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British scientists have restored the sight of blind mice by transplanting light-sensitive photoreceptor cells into their eyes.

The work is a step towards a new treatment for patients with degenerative eye diseases.

Scientists at University College London Institute of Ophthalmology injected cells from young healthy mice directly into the retinas of adult mice that had night-blindness.

The findings are published in .

The cells transplanted were immature rod-photoreceptor cells, which are especially important for seeing in the dark.

After four to six weeks up to one in six of the transplanted cells had formed the connections needed to transmit visual information to the brain.

Hidden platform

The researchers tested the vision of the treated mice in a dimly-lit water maze.

Those mice with transplanted rod cells were able to see a visual cue to find a hidden platform to enable them to get out of the water. This was in contrast to untreated mice who found the platform only by chance after lengthy exploration of the maze.

Prof Robin Ali, at UCL Institute of Ophthalmology and Moorfields Eye Hospital, who led the research said: "We've shown for the first time that transplanted photoreceptor cells can integrate successfully with the existing retinal circuitry and truly improve vision.

"We're hopeful that we will soon be able to replicate this success with photoreceptors derived from embryonic stem cells and eventually to develop human trials."

Prof Ali said the behavioural maze test was "ultimate proof" that a significant amount of vision had been restored in the treated mice.

But although the results appear promising, there are still many steps to go before such a treatment might be suitable for patients.

There are two types of photoreceptor in the eye - rods and cones. It has so far proved harder to transplant cone photoreceptors - which are crucial for human sight and tasks like reading.

The scientists also plan to experiment with photoreceptors derived from embryonic stem cells. Prof Ali said such cell lines already exist but the question is how efficiently they can transplant them.

Loss of photoreceptors is the cause of blindness in many human eye diseases including age-related macular degeneration (AMD), retinitis pigmentosa and diabetes-related blindness.

But many more animal studies will be needed before such a technique would be tried with humans.

'Great encouragement'

The research was funded by the Medical Research Council, the Wellcome Trust, the Royal Society the British Retinitis Pigmentosa Society, Alcon Research Institute and The Miller's Trust.

Dr Rob Buckle, head of regenerative medicine at the MRC said: "This is a landmark study that will inform future research across a wide range of fields including vision research, neuroscience and regenerative medicine.

"It provides clear evidence of functional recovery in the damage eye through cell transplantation, providing great encouragement for the development of stem cell therapies to address the many debilitating eye conditions that affect millions worldwide."

There are already a number of research programmes aiming to treat blindness using cell transplants.

Last year, the same research group were given the go-ahead to carry out Europe's first clinical trial involving human embryonic stem cells at Moorfields Eye Hospital.

That study involves patients with Stargardt's disease, one of the main causes of blindness in young people. Early results suggest the technique is safe but reliable results will take several years.