Stem cells taken from tiny monkey embryos and implanted in the brain reversed some of the Parkinson’s symptoms in monkeys used to study the disease, Japanese researchers reported on Monday.

Their study, published in the Journal of Clinical Investigation, supports arguments that stem cells taken from days-old embryos can be used to replace damaged tissues in a range of diseases, experts said.

But they also cautioned that the study was preliminary and needed much more follow-up.

Yasushi Takagi and colleagues at Kyoto University grew stem cells from early monkey embryos and coaxed them into becoming, or differentiating into, neurons. They then transplanted these into the brains of monkeys who had been given a Parkinson’s-like condition using chemical damage.

Parkinson’s is caused when key brain cells that produce the message-carrying chemical dopamine die off. Symptoms start with a trembling and patients can end up paralyzed. There is no cure.

Cell transplants have been a big hope of researchers, and many groups have tried transplanting brain cells into patients, including cells from the fetuses of pigs and humans.

Proponents of embryonic stem-cell research believe their field offers a good opportunity, as these cells have the ability to become a range of tissues without causing immune reactions.

Opponents say using a human embryo for such research or even treatment is unethical. Current law forbids the use of U.S. federal funds for most embryonic stem-cell research.

Jun Takahashi and colleagues coaxed monkey embryonic stem cells into becoming neurons and then added FGF20, a growth factor that is produced exclusively in the area of the brain affected by Parkinson disease.

Then they transplanted the cells into the brains of macaques. The monkeys showed reduced symptoms from their Parkinson’s-like disease and when they were killed, the transplanted cells were found to have grown in their brains, Takahashi’s team said.

Dr. William Langston of the Parkinson’s Institute in Sunnyvale, California, called the study a milestone but noted that the brain cells made by the Japanese team did not survive very well.

“Clearly the study reported here will advance research aimed at validating the use of stem cells to treat neurodegenerative disease,” Langston wrote in a commentary. “And this is most welcome, particularly for investigators working on strategies for cell replacement the United States,” he added, referring to the political controversy over embryonic stem cells.