Researchers followed 18 patients for up to three years after treatment. The studies are the first to show safety of an embryonic stem cell treatment in humans for such a long period.
"It's a wonderful first step but it doesn't prove that (stem cells) work," said Chris Mason, chair of regenerative medicine at University College London, who was not part of the research. He said it was encouraging the studies proved the treatment is safe and dispelled fears about stem cells promoting tumour growth.
Embryonic stem cells, which are recovered from embryos, can become any cell in the body. They are considered controversial by some because they involve destroying an embryo and some critics say adult stem cells, which are derived from tissue samples, should be used instead.
Scientists have long thought about transforming them into specific types of cells to help treat various diseases. In the new research, scientists turned stem cells into retinal cells to treat people with macular degeneration or Stargardt's macular dystrophy, the leading causes of blindness in adults and children.
In each patient, the retinal cells were injected into the eye that had the worst vision. Ten of the 18 patients later reported they could see better with the treated eye than the other one. No safety problems were detected. The studies were paid for by the U.S. company that developed the treatment, Advanced Cell Technology, and were published online Tuesday in the journal, Lancet.
Dr. Robert Lanza, one of the study authors, said it was significant the stem cells survived years after the transplant and weren't wiped out by the patients' own immune systems. For some of the patients, Lanza noted their improved vision changed their lives, referring to a 75-year-old horse rancher who had been blind in one eye before the treatment.
"One month after his treatment, his vision had improved (substantially) and he can even ride his horses again," Lanza said in an email. He said other patients have regained their independence with their newfound vision and said some people are now able to use their computers again, read their watches or travel on their own.
"The next step will be to prove these (stem cell) treatments actually work," Mason said. "Unless there is a sham group where you inject saline into (patients') eyes, we can't know for sure that it was the stem cells that were responsible."
AP Science Writer Malcolm Ritter in New York contributed to this report.