If our eyes are a window to the soul, then our corneas are the panes of glass in that window. All light entering the eye must first pass through these thin, clear films of epithelial tissue. Unfortunately, like a sheet of glass, the cornea is somewhat fragile; it can become cracked or clouded by way of disease or injury, in which case vision loss is usually inevitable. Making matters worse, standard treatment--a corneal transplant--typically fails.
Now, however, reseachers have developed techniques to substitute fresh, bioengineered tissue for damaged corneas--in essence, methods for replacing the window glass. In two papers published this month, two separate teams report having restored or improved vision in a number of patients whose corneal damage had previously been considered untreatable. Although the groups used slightly different approaches, they met with similar success rates. And they were sucessful enough that, after skin and cartilage, corneas may well become the third type of bioengineered replacement tissue available in the clinic.
The problem with standard corneal transplantation has had to do with corneal stem cells. This population of cells normally renews and repairs the eye's surface layer, or epithelium, on a regular basis. When the cornea is severely damaged, though, the stem cells too are often depleted. Transplanting a cornea without its complement of stem cells is setting it up to become as scarred and opaque as the one it is replacing. But obtaining a sufficient number of stem cells from a donor cornea can jeopardize it. To resolve this dilemna, researchers have been searching for ways to generate larger numbers of corneal stem cells from a small number of donor cells.
According to their report in the July 12 New England Journal of Medicine, Ray Jui-Fang Tsai of the Chang Gung Memorial Hospital of Taoyun, Taiwan, and his colleagues accomplished this goal by harvesting some corneal stem cells from the patient's healthy eye and seeding them onto an amniotic membrane matrix. There the smattering of cells grew into a sturdy layer of tissue that was subsequently stitched onto the patient's damaged eye, replacing the bad corneal tissue. Of the six patients treated, all regained their vision.
The other team, Ivan R. Schwab and R. Rivkah Isseroff of the University of California, Davis, reversed vision loss in 10 out of 14 patients. Their results appeared in the July issue of the journal Cornea. Their approach was slightly different: After harvesting stem cells from the donor, they first divided them among multiple laboratory dishes, some of which were placed on the amniotic membrane, others of which were frozen and stored for possible later use. This way, Schwab notes, if the transplant fails, the harvesting procedure need not be repeated.
For cases in which both of the patient's eyes were damaged, the UC Davis researchers also investigated the use of donor tissue from close relatives. Four of their 14 subjects received composite tissue grown from cornea cells donated by a sibling or child. And all four experienced improved or restored eyesight. None of the relatives reported pain during the five-minute donation procedure, and none experienced any complications afterwards.
That said, the results do come with caveats: long-term success rates and risks have yet to be determined. And whether or not the donor stem cells will persist, for example, is unknown. Still, the development offers great promise for treating corneal damage and, more broadly, damage to other epithelial tissues, such as the the lung and the gastrointestinal tract. "The really exciting thing is where this can take us," Schwab enthuses. "Replacing diseased tissues and organs with bioengineered tissue is rapidly moving from the realm of science fiction to reality."