Accounting for 10% of blindness worldwide, corneal blindness is the second leading cause of blindness after cataracts. It's caused by disease or trauma. In the U.S. alone, 47,000 cornea transplant are performed in the U.S. every year.
The cornea is the clear, front part of the eye where a contact lens would sit. The cornea is responsible for 75 percent of the focusing power of the eye. The cornea does not have a direct blood supply, so there are very few disease processes in a donor that would rule out cornea donation.
The cornea has 5 layers, but two of the most important are the epithelium and the endothelium.
The epithelium forms the protective, outer covering of the cornea. Like all epithelial cells, these are continually sloughed away and replaced. The biggest danger to a donated cornea is time between death and preservation. Because one stops making tears at the time of death, it does not take long for the epithelium to dry out, slough away and expose the inner layers to infection.
This is a slit-lamp micrograph of a healthy, donated cornea. Notice how it is translucent and smooth.
This is a slit-lamp micrograph of a cornea that has had a long death to preservation time. Note how a large area of the epithelium in the lower half has completely sloughed away. The folds you see are evidence of swelling. Finally, the white haze in the lower right corner is an infiltration of white blood cells. This cornea cannot be transplanted.
The endothelium is a single-cell layer on the back of the cornea. These cells' only job is to pump water out of the cornea, thus keeping it clear. When these cells fail, water builds up in the cornea and it becomes opaque. These cells are critical to a successful transplant because they do not regenerate. What you are born with is what you get! A cornea must have at least 2,000 cells/mm2 to be transplanted. High cell counts will typically be matched with younger recipients.
These endothelial cells continue to be viable for several hours after death. Delays beyond 18 hours render the cornea non-transplantable in most cases.
Specular micrograph of an endothelial layer of a donor cornea. These living cells are critical to a successful transplant, as they do not regenerate and must last the lifetime of a recipient. Extended death to preservation times can put the endothelium in danger.