The world's first clinical trial of brain stem cells to treat strokes is set to move to its next phase.An independent assessment of the first three patients to have had stem cells injected into their brain at Glasgow's Southern General Hospital has concluded it has had no adverse effect.The assessment paves the way for the therapy to be tested on more patients to find a new treatment for stroke.The hope is that the stem cells will help to repair damaged brain tissue.The trial is being led by Prof Keith Muir of Glasgow University. He told BBC News that he was pleased with the results so far."We need to be assured of safety before we can progress to trying to test the effects of this therapy. Because this is the first time this type of cell therapy has been used in humans, it's vitally important that we determine that it's safe to proceed - so at the present time we have the clearance to proceed to the next higher dose of cells."An elderly man was the first person in the world to receive this treatment last year. Since then it has been tried out on two more patients.Global trialsThe patients have received very low doses of stem cells in trials designed to test the safety of the procedure.Over the next year, up to nine more patients will be given progressively higher doses - again primarily to assess safety - but doctors will also be using this clinical trial to assess the best ways of measuring the effectiveness of the treatment in subsequent larger trials, which would not begin for at least 18 months.There are a growing number of well-regulated clinical trials of stem cell treatments now under way in various parts of the world, including one which also began last year by the US firm Geron to develop a treatment for paralysis.The development of stem cell treatments is still at an early stage and it is likely to be many years before these treatments become widely available.Strokes kill about 67,000 people in the UK every year, according to the Stroke Association.The charity says it is the third most common cause of death in England and Wales after heart disease and cancer.The stroke trial is being carried out with Reneuron Group plc. The company's chief executive officer Michael Hunt said there was a long way to go."The earliest a treatment could be widely available if everything goes very well is five years. It is very much a case of so far, so good. It is still at a very early stage but we draw great comfort from these results."