Whereas it once seemed a matter of biological fact that neurons of the central nervous system (CNS) do not regrow after being severed, advances in neuroscience research have made considerable progress in turning this dogma on its head. Another piece of evidence to contradict the doctrine that CNS lesions are irreversible was provided recently by Simona Neuman and Clifford Woolf (Neuron 23, 83– 91; 1999), who found that sectioning a peripheral nerve before its central transection can actually stimulate regrowth of the neuronal fibers in the CNS.

Axonal growth into the dorsal column of the CNS stops typically at the site of injury (arrows) when a spinal lesion occurs (upper panel). However, Neuman and Woolf made preconditioning lesions by peripheral sciatic nerve transection 1 or 2 weeks before or 2 weeks after bilateral dorsal column lesions of rat spinal cords (T6–T7).

Anterograde neuronal tracing by injection of conjugated horseradish peroxidase showed that preconditioning results in growth of the ipsilateral sciatic central axons into and across the severed spinal cord (pink fibers, lower panel). The most extensive fiber regrowth was seen with preconditioning at 1 week before central transection. No regrowth was found when peripheral lesions were made after central transection.

The findings reinforce the idea that inhibition of CNS regrowth depends at least partly on the internal state of the growing axons, rather than solely on the neuronal environment. If the way in which preconditioning lesions alter the instrinsic growth state of CNS neurons can be understood and therapeutically harnessed, this basic research could offer clinical hope to paralyzed individuals.