The Three Blind Mice of the famed nursery rhyme had little hope of ever recovering their chopped-off tails. But if they’d had access to a brand-new retinal transplant technique, they just might have stood a fighting chance. Researchers have now demonstrated success in grafting healthy eye tissue into previously-blind mice—setting the stage for retinal transplants that could one day restore sight in humans.
Your ability to read the words on this webpage relies on your retina, which is essentially the nerve center of the eye. All light passing through the lens and iris strike this layer at the back of the eyeball, which contains the light-sensing rods and cones that let you see the world. Without these light-sensitive cells, we would see only darkness. So it’s no surprise that when the retina has problems, things go badly in the sight department.
The two most common kinds of retina disorders are retinitis pigmentosa and macular degeneration, which together affect about more than 20 million Americans. In both, a person experiences the world gradually shrinking and darkening before them as they first lose the periphery of their vision, and then the center. And while some treatments can slow the progress of these types of sight-robbing disorders, so far no way has been found to halt or significantly reverse the blindness.
That hasn’t stopped researchers from trying. In addition to creating implantable bionic eyes—essentially robotic retinas—researchers have spent the past decade trying to transplant new photoreceptor cells into the retina to restore lost vision with some modest success. However, they’ve been largely thwarted by the fact that these cells do not integrate well into a person's existing retina when transplanted, thus limiting the potential for restoring sight more fully. These transplants also don’t work for people at the end stages of degeneration, who have lost the outer layer of their retinas.
In 2014, ophthalmologist Michiko Mandai of Japan's RIKEN Center for Developmental Biology set her sights on a different solution to this tenacious problem. Mandai knew that the eye didn’t seem to take well to individual cells. But what about transplanting a fully-formed piece of retinal tissue? A few years back, she helped develop retinal tissue grown from stem cells that, when grafted into mouse eyes, appeared to fully integrate with the existing retinas.
The results were stunning. "At first glance, I almost thought I was looking at a [healthy] retina, not the degenerated retina with transplants," Mandai says of her first reaction to the experimental results. "I was so surprised and excited to see these tissue could develop into a beautifully structured photoreceptor layer, with the most perfect morphology."
The problem: Mandai and her collaborators couldn’t tell whether these functional-looking retinas actually restored vision. In this newest study, she decided to tackle that question. After transplanting stem cell-grown outer nuclear layers into 21 mice that had been bred to develop degenerating retinas, Mandai and her team set about testing their new eyes.
They found that mice with the transplanted retinal tissue in one or both eyes appeared roughly 50 percent better able to recognize light signals that warned them when an electric shock was coming, compared to the mice without the transplant. Later analysis of the brain signals of the grafted mice confirmed that their eyes did appear to recognize light, according to the study published yesterday in the journal Stem Cell Reports. "We could record the robust response to light in a straightforward way, and we were very happy to see these responses," Mandai says.
The next step: human eyes. After testing the safety of their techniques, Mandai and her team hope to start human clinical trials in roughly two years, to find out if grafting retinal tissue from human stem cells can improve sight in people just as well. Mandai cautions that "we cannot expect too much from the beginning" of these trials. Patients will likely only see a small spot of light—which is still better than complete darkness. Continued improvements to the procedure, however, could lead to better and better improvements in eye function, Mandai says.
It may just be a patch of brightness worth waiting for.