Although the cornea is clear and seems to lack substance, it’s actually a very organized group of cells and proteins. The cornea is different from most tissues in the body, since it contains no blood vessels to nourish it and protect it against infection. Instead, the cornea receives its nourishment from the tears and the 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 job of the epithelium is to block the passage of foreign material, such as dust, water, and bacteria, into the eye and other layers of the cornea. The epithelium also provides a smooth surface that absorbs oxygen and the 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.”
Directly below the basement membrane of the epithelium is the transparent sheet of tissue known as Bowman’s layer. It’s made of strong, layered protein fibers called collagen. If it’s 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 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.
Located under the stroma 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. Descemet’s membrane is made of collagen fibers and can repair itself readily after an injury.
Finally, 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, or 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 the fluid being pumped out of the cornea. Once endothelial cells are destroyed by disease or trauma, they’re lost forever. If too many endothelial cells are destroyed, which could result in corneal edema and blindness, a corneal transplant is the only potential treatment.