Within four months, mice that had been transplanted with human stem cells showed long-term recovery of motor function.
Killing Human Cells
Other studies have also shown that stem cells can heal spinal damage. One recent study found that genetically engineered stem cells from rat embryos helped the severed spinal cords of rats grow back together.
To prove the mice recovered because of human stem cell injections, the UC Irvine scientists injected their test animals with diphtheria toxin, which kills only human cells and not mouse cells.
Killing the human stem cells abolished the injured mice's improved walking ability, suggesting the human neural stem cells were the main catalyst for the recovery.
"It's very critical at this stage to do side-by-side comparisons between different types of cells to see which ones are the most capable [of repairing the spinal damage]," said Brian Cummings of UC Irvine, a study co-author.
Itzhak Fischer, chair of the Department of Neurobiology and Anatomy at Drexel University in Philadelphia, Pennsylvania, says the study is significant because it provides "evidence that human stem cells prepared by this group can integrate with the host tissue and directly participate in the repair process of spinal cord injury."
Many questions, however, remain unanswered. One issue is how soon after an injury cells have to be transplanted to have the desired effect.
"These things vary in different animal models," Anderson said. "What's going to happen if you try to put these cells in right after an injury? Is it possible to use these cells in a chronic setting? Some data suggests that it might not be possible."
Still, in the last ten years scientists have made enormous progress in understanding the mechanisms of stem cells and their potential benefits for treating injuries of the central nervous system.
So will we ever see the day when damaged spinal cords are as straightforward to treat as a broken bone?
"I don't know if spinal cord injury will ever be like a broken bone," Anderson said. "The nervous system is the most complex thing you have in your body."
Cummings says scientists may get to the point where they are able to improve someone's function by fixing one segment of the spinal cord.
"We may not get people out of their wheelchairs immediately, but perhaps get them their bowel and bladder functions back," he said. "I think we're going to make smaller steps like that that are not actually walking steps."
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