Eye images courtesy
Ganglion cells are one of three types of neural cells which make up the retina. By injecting stem cells into the retina, scientists hope to encourage the eye to make more of these bad boys.
Image courtesy University of Delaware Department of Biological Sciences.
Eye diagram courtesy
The cellular layers of the retina, including retinal pigment epithelium, photoreceptors (rods and cones) and neurons.
Courtesy The Schepens Eye Research Institute.
Turning a blind eye
Cataracts can be removed. Corneas can be transplanted. Glaucoma can be treated. But dead photoreceptor cells are, well, dead. In diseases like age-related macular degeneration, retinitis pigmentosa and diabetes, doctors can do little but monitor the cell death in the retina.
Doctors have tried to transplant retinas, but with little success so far, leaving the millions of Americans with these diseases little cause for optimism. Now comes a ray of hope in the quest to restore the retina, courtesy of the all-purpose, handy-dandy stem cell . If you've been diligently reading The Why Files, you know that stem cells are the body's all-purpose cells.
Stem cells are so handy, they remind us of Vise Grips, the plier tool that does almost everything. Stem cells can transmute into many kinds of cells.
Blood stem cells become the red and white cells in your blood.
Neural stem cells can form the neurons that let you think, and the glia, the support cells that help your brain operate.
An eye on the prize
In the gathering excitement over stem cells, this is not a theoretical proposition. Results from the lab of Michael Young, at the Schepens Eye Research Institute and Harvard Medical School, demonstrate that stem cells can repopulate a damaged retina.
Eventually, this work could lead to treatment for some major blinding diseases. But that's far enough down the road to give us time to find out what he did, and what remains to be done.
Young used neural stem cells from the rat's hippocampus, a region in the brain associated with memory, and injected them in front of the retina in rats that go blind due to bad genes. The cells came from Fred Gage and Jasodhara Ray of the Salk Institute in California, who have investigated the role of neural stem cells in learning. And here's the amazing thing: Once in the eye, the cells migrated to the damaged retina, and converted into normal-looking retina cells with normal-looking nerve fibers that extend toward normal-looking locations, like the optic nerve that tells the brain what's going on in front of the eye. The cells were not, however, light detectors, but rather nerves that connect light detectors to the optic nerve.
The fine print
But let's stick with the good news for a moment. Even though there is no sign that the stem cells have become new rods and cones, the essential cells that detect light, just replacing neurons would be a major step forward in treating some of the 100-plus diseases that destroy the retina. For example, the optic nerve dies in the blinding diseases glaucoma due to pressure inside the eye. If the nerve could be regenerated simply by squirting in a few cells -- that could justify several day's wages at the lab bench.
Curiously, stem cells are smart enough to read chemical cues in their environment. In the diseased eyes, they "know" enough to migrate to where they are needed, and to differentiate into some types of neurons. In normal eyes, they do nothing. "This result is strongly supportive of other recent studies, in which stem or progenitor cells seem to respond to injury cues," Young says. "Stem cells have the potential to rewrite the rules for developmental biology and have the potential to repair damaged retina."
He adds, "This is the first definite evidence for survival, migration and nerve cell differentiation for transplanted stem cells in diseased eyes of mature rats." More broadly, it's also "one of the first demonstrations that stem cells can help repair the diseased, mature central nervous system."
But we found more evidence. A group at Washington University has helped repair damaged spinal cord in rats, nine days after the damage, using neural stem cells.
Returning to the vision research, before you set off any fireworks, remember that nobody knows if the new cells are working. "Now we have to determine the level of information being transferred," as Young says. Let's recap what remains to be done:
Find evidence for new rods and cones, the light-detecting cells in the retina.
Demonstrate the technique in people (the next step is to try it with pigs).
We're keeping our eye on the prospect that hearts could grow new arteries on demand.
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