See, blind mice.

Mice born without eyesight are now seeing the light thanks to teams of researchers in the U.S. and England. The scientists were able to repair the blind mice's vision by replacing damaged eye cells, called photoreceptors, with healthy ones.

Millions of light-sensitive photoreceptor cells can be found in the eye's retina, a thin layer of tissue at the back of the eye. "The retina contains two types of these photoreceptors--rods and cones. Rods help you see at night, and cones allow you to have color vision and see in the daytime," says Anand Swaroop, a professor of ophthalmology at the University of Michigan. In Swaroop's study, none of the mice's rod cells were working.

Eyes don't repair damaged photoreceptors because usually there are millions of others to pick up the slack. For the blind mice, Swaroop and other researchers would have to figure out how to make new rod cells. To do this, they took immature cells that were on their way to becoming photoreceptors from the eyes of young mice. Then they transplanted these cells into the retinas of the blind mice. The cells successfully developed into healthy rod cells.

Since blindness in people can also be caused by nonworking rod cells, the treatment could be a step toward sight for humans too, says Swaroop.