strong layered protein fibers called collagen. If injured, Bowman's layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur. Stroma Beneath the Bowman's layer is the stroma, which comprises about 90 percent of the cornea's thickness. It consists primarily of water (78 percent) and collagen (16 percent), and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and form. The collagen's unique shape, arrangement, and spacing are essential in producing the cornea's light-conducting transparency. Descemet's Membrane Under the stroma is the Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against eye infection and injuries. Descemet's membrane is composed of collagen fibers which are different from those of the stroma, and is made by the endothelial cells that lie below it. Descemet's membrane is regenerated readily after injury. Endothelium The endothelium is the extremely thin, innermost layer of the cornea. Endothelial cells are essential in keeping the cornea clear. Normally, fluid leaks slowly from inside the eye into the middle corneal layer (stroma). The endothelium's primary task is to pump this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque. In the healthy eye, a balance is maintained between the fluid moving into the cornea and fluid being pumped out of the cornea.  Once endothelium cells are destroyed by disease or trauma, they are lost forever. If too many endothelial cells are destroyed, corneal edema and blindness ensue, with corneal transplantation the only available therapy. The Corneal Function Since the cornea is smooth and clear as glass but is strong and durable, it helps the eye in two ways: It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the sclera, or white part of the eye. The cornea acts as the eye's outermost lens.  It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye's total focusing power. When light strikes the cornea, it bends, or refracts, the incoming light onto the lens. The lens further refocuses that light onto the retina, a layer of light sensing cells lining the back of the eye that starts the translation of light into vision. For you to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images. The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lens and the retina would be susceptible to injury from UV radiation. The Cornea and Injury The cornea copes very well with minor injuries or abrasions. If the highly sensitive cornea is scratched, healthy cells slide over quickly and patch the injury before infection occurs and vision is affected. If the scratch penetrates the cornea more deeply, however, the healing process will take longer, at times resulting in greater pain, blurred vision, tearing, redness, and extreme sensitivity to light. These symptoms require professional treatment. Deeper scratches can also cause corneal scarring, resulting in a haze on the cornea that can greatly impair vision. In this case, a corneal transplant may be needed.  Be sure to visit your eye doctor if you feel you have any impaired vision. Cornea Diseases and Disorders Some disorders of the cornea are: Allergies - Allergies affecting the eye are fairly common. The most common allergies are those related to pollen, particularly when the weather is warm and dry. Symptoms can include redness, itching, tearing, burning, stinging, and watery discharge, although they are not usually severe enough to require medical attention. Antihistamine decongestant eyedrops can effectively reduce these symptoms, as does rain and cooler weather, which decreases the amount of pollen in the air.  An increasing number of eye allergy cases are related to medications and contact lens wear. Animal hair and certain cosmetics, such as mascara, face creams, and eyebrow pencil, can cause allergies that affect the eye. Touching or rubbing eyes after handling nail polish, soaps, or chemicals may cause an allergic reaction. Some people have sensitivity to lip gloss and eye makeup. Allergy symptoms are often temporary and can eliminated by not having contact with the offending cosmetic or detergent. As a routine matter, always protect your eyes whenever you can and be sure to visit your eye doctor regularly for a checkup.