hey are completely severed, but researchers believe that in a partial injury, surviving nearby axons that serve the same general body area and function can "sprout" new connections to those injured nerve cells that have lost some of their axons. In studying spinal cord injury in rats − the usual model for this kind of investigation − researchers had thought that younger rats ("pups") regain function faster because this sprouting occurs more quickly and proficiently than in older rats. "Just as young trees grow more quickly if you prune them than do older trees, we thought than in young animals, surviving axons would sprout new, and longer, axonal connections more readily," Wrathall said.

But their findings surprised them. "We didn't see that sprouting was faster or better in younger than in adult rats after a partial spinal cord injury," she said. Instead, they saw distinctions in what occurred in cells within the spinal cord at the site of injury. Leung and Wrathall specifically discovered that in the pups, specialized neural stem cells grew vigorously after injury and within one week, many oligodendrocytes, cells whose function is to provide a protective myelin sheath to axons, were produced..

The researchers believe that these activated cells wrap nearby surviving axons with extra myelin sheathing in order to protect them and support their function after injury.

"The ability of axons to transmit their signals is greatly dependent on the insulation provided by their myelin sheaths, and we know that axons near the site of injury eventually can die due to loss of this myelin," Wrathall said. "So we believe these stem cells work to protect healthy axons against toxic factors in the microevironment."

Adult rats do not activate these specialized cells to the same extent as the pups do after injury, for reasons that are not understood, she added.

"We hadn't expected these results, but they are exciting for the
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Contact: Laura Cavender
lsc6@georgetown.edu
202-687-5100
Georgetown University Medical Center
8-Nov-2006

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