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Scientists have restored nerve cells destroyed by a condition similar to Parkinson's disease, in monkeys.

The Japanese team hope their work could lead to stem cell trials in human patients before the end of 2018.

Parkinson's disease causes the progressive loss of nerve cells that release dopamine, a chemical that helps control body movement.

The researchers triggered a similar loss of cells in macaque monkeys, then used human stem cells to replace them.

The animals showed significant improvement in their symptoms two years after having precursor dopamine neurons derived from human stem cells transplanted into their brains.

These cells - known as induced pluripotent stem (iPS) cells - are created by genetically reprogramming ordinary adult cells, so that they revert to an embryonic-like state.

In this state, they can then be coaxed to develop into many different adult cell types - in the case of this experiment, dopamine neurons.

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Researcher Prof Jun Takahashi, from Kyoto University, said the work, published in the journal Nature, showed that the artificially created cells were as effective as those created naturally in the brain of the monkeys.

He said: "Because iPS cells are easy to obtain, we can standardise them to only use the best iPS cells for therapy."

Brain scans confirmed the cells were functioning as expected and not triggering a damaging immune response, or the growth of cancerous tumours, which can be associated with experimental stem cell therapies.

The researchers also found that the quality of the donor cells was the key factor - not how many each monkey received.

Dr Tilo Kunath, a Parkinson's UK-funded researcher at the Medical Research Council Centre for Regenerative Medicine at the University of Edinburgh, said: "This is extremely promising research demonstrating that a safe and highly effective cell therapy for Parkinson's can be produced in the lab.

"Such a therapy has the potential to reverse the symptoms of Parkinson's in patients by restoring their dopamine-producing neurons.

"The next stage will be to test these therapies in a first-in-human clinical trial."

Prof Tom Foltynie, of the National Hospital for Neurology & Neurosurgery, said there would be concerns about how stem cell transplants might pan out in human Parkinson's patients, but the latest work had addressed some of the previous uncertainty about whether it was safe to use iPS cells.

He said "Overall, the results they report in the short term are encouraging and provide some reassurance that these cells can function as sources of dopamine to address those movement symptoms of Parkinson's disease that relate to dopamine loss.

"No-one expects that transplants will address the non-dopamine, non-movement aspects of Parkinson's disease such as dementia and falls, which ultimately emerge with very long-term follow up."