|The cornea functions as a window to the eye’s internal structures.|
|Each layer of the cornea has a different function.|
|The cornea contains two-thirds of the eye’s total focusing power.|
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The cornea””the transparent dome-shaped part of the eye””is an important component of the visual system. It not only allows light into the eye and is a site of refraction, but it functions as a window to the eye’s internal structures. Each of its six layers has a different function which contributes to the cornea’s overall functionality.
The cornea is approximately .5mm thick and contains two-thirds of the eye’s total focusing power. As light enters the eye, it is refracted (bent) by the cornea and the lens until it refocuses on the retina. The convex surface of the cornea is thicker in the periphery than in the center, and the size remains static after the first year of life.
Since the cornea does not contain a blood supply, it obtains oxygen from the tears that coat the eye. The tears of the eye obtain oxygen from air which is distributed throughout the tear film. The oxygen assists in maintaining overall corneal health by keeping the surface clear and shiny. Even though there are no blood vessels, the cornea is very sensitive due to the numerous nerve endings extending into it.
For many years, the cornea was known to contain five layers; however there has been a recent discovery of a sixth layer. What follows is a description and discussion of the various layers as well as their functions.
The epithelium is a five-to-seven-level-thick structure that assists in maintaining corneal optical clarity by constant regeneration. It is composed of flattened cells that are replaced continuously and provide a smooth surface for the tear film. The tear film helps provide nutrients to the front surface of the cornea and washes out foreign objects from the eye. Another function of the epithelium is to block foreign material from entering the area.
There are thousands of nerve endings in the epithelium that cause the cornea to be extremely sensitive to pain. It is bathed by the tears of the eye, which keep the cornea moist. Other functions of the tear film are to maintain the smooth surface of the epithelium, assist in optical clarity, and provide nutrition to the cornea. Injury to the corneal epithelium causes sloughing off of the epithelial cells resulting in corneal abrasion. Since these cells regenerate quickly, the cornea is able to heal itself rapidly. Deeper penetration into the cornea by an injury can lead to scars which can cause the cornea to lose its optical clarity and visual acuity.
The very tough condensed layer of cells that lie below the epithelium of the cornea is called the Bowman layer. It is composed of collagen fibers that are difficult to penetrate. As a result, this layer aids in protecting the cornea from deeper injury. An injury to this area of the cornea results in scarring, which if located in the visual axis can affect acuity.
Lying directly beneath Bowman’s layer is the stroma. This is the thickest part of the cornea and is composed of 16% collagen and 78% water. The collagen in this layer run parallel to one another and is very strong and elastic. The fibers here do not contain blood vessels. They are evenly spaced and contribute in maintaining corneal transparency as well as the cornea’s shape.
Located just below the stroma is the newly discovered corneal layer called Dua’s layer. This tier was discovered in May 2013 by Harminder Dua, MD, professor of ophthalmology and visual sciences at the University of Nottingham.
The layer was discovered by Dr. Dua’s team while doing research on corneal transplantation on donated eyes. Dua’s layer is very thin””15 microns thick””and is impervious to air which Dr. Dua and his colleagues believe may play a major role in patients undergoing corneal transplant surgery. Perforation and tears to this layer can cause fluid buildup in the cornea and result in corneal hydrops. Therefore, further research in the study of this layer can be useful in the diagnosis of patients with keratoconus.
The thin, very elastic fifth layer of the cornea is Descemet’s membrane which acts as a protective barrier against infection. It is comprised of very elastic collagen fibers that are different from those in the stroma which retract if cut. It also serves as a protective barrier against infection and injury to the eye. This membrane is 8″“10 microns in thickness, is extremely resilient, and will regenerate if damaged.
This membrane is located beneath the stroma and supports the endothelium and is important to the health of the endothelium.
The sixth and most posterior layer of the cornea is the endothelium. This level lies adjacent to the anterior chamber and is the innermost layer of the cornea. This single layer of cells maintains the balance of water in the cornea by keeping it in a partially dehydrated state, by pumping water from the stroma. This fluid balance is essential for the clarity of the cornea.
If the endothelium is unable to maintain the balance of fluid in the cornea, the stromal layer will swell, become hazy, and result in reduced visual clarity. At the same time, the endothelium also allows nutrients from the aqueous in the anterior chamber to reach the other layers of the cornea. If the cells of this layer are damaged, they will not regenerate, but the remaining cells will grow larger to compensate for the devitalized cells. Cells of this layer can be lost due to age, or through diseases such as Fuchs Dystrophy, and can be an indicator of the overall health of the cornea.
Janet Hunter is president of Eye Source LLC, and specializes in ophthalmic technician training.