The longest-running trial of stem cells derived from a human embryo found that the cells caused patients none of the problems scientists feared, such as forming tumors, and reversed partial blindness in about half the eyes receiving transplants, researchers reported on Tuesday.

The results, published in The Lancet, could help re-invigorate the controversial quest to harness stem cells, which have the ability to turn into any of the 200 kinds of human cells, to treat diseases.

In an accompanying commentary, Dr. Anthony Atala of the Wake Forest Institute for Regenerative Medicine called the work "a major accomplishment."

After intense excitement among scientists and the public about the promise of stem cells and ethical debates about destroying human embryos to obtain them, the field stumbled when a high-profile trial for spinal cord injury was halted by Geron Corp in 2011 and the interest of other companies waned.

The small study's main goal was assessing the safety of the transplanted cells. Called retinal pigment epithelial cells, they were created by taking stem cells from a days-old embryo created in a fertility clinic and inducing them to differentiate into the specialized cells.

The study "provides the first evidence, in humans with any disease, of the long-term safety and possible biologic activity" of cells derived from embryos, said co-author Dr. Robert Lanza, chief scientific officer of Advanced Cell Technology, which produced the cells and funded the study.

Nine patients with Stargardt's disease (which causes macular degeneration in childhood) and nine with dry age-related macular degeneration (a leading cause of adult blindness) received implants of the retinal cells in one eye. The other eye served as a control.

Four eyes developed cataracts and two became inflamed, probably due to the patients' age (median: 77) or the use of immune-supressing transplant drugs.

The retinal cells, which help keep the eye's rods and cones alive and functional, survived in all 18 patients, most of whose vision improved. In those with macular degeneration, treated eyes saw a median of 14 additional letters on a standard eye chart a year after receiving the cells, with one patient gaining 19 letters. The untreated eyes got worse, overall. The Stargardt's patients had similar results.

In real-life terms, patients who couldn't see objects under 12 feet (4 meters) tall can now see normal-size adults.

The vision of one 75-year old rancher who was blind in the treated eye (20/400) improved to 20/40, enough to ride horses again, Lanza said. Others became able to use computers, read watches, go to the mall or travel to the airport alone for the first time in years.

While calling the results "encouraging," stem cell expert Dusko Ilic of Kings College London, who was not involved in the work, warned that even if the larger clinical trial planned for later this year is also successful, "it will take years before the treatment becomes available."