Lt Col Reynolds
The cornea is the outermost, transparent layer of the eye, located in front of the iris. Although the cornea is clear and seems to lack substance, it’s actually a very organized group of cells and proteins. Dr. Patel, can you tell us more about the cornea?

Dr. Patel
Sure, Dr. Reynolds. The cornea is different from most tissues in the body because it contains no blood vessels to nourish and protect it against infection. Instead, the cornea receives its nourishment from tears and a liquid called aqueous humor that fills the space behind it.

The cornea has five basic layers, each of which has an important function. The epithelium is the cornea’s outermost region, making up about 10 percent of the cornea’s thickness. The epithelium’s main function is to keep foreign material, such as dust, water, and bacteria, from entering the eye and other layers of the cornea. The epithelium also absorbs oxygen and cell nutrients from tears, and distributes these nutrients to the rest of the cornea. Thousands of tiny nerve endings in the epithelium make the cornea extremely sensitive to pain when rubbed or scratched. The foundation of the epithelium, where the epithelial cells anchor and organize themselves, is called the basement membrane.

Directly behind the basement membrane of the epithelium is a transparent sheet of tissue known as Bowman’s layer, which is made of strong protein fibers called collagen. An injury to Bowman’s layer may cause scar tissue to form. Scars in Bowman’s layer that are large and centrally located can cause some vision loss.

Behind Bowman’s layer is the stroma, which makes up about 90 percent of the cornea’s thickness. It consists primarily of collagen, and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and shape. The stroma is the primary location where refractive surgery takes place.

The next layer is a tissue called Descemet’s membrane. This is a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries.

Finally, the innermost layer of the cornea is the endothelium, which is extremely thin. Endothelial cells are essential in keeping the cornea clear by pumping excess fluid away from the stroma. Without this pumping action, the excess fluid would cause the stroma to swell, and the cornea would become hazy and ultimately opaque.

In a healthy eye, a perfect balance is maintained between the fluid moving into the cornea and the fluid being pumped out of the cornea. If endothelial cells are destroyed by disease or trauma, they’re lost forever. The loss of too many endothelial cells could result in corneal edema and blindness. If this happens, a corneal transplant is the only potential treatment.