"I wouldn't say the data is super convincing that these cells are doing much," says Eggan, also a principal investigator with the Harvard Stem Cell Institute. Only two patients were studied, and in one of the patients, the cells appeared to have had no biological effect, despite her report of improvement. "For myself, I would have felt a lot more comfortable if I had several people telling the same story."

This research is important, Eggan says, because it shows that at least some patients can safely receive cells derived from human embryonic stem cells. The implanted cells did not grow uncontrollably and did not trigger development of a teratoma—a benign tumor that contains other bodily tissues. "The cells are there, they're surviving, and they're persisting. They're not growing out of control, and they're not growing a teratoma."

In the study, human embryonic stem cells were coaxed into becoming retinal pigment epithelium, or RPE cells. They were carefully examined for impurities or potential dangers, and then injected into one eye in each patient. The two received 50,000 RPE cells each. The next round of patients will receive 100,000 RPE cells, and later patients in the trial will be given 200,000 cells each.

Of the 24 people in the trial, half have dry, age-related macular degeneration—the most common cause of vision loss in older Americans—and half have Stargardt macular dystrophy, a far rarer disorder that strikes children and teenagers. There are no effective medical treatments for either condition. Lanza says the company will soon begin another round of research in the U.K. involving 24 patients with the same conditions.

The therapy is aimed at preventing loss of photoreceptors in the eye. Because the trial patients have already lost most of their receptors, they are less likely to benefit than people who have retained more vision, Lanza says.

The eye is a logical organ for testing stem-cell therapies because it is immunoprivileged, meaning it is unlikely to mount an immune response to foreign cells. The eye is also easy to see into, to check for progress and problems.

Ellen Feigal, senior vice president of research and development for the California Institute of Regenerative Medicine, describes the new study as "good for the field."

"It's a very good initial step in the process to evaluate a therapy for its safety and effectiveness," she says.