TUESDAY, Oct. 14, 2014 (HealthDay News) -- A new study is the first to show the long-term safety of embryonic stem cell transplants to treat human disease.

The research involved 18 people who received the transplants to treat forms of macular degeneration, a leading cause of vision loss.

The transplants, which restored some sight in more than half of the patients, appeared safe up to three years after the procedure.

The study, funded by a U.S.-based company called Advanced Cell Technology, was published Oct. 14 in The Lancet.

"Embryonic stem cells have the potential to become any cell type in the body, but transplantation has been complicated by problems," lead author Dr. Robert Lanza, chief scientific officer at Advanced Cell Technology, said in a journal news release. Those problems include the rejection of the transplanted cells by the patient's immune system, as well as the danger that the cells might spur certain types of cancers called teratomas.

A teratoma is a type of cancer that occurs when stem cells develop into multiple types of cells and form incompatible tissues that can include teeth and hair.

As Lanza explained, because of these issues, scientists interested in embryonic stem cell therapy have tended to focused on sites in the body that typically do not produce a strong immune response. The eye is one such spot.

In the new study, human embryonic stem cells were first prompted to develop into eye cells called retinal pigment epithelial cells. They were then transplanted into nine people with Stargardt's macular dystrophy, and another nine with dry atrophic age-related macular degeneration.

Patient outcomes were tracked for up to three years after transplant. No signs of either cancer-like cell growth (hyperproliferation) or immune system rejection were found in any of the treated eyes after a median follow-up of 22 months, and the only adverse events were linked not to the transplanted cells, but to the eye surgery or immune system suppression needed for the transplant.

Overall, 10 of the 18 patients said they had significant improvements in their vision, and this improvement was only seen in the eyes that had received the stem cell treatment.

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Embryonic Stem Cell Therapy Shows Long-Term Effectiveness, Safety