These transplants do not involve transplanting the entire retina, which is unlikely to ever be possible as it would be as complicated as being able to transplant part of the brain. The transplanted tissue is placed at the central retina (the macula), the area used for detailed reading vision and central visual field.
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Some of these studies have assessed transplanting human donor retina into animal models of retinal diseases with limited success. In conditions such as age-related macular degeneration, where the disease mainly affects the macula and the peripheral retina is relatively normal, transplants taken from patients’ own peripheral retina and placed at their ailing macula have also been undertaken with variable success. This technique would not be appropriate in retinitis pigmentosa and similar conditions since the peripheral retina is also affected. Other studies are assessing the transplantation of developing sheets of retinal cells grown in the laboratory – again to date this is not proving likely to be successful.
A small trial has been conducted in the USA assessing the potential benefit of transplanting developing retinal cells. Researchers reported that implanting a sheet of developing foetal retinal and retinal pigment epithelial (RPE) cells under the macula of six patients with RP and four patients with age-related macular degeneration improved visual acuity in three of the RP and two of AMD patients. It was thought that transplanting retinal cells with their underlying supportive RPE cell layer might be more effective than implanting a sheet of retinal cells alone.
This study has not been repeated or extended suggesting that it has limited utility for several reasons, including the challenges of obtaining sufficient foetal material for transplantation. It is also currently not known how this treatment works. It may be via the production of growth factors by the transplanted foetal tissue that nourishes the remaining retinal cells.
The majority of researchers in the field do not believe that this is likely to be a viable and effective strategy to replace lost retinal cells in the future.