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New transplant technique may restore eyesight.

New Transplant Technique May Restore Eyesight

Of all the vagaries that the future can hold, blindness may be the most frightening and horrible to conceive.

Investigations being conducted at Washington University Medical Center in St. Louis, however, may eventually change the way we think of and talk about blindness. Studies at the University's Central Institute for the Deaf (CID) have resulted in the first successful transplantation of photoreceptor tissue into blind eyes, according to researchers whose findings were published in the August issue of Investigative Ophthalmology and Visual Science.

The work holds no immediate prospect for clinical application but does leap several hurdles impeding photoreceptor transplants.

Most neural blindness is caused by damage to and degeneration of the eye's rods and cones, the photoreceptor cells that begin the conversion of light into nerve impulses. Perhaps 15 percent of adults lose some or all of their vision to age-related macular degeneration, a syndrome that especially affects photoreceptors, in the fovea, the tiny area at the retina's center most responsible for sharpness of vision. Add to that population the many victims of retinitis pigmentosa, a degenerative disease affecting younger adults, and those who damage their eyes by overexposure to sunlight or other bright sources. Blind people whose vision loss is attributed to impaired photoreceptors number in the millions.

CID scientists Martin S. Silverman, Ph.D., and Stephen E. Hughes, Ph.D., pioneered techniques to replace missing photoreceptors with transplanted photoreceptors in the hope that they might take over the task of responding to light. Though the experiments showed that it is possible to place the cells in the correct location within the retina and to do so gently enough that the cell's characteristic shapes are generally preserved, it remains to be seen whether the transplanted cells will survive indefinitely and whether they will make their essential connections with adjacent layers of the retina.

The concept of replacing damaged photoreceptors is revolutionary, but "once we had the idea, it was really just a matter of overcoming the technical problems," Silverman says. Silverman's thesis for solving those problems sprang from his work at CID transplanting the sensory "hair cells" of the inner ear. The hair cells are the first-line cells responsible for the sense of hearing, and Silverman knew they were similar in some respects to the retina's photoreceptors. A few diseases, such as Usher's syndrome, even attack both.

Leaping Technical Hurdles

Silverman is intrigued with complex questions that might ultimately have relatively simple answers, so he wasn't deterred by the long-standing belief that it is impossible to restore vision after the loss of photoreceptors. The first hurdle he faced was isolating the microscopic photoreceptor cells from the donor retina. Because their strict organization is as important to sight as are the photoreceptors themselves, traditional cell-harvesting techniques that dissociate and purify tissue were inappropriate.

As an alternative, Silverman and Hughes developed a technique to flatten an intact donor retina and affix it to a block of gel to stabilize it. By sectioning down through and microscopically examining each of the five layers of the retina, they knew when they had reached the photoreceptor layer. There, they took a thicker section of approximately 200 microns - about as thick as a three-by-five index card - of perfectly organized photoreceptors. This section of tissue became the transplant.

More Studies Needed

To date, Silverman and Hughes cannot say that sight is being restored absolutely as a result of the transplants. While in their judgment the transplanted cells appear to be alive and contain the photopigment required for vision, they say it remains to be seen whether these cells link up with neurons in the other layers of the retina. "There is a suggestion of retinal activation by light," Silverman says. "But the transplant could be releasing neurotransmitters without making appropriate synaptic contact." Only examination of the cells under electron microscopy will reveal the existence of the connections. That's the next step.
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Title Annotation:photoreceptor transplants
Publication:Nutrition Health Review
Date:Jun 22, 1990
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