In a scientific feat that borrows from fiction, once-blind mice can see (and run - keep reading to see how) after receiving a retinal injection of light-sensitive cells - an approach that could one day help humans with disabling eye diseases.

"If we transplant cells in large enough numbers at the same developmental stage, these cells are able to interact with themselves and with the retinal environment to re-form the light-sensing layer," said Robert MacLaren, a professor of ophthalmology at the University of Oxford and lead author of a study published in the Proceedings of the National Academy of Science.

Using a mouse model of retinal degeneration, MacLaren and colleagues tested the effects of transplanted photoreceptor precursor cells - an intermediate between stem cells and the light-sensing cells of the adult eye.

"The cells are transplanted through a very, very fine needle," said MacLaren, describing the "seeding" hundreds of thousands of cells suspended in liquid between the two planes of the retina. "It's like lifting up topsoil, sowing the seeds and then rolling it back into position."

After two weeks, the transplanted cells had replaced the retina's light-detecting layer, complete with connections to the optic nerve. A pupil constriction test found that 10 of the 12 mice showed an improved response to light, suggesting the once-blind mice could actually see - a finding bolstered by behavioral tests in which the mice, which are nocturnal, ran away from light. Before the transplant, they stayed put.

"These mice represent the oldest, most end-stage generation you could get in a patient - someone who has been blind for years," said MacLaren, describing how the technique might be able to restore vision in people with decades-old degenerative eye disease. He added that the photoreceptor precursor cells could even be prepared from the patient's own tissue using induced pluripotent stem cell technology, which reprograms skin or blood cells into stem cells that can become multiple tissue types.

"Once we can address the safety issues, we can start clinical trials in patients," MacLaren said. "If we can get to that stage, there is every potential that patients that are completely clinically blind will be able to see again to some extent."