Corneal Blindness

The cornea is the eye’s outermost layer. It is the clear, dome-shaped surface that covers the front of the eye.

Although the cornea is clear and seems to lack substance, 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. Instead, 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 tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas.

The cornea is as smooth and clear as glass but strong and durable and 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 photo-sensitive 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 refractive process is similar to the way a camera takes a picture. The cornea and lens in the eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image. 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 highly susceptible to injury from UV radiation.

Volunteers with a little technical background can support the Foundation to update patient information and also carry out periodic research related to reversible blindness.

The epithelium is the cornea’s outermost region, comprising about 10 % of the tissue’s thickness. The epithelium functions primarily to: (a) to block the passage of foreign material, such as dust, water, and bacteria, into the eye and other layers of the cornea; and (b) to 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 extremely 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.

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. Once injured, Bowman’s layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur.

Beneath Bowman’s layer is the stroma, which comprises about 90% of the cornea’s thickness. It consists primarily of water (78%) and collagen (16 %), 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.

Lying beneath the stroma, Descemet’s membrane is a thin but strong sheet of tissue that acts as protection against infection and injuries. It is composed of collagen fibers (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.

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 a healthy eye, a perfect 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 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. Corneal disease also occur due to genetic as well as environmental factors. Infection and injury due to chemical (acid or alkali) and fire burns are also common in India besides Vitamin A deficiency in children. This vitamin deficiency is mainly due to inadequate nutrition, diseases such as measles or debilitation. Poor sanitation methods also add to the burden of eye care in this country.

Since the artificial cornea respond to chemicals and other irritants in much the same way as human eye tissue, it might reduce the need for testing the safety of drugs, cosmetics and other products in the eyes of live animals. The effort to develop an artificial cornea grew out of research on how the eye heals after it is injured, according to Rejean Munger, of the University of Ottawa in Canada. Ideally, the researchers wanted to study donated human corneas, but all good corneas were being used for transplants according to Munger.

Under the leadership of Dr. May Griffith, the research team began the project by taking cells from the three layers of the human cornea. These cells were then infected with a harmless virus containing a gene that triggered the cells to continuously produce new cells. The investigators then used these “immortalized” cells to grow into an artificial version of the cornea. After about 2 weeks, they compared the structure and function of the artificial corneas to natural corneas. The newly formed corneas resembled human corneas in several ways (reported in the journal Science). When exposed to a mild eye irritant, the artificial corneas responded in much the same way as human corneas. Also, the cells were as transparent as natural corneas.

These artificial corneas are “functionally similar” to human corneas, according to Munger. Eventually, artificial corneas might be transplanted into people, according to Munger, but before that can happen, researchers will have to refine the tissue. For example, even though artificial corneas are transparent and respond to irritants as human eyes do, they do not have the proper shape needed to allow people to see. Although meaningful, artificial corneas cannot replace donor tissue as of now.