Embryonic stem cell transplants intended to treat a leading cause of vision loss have been shown to be both effective and safe in a first study of the long-term effects, researchers say.

In a study, 18 people suffering from macular degeneration -- an affliction that causes a progressive loss of vision -- were given the stem cell transplants, with more than half of them having some sight restored by the procedure.

The transplants appeared both effective and safe for up to 3 years after they were performed, researchers reported in The Lancet.

In macular degeneration, tissue at the center of the retina of the eye becomes thin and begins to break down.

While there are presently no approved treatments for the condition, stem cells have shown promise as a way to rebuild this retinal tissue, the researchers say.

"Embryonic stem cells have the potential to become any cell type in the body, but transplantation has been complicated by problems" including tissue rejection and tumor formation, explained lead author Robert Lanza of Advanced Cell Technology, which funded the study.

Because of those problems, stem cell research has focused so far on areas in the body that don't normally trigger a strong immune response, Lanza says -- including the eye.

Study participants who showed improvement in their vision -- on average, an improvement of three lines on a standard eye chart -- showed no evidence of rejection or tumors during 3 years of follow-ups, the researchers said.

"It's made a huge difference in the quality of their life," Lanza says.

A control group of patients with macular degeneration who did not receive a stem cell transplant did not show any similar improvement in their vision, the researchers noted.

Lana and his research colleagues, including study co-author Steven Schwartz of the Jules Stein Eye Institute in Los Angeles, say they are preparing for a second phase of clinical trials that will involve a much larger number of participants.

Stein says he feels cautiously optimistic.

"This is a significant first step, but by no means are we there yet," he says. "This treatment is not ready for prime time; lots of work needs to be done... Regenerative medicine holds great hope and promise for people suffering from untreatable conditions. This study is the first small step towards achieving that promise."

Other cell researchers were quick to express interest in the study's results.

"The million-dollar question about stem cell research is, will it actually affect cures?" said Dr. Stephen Chang of the New York Stem Cell Foundation. "This trial, while small and limited, shows that embryonic stem cells will potentially change the course of disease -- and in this case vision -- dramatically."