Doctors at Moorfields Eye hospital in London have been given the go-ahead to carry out Europe's first clinical trial using human embryonic stem cells.They will inject retinal cells into the eyes of 12 patients with an incurable disease, Stargardt's macular dystrophy, which causes progressive sight loss.The disease develops in childhood and affects around one in 10,000 people.It causes the gradual loss of central vision leaving only peripheral sight.The trial will test the safety of using replacement retinal cells known as retinal pigment epithelial cells, derived from human embryonic stem cells.'Milestone'The trial is a partnership with an American bio-tech company Advanced Cell Technology (ACT) which has already begun treating patients in California.Gary Rabin, chairman and CEO of ACT described the Moorfields trial as "another milestone for the field of regenerative medicine".Stem cells are the body's master cells - those from human embryos have the potential to turn into any of the 200 or so tissue types in the body.The stem cell line was created in the United States several years ago using a donated embryo of just a few cells, smaller than a pinhead.Supporters say embryonic stem cell therapy has the potential to treat not only blindness but a huge range of disorders, from heart disease to cancer.Opponents object to the procedure because it began with the destruction of a human embryo.Another US company, Geron, has also used embryonic stem cells to treat patients with spinal injuries.The Moorfields trial and those in the US involving ACT and Geron are all to test whether the procedure is safe.If it is, then progressively larger numbers of stem cells will be used to check for effectiveness.Prof James Bainbridge from Moorfields Eye Hospital said: "There is real potential that people with blinding disorders of the retina including Stargardt's disease and age-related macular degeneration might benefit in the future from transplantation of retinal cells."Prof Chris Mason, Chair of Regenerative Medicine Bioprocessing, Advanced Centre for Biochemical Engineering, University College London, said: "Whilst principally a safety study, it will undoubted significantly add to the growing core of knowledge on cell therapies, thus helping advance the entire field."Earlier this year the National Institute of Health Research (NIHR) awarded Moorfields and UCL Institute of Ophthalmology £26m for work aimed at the translation of scientific advances into new treatments for people with sight-threatening disease.