The cornea is the eye’s outermost layer. It is the clear, domeshaped surface that covers the front of the eye. It plays an important role in focusing your vision. Although the cornea may look clear and seem to lack substance, it is a highly organized tissue. 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 tears and the aqueous humor (a fluid in the front part of the eye that lies behind the cornea).
The tissues of the cornea are arranged in three basic layers, with two thinner layers, or membranes, between them. Each of these five layers has an important function. These layers are:
Epithelium The epithelium is the cornea’s outermost layer. Its primary functions are to:
block the passage into the eye of foreign material, such as dust, water, and bacteria; and
provide a smooth surface to absorb oxygen and nutrients from tears, which are then distributed to the other layers of the cornea.
The epithelium is filled with thousands of tiny nerve endings, which is why your eye may hurt when it is rubbed or scratched. The part of the epithelium that epithelial cells anchor and organize themselves to is called the basement membrane.
Bowman’s membrane The next layer behind the basement membrane of the epithelium is a transparent film of tissue called Bowman’s layer, composed of protein fibers called collagen. If injured, Bowman’s layer can form a scar as it heals. If these scars are large and centrally located, they may cause vision loss.
Stroma Behind Bowman’s layer is the stroma, which is the thickest layer of the cornea. It is composed primarily of water and collagen. Collagen gives the cornea its strength, elasticity, and form. The unique shape, arrangement, and spacing of collagen proteins are essential in producing the cornea’s light-conducting transparency.
Descemet’s Membrane Behind the stroma is Descemet’s membrane, a thin but strong film of tissue that serves as a protective barrier against infection and injuries. Descemet’s membrane is composed of collagen fibers that are different from those of the stroma, and are made by cells in the endothelial layer of the cornea (see above). Descemet’s membrane repairs itself easily after injury.
Endothelium The endothelium is the thin, innermost layer of the cornea. Endothelial cells are important in keeping the cornea clear. Normally, fluid leaks slowly from inside the eye into the 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 and become thick and opaque. In a healthy eye, a perfect balance is maintained between the fluid moving into the cornea and the fluid pumping out of the cornea. Unlike the cells in Descemet’s membrane, endothelial cells that have been destroyed by disease or trauma are not repaired or replaced by the body.
Keratoconus Is a progressive eye disease in which the normally spherical cornea thins and begins to bulge into a cone-like shape. The cornea is a clear organ that lies in the central portion of the surface of the eye. In patients with keratoconus, the cone-shaped cornea cannot focus light rays properly and causes distorted vision.
CORNEAL TRANSPLANT SURGERY Recommended for a patient whose cornea is damaged due to disease, infection, or injury. Common indications for corneal transplantation include but are not limited to the following:
Central corneal scars caused by keratoconus, chemical or mechanical injury, corneal ulcers, and herpes simplex virus keratitis
Corneal edema (swelling)
A corneal transplant involves the replacement of the damaged cornea with a healthy donor cornea (obtained through an eye bank).
There are several different corneal transplant procedures. The traditional full thickness corneal transplant procedure (penetrating keratoplasty) involves the replacement of the entire damaged cornea with a healthy cornea from a human donor tissue that is obtained from an eye bank. However, technological advances have allowed for the development of specialized procedures that replace only the damaged portion of the cornea, leaving the remainder of the cornea intact.
Descemet's Stripping and Automated Endothelial Keratoplasty (DSAEK) replaces only the innermost layers of the cornea. This allows the procedure to be performed through a much smaller incision with shorter recovery times and fewer risks than a traditional corneal transplant.
DSAEK is most commonly performed on patients with Fuchs' Dystrophy, an inherited eye disease in which the cells of the endothelium deteriorate, resulting in distorted vision and corneal swelling. As this condition progresses, the vision and discomfort will worsen.
DSAEK Procedure During the DSAEK procedure, a small incision is used to access the inner portion of the cornea and remove the damaged endothelial tissue. The donor tissue is then inserted into the eye. The donor tissue is then centered and held in place by an air bubble. The incision site is then closed with a few stitches.
Penetrating Keratoplasty Procedure During the procedure, a circular incision is made into the cornea. A segment of the diseased corneal tissue is removed and replaced with healthy tissue that is sutured into place.