Corneal Transplantation The cornea is the clear window in the front of the eye. Disorder, injuries, diseases, ulcer, and heredity can all influence the clarity of the cornea.

 

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Corneal Transplantation

The cornea is the clear window in the front of the eye. Disorder, injuries, diseases, ulcer, and heredity can all influence the clarity of the cornea

 

In order to understand causes of disease and the reasons for corneal transplantation, one needs to know a little about the anatomy of the cornea. There are five layers to the cornea. On the outside is the epithelium, a soft five cells thick protective and lubricating layer. Below that is a clear layer called Bowman's membrane. Next is the stroma, an incredibly strong and firm, but clear layer. This is composed of strong protein cross linkages. Below that is another clear layer called Descemet's membrane.

Finally, on the inside surface, the endothelium. This layer is responsible for keeping the cornea clear. It is only one cell layer thick.

Technically, the endothelium pumps water out of the rest of the cornea by a mechanism called the Na+ -P+ ATP-ase pump. This keeps the cornea lustrous and crystal clear. It is so clear that one can easily see through it to the colored part of the eye, the iris.

Oxygen for metabolism comes primarily from the air. The energy substrate is glucose which is supplied from the aqueous humor. Proteins are also supplied via this route.

The endothelium is only about 5 microns thick, and represents only one cell layer. When these cells are observed with a flat mount preparation of the cornea under the specular microscope they look like this.

 

 

           

                Higher Cell Density                                     Lower Cell Density and Polymorphism

    Various Magnifications of Lens Endothelial Cells

When we are born there are about 4,500 cells per square millimeter. As we age the cell density declines to about 2, 500 per square millimeter by the time we are eighty. When the cell density declines to about 500 per square millimeter, and the cells become very large (polymegathism) and very irregular (polymorphism), the corneal endothelium cannot keep the water pumped out of the remainder of the cornea, and it becomes cloudy or hazy. Vision declines. The cells cannot regenerate themselves, and a corneal transplant needs to be performed.

 

Other causes for the need of a corneal transplant are corneal scars.

Hereditary corneal dystrophies also can cause a need for a transplant.

Trauma, as well, may necessitate a corneal transplant.

Treatment

A circular knife (trephine) is used to remove the diseased tissue. A  similar knife is used to remove a "button" from a donor's cornea (They have to be dead, but not too long.) The button is then sewn into the hole left by the circular knife on the patient's eye. The sutures have a width three times the width of a red blood cell (20 microns).

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 Corneal Transplant (Courtesy of Richard Beatty, MD)

 

 

 

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