For the first time, researchers have witnessed severed spinal cord fibers regrow on their own in adult mammals. The findings, reported in the May issue of Neuron, raise hopes for nerve regeneration as a treatment someday for spinal cord injuries.
Most neurons receive and transmit signals via a single axon and many short dendrites. The axons extending to the brain are unable to repair themselves. "Previously, we knew that there were two major barriers to regeneration of these axons: the environment that the axons have to grow through and the neuron's failure to turn on some of the genes that are needed for axon growth," explains Pate Skene, a neurobiologist at Duke University.
Clifford Woolf, an anesthesiology researcher at Massachusetts General Hospital in Boston, and his colleagues questioned the assumption that the cellular environment blocks axon regrowth. To test the hypothesis, the researchers examined neurons with two axons. They cut both the axons leading to the sciatic nerve (the main sensory nerve in the leg) in 10 rats and snipped the other axons that lead to the spinal cord. This second cut, they reasoned, might allow whatever growth factors heal the peripheral nerve to also repair the spinal cord axon. After 6 to 8 weeks, they found, all the rats had numerous axons regrowing near the spinal cord injury.
In a second experiment, the researchers injured the peripheral nerve a week before injuring the spinal cord; the axon fibers regrew completely across both injury sites. In half the rats, axons had even extended halfway to the brain. "A complete regeneration across the injury site had been achieved," says Woolf.
"This is a very robust phenomenon," says Skene. The next step, Woolf adds, is to find out what growth signals may have come from peripheral axons. "Once we know that," he says, "we can then think of applying this to humans."