To be able to produce some degree of recovery years after a stroke would be a remarkable achievement

Dr Justin Zivin, University of California, San Diego

At present, there is no direct treatment for reversing the damage to the brain caused by a stroke.

The only treatment for these patients is rehabilitation through physical or occupational therapy.

In the study, transplants were given to 12 people had suffered a stroke in the previous six years.

Each patient had a major problem with movement, and some had a paralysed arm or leg.

None experienced complications or other problems related to the transplant.

Lead researcher Dr Douglas Kondziolka said: "This study was designed to evaluate if it's feasible to put these cells into the brain and whether the process is safe.

"With these positive results, now we can move on to a larger study with more patients to find out whether these transplants really help patients recover their lost abilities."

The patients received either two million or six million cells transplanted into three sites within and around the stroke-damaged areas of the brain.

The cells, called LBS-Neurons, originate from human tumour tissue composed of embryonic-like cells.

In the laboratory, scientists have developed a process that uses several chemicals to transform these cells into mature brain cells, or neurons.

Substantial improvements

Six of the 12 patients improved substantially when they underwent test of their ability to walk or move their arm or leg.

Scans also provided evidence that that the transplanted cells were integrating well with existing tissue.

Dr Kondziolka said it was impossible to draw any firm conclusions as the technique had only been tested on a small number of patients.

However, he added: "There were some trends indicating that patients were improving."

Dr Kondziolka said that the signs of improvement were not consistent.

While this research is potentially exciting, there are a lot steps to go through

Eion Redahan, Stroke Association

Some patients had worse scores on the tests used to measure patients' disability after six months than they did at the time of the transplant, and some patients' scores stayed the same over the six months.

Dr Justin Zivin, of the University of California, San Diego, said: "To be able to produce some degree of recovery years after a stroke would be a remarkable achievement.

"But it's too soon to know whether these transplants promise hope for patients disabled with stroke."

Eoin Redahan, of the Stroke Association, also urged caution.

He said: "A lot of people may think this will solve all their problems, and while this research is potentially exciting, there are a lot steps to go through."

Mr Redahan said it was possible that the patients had improved because they thought they were getting a therapy that was doing them some good - what is known as the placebo effect.

He also warned that the use of cells from tumours was potentially fraught with danger, as they tended to divide in an uncontrolled fashion.

The research is published in Neurology, the scientific journal of the American Academy of Neurology.