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Turning a Blind Eye
the Why Files
University of Wisconsin
|Eye images courtesy Brian's Eye.||
There are lots of ways vision can turn sour -- eye injury, scarred cornea
and cataracts come to mind. Each type of blindness is bad, but the worst
involves disease of the retina where a fiendishly complex set of nerve
cells convert photons of light into nerve signals in thousandths of a
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.
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.
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
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.
If you thought we had lost our thread here, we haven't. Neural stem cells return us directly to vision -- the retina is part of the central nervous system, and neural stem cells can become retinal cells.
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.
|Eye diagram courtesy Brian's Eye.||
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
The fine print That's a lot of normalcy for us Why-Filers, but we hasten to add that nobody knows if the cells are normal. You read that caution correctly. Young does not claim to have answered the crucial question: Do these cells help the rat see? He's hooking up with an expert in the obscure field of rat-vision to answer that one.
| The cellular
layers of the retina, including retinal pigment epithelium, photoreceptors
(rods and cones) and neurons.
Courtesy The Schepens Eye Research Institute.
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:
Date last modified January 29, 2000