Researchers will report today that cells grown from human embryonic stem cells slowed vision loss when injected into the eyes of rats with a disease similar to macular degeneration, the leading cause of blindness in people older than 55.

The experiments do not prove that the cells, obtained through the destruction of human embryos, will work in people. But by showing that the cells have the potential to fill in for failing cells in the retina, experts said, the work may help justify trying the technique in humans.

Raymond D. Lund, then at the University of Utah's John A. Moran Eye Center in Salt Lake City, and Robert Lanza of Advanced Cell Technology Inc. (ACT) in Worcester, Mass., started by developing a reliable method for turning embryonic stem cells into retinal pigment epithelium cells, which nourish the light-sensitive "photoreceptor" cells in the eye. In macular degeneration, the pigment cells gradually disappear.

The researchers achieved the transformation in all 18 stem cell lines they worked with -- including some provided by the National Institutes of Health and others developed privately at Harvard University and at ACT -- proving that their approach can consistently produce the crucial pigment cells. Then they injected the cells, about 20,000 per eye, into the retinas of 14 rats with a genetic disease similar to macular degeneration. Eight control rats received eye injections without any cells.

Forty days after treatment, the team measured retinal electrical activity in response to flashes of light, and it found that the treated rats were twice as responsive as the untreated ones, which by then were going blind. A separate test -- which tracks eye and head movements in response to a moving display, a measure of an animal's ability to discern fine details -- showed that the treated rats had twice the visual acuity of the untreated rats nearly three months after treatment.

Microscopic examination of the retinas at autopsy showed that the treated eyes had healthy photoreceptor layers five to seven cells thick, while the untreated eyes had an average thickness of just one cell. (Healthy rats have layers 10 to 12 cells thick.) None of the cells divided abnormally or grew into tumors, the team reports in today's issue of the journal Cloning and Stem Cells.

"It's important and pretty exciting work," said Lucian V. Del Priore, an expert in retinal cell transplants at Columbia University. A steady source of safe, laboratory-cultivated cells for retinal transplants would be a welcome advance, he said.

But Del Priore cautioned that the rat disease is not identical to macular degeneration (no animal disease is), and it remains uncertain how long the new cells will survive and work. Immune-system responses, while generally suppressed inside the eye, can eventually lead to rejection, he said.

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