In mice, researchers have successfully transplanted stem cells that were generating photoreceptors, light-sensitive cells that appear in red and brown in the magnified view of the retina at upper right. The effort could lead to treatment in humans for photoreceptor loss, the most common cause of blindness. Image Credit: Gunilla Elam / Photo Researchers, Inc.
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Three Blind Mice: See How They . . . See
By Scott P. Edwards
 January 15, 2007

In a scientific first, researchers based in London have restored vision in mice by transplanting light-sensitive photoreceptor cells in their eyes. Although human application is not assured, the finding is a significant first step.

The scientists used stem cells from a stage when the retina is forming to restore vision in the mice, which were affected by photoreceptor loss, the most common cause of blindness in humans. Photoreceptors, commonly known as rods and cones, are specialized nerve cells located at the back of the eye in the retina.

Rod cells are most sensitive to light and dark changes, while cone cells respond more to colors. Photoreceptor loss leads to diseases such as retinitis pigmentosa and age-related macular degeneration.

“Photoreceptors are the first cells of vision,” says Anand Swaroop of the University of Michigan Medical School, who collaborated on the British study. “These cells sense light. If they die, you go blind.”

Timing Determines Cell Fate

Scientists believe the retina is an ideal spot to try stem cell therapy because wiring to the brain remains intact, at least initially, after photoreceptor loss. Therefore, researchers at the University College London (UCL) Institute of Ophthalmology thought transplanted photoreceptors would need to make only one short connection to restore vision in the mice, which lacked photoreceptors because of genetic defects.

Previous attempts to restore vision via stem cell transplants had proved fruitless, however, because the stem cells did not develop enough to integrate into their new environment and grow into functioning photoreceptors. The British researchers, led by Robin Ali of UCL and Robert MacLaren, an eye surgeon at London’s Moorfields Eye Hospital, transplanted cells at a later stage of development when they were more likely to develop into photoreceptors. The cells were taken from healthy mice only after they had begun to produce rhodopsin, a light-absorbing pigment, they reported Nov. 9 in Nature.

“We worked on the theory that cells at a later stage of development might have a higher probability of success upon transplantation,” Ali says. “We showed that cells taken from the peak rod genesis stage of development, when the retina is about to be formed, can be successfully transplanted and integrated into the adult or degenerating retina.”

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