Image caption Nerve regeneration in the rat

US scientists say they have made progress in repairing spinal cord injuries in paralysed rats.

Rats regained some bladder control after surgery to transplant nerve cells into the spinal cord, combined with injections of a cocktail of chemicals.

The study, published in the Journal of Neuroscience, could raise hopes for one day treating paralysed patients.

But UK experts say it will take several years of research before human clinical trials can be considered.

Scientists have tried for decades to use transplants of nerve cells to restore function in paralysed animals by bridging the gap in the broken spinal cord.

However, coaxing the cells to grow and form new connections has proved elusive.

One problem is the growth of scar tissue as the body's responds to injury, which seems to block cell regeneration.

If we can show in a larger animal that our technique works and does no additional harm I see no reason why we couldn't move rapidly in humans Dr Jerry Silver, Case Western Reserve Medical School, Cleveland, Ohio

US scientists carried out complex surgery to transplant nerves from the rodents' ribs into the gap in the middle of their spinal cord.

They also used a special "glue" that boosts cell growth together with a chemical that breaks down scar tissue in an attempt to encourage the nerve cells to regenerate and connect up.

The researchers found for the first time that injured nerve cells could re-grow for "remarkably long distances" (about 2cm).

They said that while the rats did not regain the ability to walk, they did recover some bladder function.

Lead author Dr Jerry Silver of Case Western Reserve Medical School, Cleveland, Ohio, said: "Although animals did not regain the ability to walk, they did recover a remarkable measure of urinary control."

Co-author Dr Yu-Shang Lee of the Cleveland Clinic, Ohio, added: "This is the first time that significant bladder function has been restored via nerve regeneration after a devastating cord injury."

The findings may help future efforts to restore other functions lost after spinal cord injury.

They also raise hope that similar strategies could one day be used to restore bladder function in people with severe spinal cord injuries.

Dr Silver said further animal experiments will be needed to see if the technique could work in humans.

He told BBC News: "If we can show in a larger animal that our technique works and does no additional harm I see no reason why we couldn't move rapidly in humans."

'Remarkable advance'

Commenting on the study, Dr Elizabeth Bradbury of King's College London said several challenges must be overcome before the therapy can be trialled in patients.

"There are a number of challenges before this therapy can be brought to the clinic," she said.

"Nevertheless this is a remarkable advance which offers great hope for the future of restoring bladder function to spinal injured patients and if these challenges can be met we could be reaching clinical trials within three to five years."

Dr John Williams, head of neuroscience and mental health at the Wellcome Trust, said the implications for people are not yet clear.

"This is one of a number of ways that one can approach restoration of bladder function in paralysed patients, but careful studies will be needed to optimise which of the technologies under investigation might be of most benefit to patients."