Stem Cells Repair Damaged Spinal Cords in Mice

Stefan Lovgren
for National Geographic News
September 20, 2005
A new study shows that human stem cells injected into mice can repair damaged spinal cords and help partially paralyzed mice walk again.

Although many questions remain unanswered, the research raises the hope of using stem cells to help people with spinal cord injuries.

"We're very excited about these stem cells," said Aileen Anderson of the Department of Physical Medicine and Rehabilitation at the University of California, Irvine. "We're really on the cusp of making some big leaps forward."

Previous research has suggested that human stem cells can help rodents recover from spinal injuries. But the new study marks the first time that scientists have shown that human stem cells make connections with the nervous systems of the mice and are thus key to recovery from spinal cord injuries.

The findings are reported today in the journal Proceedings of the National Academy of Sciences.

Making Connections

Stem cells are primitive cells with the power to transform into the various cells and tissues found in the human body. There are many types of stem cells, from bone marrow to fetal tissue to embryos.

For their experiment, the UC Irvine researchers used fetal brain stem cells provided by StemCells Inc., a Palo Alto, California-based company. The scientists injured the spinal cords of mice and nine days later injected them with the stem cells.

Using mice, scientists can easily mimic the damage caused to humans in a car or diving accident, making mice an excellent model to study neurological diseases.

The researchers found that the stem cells migrate up the spinal cord and develop into multiple different neural cell types, including neurons and oligodendrocytes, the cell type that forms insulating myelin sheaths around nerves.

Myelin is a nerve-fiber coating that plays a critical role in maintaining the nervous system's electrical conduction. Injuries or disease that strip away this protective layer can lead to sensory or motor deficiencies and sometimes paralysis.

"We show that these [stem] cells make connections with the nervous system of the mouse in a way that is appropriate … and could mediate recovery," Anderson said. "That's a big thing."

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."

Broken Bones

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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