Stem cells show potential for treating the debilitating nerve condition motor neurone disease, research suggests.
Scientist Stephen Hawking has motor neurone disease
A US team found injecting rats with stem cells delayed the onset of MND.
Writing in the Transplantation, the researchers from Johns Hopkins Medical Institutions warned clinical use of stem cells was still a long way off.
But they said their findings would help scientists to better understand how stem cells behaved when they were transplanted into the body.
Motor neurone disease (MND) affects about 5,000 people in the UK.
It is a progressive disorder caused by the break-down of the nerve cells, called motor neurones, which control the muscle activity.
It is characterised by muscle-wasting, loss of mobility, and difficulties with speech, swallowing and breathing.
To investigate whether stem cells - cells that can transform into any type of cell in the body - could help MND sufferers, scientists injected rats, bred to carry the most common form of MND, amyotrophic lateral sclerosis (AMS), with live human stem cells into their lower spines.
They found that 70% of the transplanted cells developed into new nerve cells, and many of them had grown new endings connecting with other cells in the rats' spinal cords.
The onset of the disease, marked by weight loss, was also delayed.
It began on average at 59 days, in the rats injected with live stem cells, compared with 52 days for control rats that had been injected with dead, and therefore inactive, stem cells.
They also discovered the rats with live stem cells grew weaker more slowly and lived longer than those that had received dead stem cell transplants.
Dr Vassilis Koliatsos, from Johns Hopkins Medical Institutions, who worked on the study, said: "We were extremely surprised to see that the grafted stem cells were not negatively affected by the degenerating cells around them, as many feared introducing healthy cells into a diseased environment would only kill them.
"These stem cells differentiate massively into neurons, a pleasant surprise given that the spinal cord has long been considered an environment unfavourable to this type of transformation."
The researchers said their findings were a "proof of principle", and cautioned that more work would need to be done on rats and humans before stem cell treatments could move into the clinic.
Belinda Cupid, research manager at the MND Association, said: "There is an urgent need to conduct research into MND. The causes of the disease are unknown and there is no cure. Therefore the MND Association welcomes the findings of this new research.
"Exactly how stem cells may help motor neurones in MND is very complex. It might be that they can replace the motor neurones or they may improve their environment. We hope that this research will provide some answers to these questions."