US researchers have for the first time injected human stem cells into monkeys with Parkinson's symptoms, seen as a key step in the fight to find a cure.
Stem cells are seen as offering grounds for great hope
The stem cells, which have been injected into rodents in the past, initially stopped the monkeys' damaged brain cells from deteriorating.
The primates' condition did however start to slide after four months, the study in the PNAS journal said.
Stem cells offer great hope for cures, but a breakthrough remains elusive.
"We are still talking about years," said Dr Richard Sidman, one of the co-authors of the study. "But it's a start, and we may be looking at applications for a number of diseases other than Parkinson's."
Primates v people
It surprised the team that the stem cells - rather than replacing the damaged cells as anticipated - actually worked to protect them, preventing further deterioration.
But while the monkeys fared well in the initial months of the trial, four months in they started once again to show the symptoms of the disease.
The researchers speculate this may be to do with the monkeys beginning to reject foreign tissue, and suggested that further research would need to be done suppressing their immune systems.
Further trials are needed to establish whether similar results are seen in people who have Parkinson's
Parkinson's Disease Society
Dr Kieran Breen, Director of Research and Development for the Parkinson's Disease Society, urged caution.
"These results are from a very early stage pre-clinical trial using an animal model of Parkinson's," he said.
"Further trials are needed to establish whether similar results are seen in people who have Parkinson's."
Dr Stephen Minger, Director of King's Stem Cell Biology Laboratory, said he welcomed any research which took the search for a cure further but similarly cautioned against great hopes.
More than four million people worldwide are estimated to suffer from Parkinson's disease, making it the most common brain degenerative disease after Alzheimer's disease.
It is a disease of the nervous system that generally affects both men and women who are more than 40 years old. It is associated with trembling of the arms and legs, stiffness and rigidity of the muscles and slowness of movement.
The progressive decline brought on by the condition is caused by a loss of brain cells which produce a chemical called dopamine.
Stem cells are seen as providing one of the major avenues of hope for a cure.
The body's "master cells", stem cells are created shortly after conception. They have the capacity to turn into any kind of tissue in the body.