The prospect of reversing blindness has made a significant leap, according to scientists in the UK.
An animal study has shown the part of the eye which detects light can be repaired using stem cells, in what is being described as "significant breakthrough" in treating blindness.
The team at Moorfields Eye Hospital and University College London say human trials on a possible treatment for impaired vision are now, for the first time, a realistic prospect.
Stem cells rebuild light signals to brain
Researchers used a new technique for building retinas in the laboratory from where they collected thousands of stem cells from these retina.
These cells were then primed to become photoreceptors, which react to light and convert it into an electrical signal sent to the brain, and injected into the eyes of blind mice. The key part of the breakthrough came as these cells hooked up with the existing architecture of the eye and began to function.
Photoreceptor cells frequently die off in some causes of blindness such as Stargardt's disease and age-related macular degeneration.
Lead researcher Prof Robin Ali explained: "This is a real proof of concept that photoreceptors can be transplanted from an embryonic stem cells source and it give us a route map to now do this in humans.
"That's why we're so excited, five years is a now a realistic aim for starting a clinical trial."
'Few cells can make a big difference'
However, the effectiveness is still low. Only about 1,000 cells out of a transplant of 200,000 actually hooked up with the rest of the eye. Yet as the immune system is also very weak in the eye so there is a low chance of the transplant being rejected.
Prof Ali contined: "A few cells can make a big difference in the eye. Tens of thousands of stem cells in the eye could improve vision, but that number of stem cells would not regenerate a much larger organ such as a failing liver."
There are already trials in people to use stem cells to replace the "support" cells in the eye which keep the photoreceptors alive. But this is the first study to suggest replacing the cells themselves.
Prof Chris Mason, from University College London, is enthusiastic about the breakthrough but urges caution. "I think they have made a major step forward here, but the efficiency is still too low for clinical uses," he said.
"At the moment the numbers of tiny and it will take quite a bit of work to get the numbers up and then the next question is: can you do it in man?"