Keirstead has been turning stem cells into specialized cells that help the brain's signals traverse the spinal cord. Those new cells have repaired damaged rat spines several weeks after they were injured.

For the last two years, he has shown dramatic video footage of healed rats walking to scientific gatherings and during campaign events to promote California's $3 billion bond measure to fund stem cell work, which passed in November.

Keirstead and his colleagues are continuing to experiment with rats to ensure the injected cells do what they're supposed to without any side effects.

"You don't want toenails growing in the brain," he said....

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