Stem Cells Get Crippled Rats Back on Their Paws

Rats with paralyzed hindlimbs can almost walk again after receiving injections of immature nerve cells. The finding, reported in the December issue of Nature Medicine, suggests that stem-cell therapies might someday lead to treatments for patients with spinal cord injuries they received long ago. However, experts caution that no such therapy is anywhere near the clinic.

The experiments were inspired by the 1996 discovery that mouse embryonic stem cells could be chemically coaxed to become nerve cells in a lab dish. Another research team--neurologists Dennis Choi and John McDonald, both of Washington University in St. Louis, and their colleagues--went on to test whether these cells would survive if transplanted into the nervous system. Once mouse stem cells had developed into precursors of nervous tissue, Choi's team injected them into the spinal cords of 22 adult rats with 9-day-old injuries.

Several weeks later, the researchers found that many of the implanted cells had matured after spreading throughout the rats' injured spinal cords. And almost miraculously, within a month the rats could lift their rear ends and step awkwardly with their hindlimbs. By contrast, rats that had received sham injections could only drag their behinds. "We didn't believe the behavioral recovery when we first saw it," McDonald recalls. But a repeat experiment on a second group of rats confirmed the recovery.

Exactly what accounts for the improvement is still unclear. One idea is that the new mouse neurons wired themselves to rat neurons, thus partially restoring the spinal cord's ability to transmit nerve signals. Another possibility is that some of the mouse-derived cells rebuilt the insulating myelin sheaths around battered spinal cord nerves, enabling them to conduct impulses again. Or perhaps the implanted cells simply secreted chemicals that repair damaged neurons or keep them alive.

Choi's team is now examining the various possibilities so that they can get better results. But even this finding is "compelling," says Oswald Steward, a spinal cord researcher at the University of California, Irvine, College of Medicine. The work, he says, represents "an obligatory first step toward a transplantation therapy for spinal cord injury" based on embryonic stem cells.