Stem cells work in mysterious ways, and that’s the tantalizing finding from scientists who treated monkeys with Parkinson’s disease using fetal stem cells.

Their results mark the first successful stem cell therapy for Parkinson’s in primates. The big news, however, is not simply that the treatment worked, but how it worked: by rescuing and rejuvenating, rather than replacing, diseased cells.

According to Richard Sidman, a Harvard Medical School neuroscientist and co-author of the research, it’s a different principle of stem cell action from what everyone’s thinking about. The study is a landmark, both for treating Parkinson’s disease and for highlighting a new therapeutic approach to stem cells. While most scientists are struggling to change stem cells into the types of cells they need — neurons, insulin-producing cells, heart cells, etc. — the new work shows that stem cells can perform the remarkable task of saving damaged cells.

The findings, which will soon be published in the Proceedings of the National Academy of Sciences, show that neural stem cells have “therapeutic mechanisms other than replacement,” said Cesar Borlongan, a Medical College of Georgia neurologist. Borlongan said he has observed similar effects when using stem cells to treat Parkinson’s symptoms in rodents.

At the time of the injections, the monkeys couldn’t feed themselves or walk without assistance, and alternated between periods of absolute stillness and uncontrollable tremors. Two months after the treatment, they were able to walk and eat. The tremors had disappeared.

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