Eighteen patients with deteriorating eyesight received embryonic stem cell transplant. Some vision was restored in more than half the volunteers, in the longest study into the fledgling technology reported Tuesday.
Stem cells derived from embryos "could provide a potentially safe new source of cells for the treatment of various unmet medical disorders requiring tissue repair or replacement," its authors said.
The study marks a new chapter in the long story of embryonic stem cells, which, after their discovery in the 1990s, were hailed as a miracle cure but then ran into problems.
Published in The Lancet, the paper looked at a US trial of stem cells among 18 patients suffering from two tragic, degenerative diseases of the retina.
Nine had a condition called Stargardt's macular dystrophy, a leading cause of juvenile blindness, and nine had dry atrophic age-related macular degeneration, which occurs among the middle-aged and elderly.
There is no conventional treatment for either condition, which eventually leads to complete blindness as the retina's light-receiving cells die out.
The participants were injected with one of three different doses of retinal cells derived from early-stage embryos -- 50,000, 100,000 or 150,000 cells.
The transplants were placed in a space under the retina of the worst-affected eye.
The patients were monitored for up to 37 months, for an average of 22 months.
Out of the 18 treated eyes, 10 showed substantial improvements in vision, as measured by the ability to read letters on a board. Of these, eight patients were able to read 15 additional letters in the first year after transplant.
In seven other patients, vision in the treated eye remained stable or improved.
But in one patient, vision decreased, falling back by more than 10 letters -- though the study did not clarify whether this was the expected degeneration associated with the disease's progression.
Untreated eyes did not show any improvement, thus pointing to the apparent benefit of the transplant, the researchers said.
And there were no side-effects or evidence of rejection.
The investigation is the first to give a longer-term validation to embryonic stem cells.
Extraordinarily versatile, embryonic stem cells can differentiate into any tissue of the body -- an ability that has thrown up tantalizing hopes of using them to replace tissue lost through disease, accident or war.
- Rewards and hurdles -
But scientists have had to face two major obstacles.
One is that donated stem cells, provoking an immune response, can be rejected by the body or cause cancer.
The eye is considered a promising site for transplants as it is located at a shield called the blood-ocular barrier, where there is not a strong immune response.
The other is ethical, with some arguing that an embryo is a human life.
The study was led by Robert Lanza, chief scientific officer at a Massachusetts biotech firm, Advanced Cell Technology Inc. The Lancet is a peer-reviewed journal, meaning that studies are scrutinized by outside specialists before they are published.
In a comment carried by The Lancet, Anthony Atala, an expert in regenerative medicine at the Wake Forest School of Medicine in North Carolina, hailed the work as a "major accomplishment".
The next step should be to fine-tune the dosage, to see whether bigger transplants make a greater difference to vision.
Embryonic stem cells have been joined in clinical trials by induced pluripotent stem cells, called iPS.
These are adult cells that have been reprogrammed to a youthful, versatile state. Their perceived advantage is that they are less likely to be rejected, as the cells can be derived from the patient himself rather than from a donor, and do not carry the moral baggage of embryonic stem cells.
In September, Japanese researchers carried out the world's first iPS implant into a woman in her 70s with macular degeneration. The trial will be conducted on five other patients.
"Much work remains before (embryonic stem cells) and induced pluripotent stem cell therapies go beyond regulatory trials, but the path is now set in motion," Atala said.