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The cornea is the clear tissue at the front of the eye. It focuses light onto the lens which then focuses the light onto the retina. The cornea provides two-thirds of the eye’s focusing power.
There are no blood vessels in the cornea. Because of this, the cornea must obtain its nutrients from another source, namely tears and the fluid inside the front of the eye (aqueous humor).
This absence of blood vessels allows the cornea to be transparent and to focus light properly.
The cornea consists of five layers, each with a specific role in regulating vision.
The epithelium is the outermost layer of the cornea. It prevents foreign material, such as bacteria, water, and dirt, from entering deeper into the cornea. The epithelium absorbs oxygen and nutrients from the air and tears, and then passes on this oxygen and nutrients to the other layers of the cornea. The epithelium also contains nerve endings, which can signal pain when the corneal has been disturbed due to swelling, trauma, or drying.
Bowman's Layer consists of transparent layers of protein fibers, known as collagen, located directly behind the epithelium. If injured, Bowman's Layer often creates scars, which can cause vision loss.
Most of the cornea consists of the stroma. The stroma is composed mostly of water and collagen. The durable collagen gives the cornea its strength, structure, and elasticity. The collagen also plays a important role in providing the cornea with its transparency.
Behind the stroma is a very thin layer of tissue known as Descemet's membrane. Descemet's membrane is composed of a different type of collagen fiber than that of the stroma. This special collagen fiber is what makes Descemet’s Membrane the stiff and rigid part of the cornea. (This is a good thing.) It is Descemet’s membrane that becomes thickened in Fuchs Dystrophy.
The endothelium is a single layer of hexagonal cells at the front of the cornea. It is the layer that deteriorates as a result of Fuchs' dystrophy. One function of the endothelium is to remove excess fluid from the stroma. Because fluid is constantly entering the stroma from inside the eye, removal of fluid is essential in maintaining proper vision. Too much water in the stroma causes the stroma to swell, leading to hazy and distorted eyesight. Moreover, once endothelial cells die, they cannot be regenerated. Instead, neighboring cells try to fill these voids, resulting in enlarged endothelial cells (polymegathism) and irregularly shaped cells (pleomorphism). Corneal edema (swelling) and vision loss often develop. The average person is born with 4000 endothelial cells per millimeter squared. This number is usually decreased to roughly 2000 by the age of ninety through natural aging. Those affected with Fuchs’ will certainly reach this number at a far earlier age.
Figure is from Doughty, copyright Blackwell Publishing. Used by permission of the publisher and author.