Definition & Function
The cornea, the eye’s outermost layer, is the clear dome-shaped window at the front of the eye. The cornea is the most important optical element in the eye. It is curved to help focus rays of light on the retina (the nerve layer that lines the inside of the back of the eye). 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 corneal tissue is a highly organized group of cells and proteins comprised of five layers. Each layer provides a different function. The outside layer of the cornea, the epithelium, is protective, like very thin, fine skin. The cornea has the highest concentration of pain fibers in any part of the body and is therefore extremely sensitive. The pain fibers are just below the epithelium . Thus, the smallest particle of dirt can feel like a cinder and a small scratch of the surface epithelium can be very painful. Fortunately, this outer layer heals rapidly from most minor injuries.

The innermost layer, the endothelium , is one cell layer in thickness. Its function is to keep the cornea clear by continuously pumping fluid out of the cornea. This layer does not replicate. The cells with which you are born are the only cells you will have during your entire lifetime. As we get older the number of cells slightly decreases. If the number decreases too much, the cornea begins to swell with fluid and becomes hazy. This swelling of the cornea is called “corneal edema.” The most common reason for a corneal transplant is the depletion of the endothelial cells, which maintain corneal clarity.