May 3, 2000 (Ft. Lauderdale, Fla.) — Mice that glow green may sound like something out of The Wizard of Oz, but little rodents that shine like the Emerald City (under ultraviolet light) could hold the key to a treatment that could one day help to repair eyes damaged by common blinding diseases.

A team of Boston researchers has shown that stem cells — primitive “master” cells that can grow into any type of cell in the body — can, in animal models, start a new population of apparently normal vision cells known as photoreceptors. Diseases such as age-related macular degeneration and diabetic retinopathy can cause gradual deterioration of the photoreceptors, accompanied by a usually irreversible loss of sight in the central field of vision.

But, as Michael J. Young, PhD, and colleagues from Schepens Eye Research Institute in Boston reported this week at a major eye research meeting here, it may be possible to one day replace defective photoreceptors with a new, homegrown population.

As you may remember from high school biology class, the eye has cells known as rods and cones — both are types of photoreceptors — that allow us to see motion, shapes, and colors. The photoreceptors live in the central portion of retina, the delicate tissue covering the inside of the eyeball that serves as a screen for receiving and transmitting images to the brain.

Young and his fellow researchers have found in experiments that it may be possible to help damaged retinas heal themselves by injecting a special type of stem cell called a retinal progenitor cell into the eye. The cells would then, ideally, undergo a metamorphosis and take on the characteristics of rods and cones; cell scientists call this type of transformation “differentiation.”

But, choosing the right type of stem cell is of critical importance. “In stem cell research, there is the question of ‘When do you start?’ Some people start right at the beginning with embryonic stem cells that have no fate, and then try to control their differentiation,” Young tells WebMD. “With repair, I think the most important thing is to start at the right point. There are all these branch points, and you want to know, ‘What’s my target, and how do I get there?’ We have no idea how to take an embryonic stem cell and turn it into a photoreceptor; we know a lot about how to take a retinal progenitor cell and turn it into a photoreceptor.”

And that’s where the shiny verdant mice come in. The researchers used a strain of laboratory mice that have been bred to have virtually every cell in their body contain a chemical that will glow green under ultraviolet light. The researchers then teased retinal progenitor cells out of the retinas of these mice and implanted the cells into tissues in the laboratory and into the eyes of other mice. Using ultraviolet light, they were then able to trace where the transplanted cells went and what happened to them.

They found that the transplanted cells, as they grew, began to look and act very much like photoreceptors. The new cells even emitted the chemical rhodopsin (also known as “visual purple”), which is found in normal rods and helps the eye discern movement.

Young is quick to caution that the results, while highly promising, are very early. “We’re quite far from clinical trials of these cells,” he tells WebMD. “First of all, we have to show that these cells are safe in animal models, and we haven’t shown that yet. And we haven’t really shown yet that they’re effective. We still have to show that these cells can actually improve vision in an animal model before we even consider clinical trials.”