Stem cell therapy came one step closer to being tested for the first time in people this week, as a US company applied to the Food and Drug Administration (FDA) for a licence to start a clinical trial. The embryonic stem (ES) cell therapy is being developed to treat Stargardt, a currently incurable disease that causes blindness in young people.

Advanced Cell Technology, a Massachusetts-based firm, has tested the treatment on rats and mice which model the human Stargardt disease, and found it prevented further vision loss without adverse side effects.

Stargardt's macular degeneration destroys a part of the retina, leading to loss of vision. The treatment involves a single injection of a type of retinal cell that supports the photoreceptors needed for vision. The researchers have generated the retinal cells from ES cells derived from day-old human embryos left over from IVF (in vitro fertilisation) treatment and donated for research purposes. ES cells have the potential to develop into any type of cell in the body and, as such, scientists envisage they could be used to mend any broken part of the body, from kidney and liver to brain. They hold promise as a treatment for a range of diseases from Parkinson's to heart disease. However, using ES cells in medical research is controversial as many view the embryo as sacrosanct and inviolable, and oppose the use of human embryos in research.

'After years of research and political debate, we're finally on the verge of showing the potential clinical value of embryonic stem cells,' said Dr Robert Lanza, the chief scientific officer at Advanced Cell Technology. He added: 'We've seen absolutely no adverse effects whatsoever in any of the preclinical experiments and our cells are more than 99.9 per cent pure.'

'Our research clearly shows that stem cell-derived retinal cells can rescue visual function in animals that otherwise would have gone blind. We are hopeful that the cells will be similarly efficacious in patients,' Dr Lanza continued.

The trials would involve 12 Stargardt patients, and could begin early next year if the FDA grants approval. The clinical trial could be the first in the world as US company Geron, the only company so far to have received a licence from the FDA, has delayed the start of its clinical trial until the end of next year due to safety problems.

A similar proposal to treat age-related macular degeneration with ES cells is being developed by scientists in the UK led by Professor Pete Coffey of University College London, but this clinical trial is unlikely to start until early 2011.