Adult sensory neurons regenerate axons through adult CNS white matter: Implications for functional restoration after SCI

It has been a long-held belief in the CNS regeneration scientific community that, in addition to intrinsic neuronal limitations and physical/molecular barriers presented by glial scarring at an injury site, the whole surrounding environment of the adult brain and spinal cord is generally nonpermissive for axon growth. Until recently, it was very difficult to separate and scientifically assess the relative contributions of each of the above to the ultimate regeneration failure of cut adult axons after SCI. However, with the advent of a novel, minimally traumatic microtransplantation system, it has been possible to test the ability of normal and degenerating CNS pathways in the brain and spinal cord to support the growth of axons from grafted adult neurons in the absence of glial scarring. Remarkably, in the absence of scarring, it was shown that normal and even degenerating adult CNS white matter supported rapid and long-distance growth of axons from the microtransplanted adult neurons. Axon regeneration failure only occurred upon contacting scar tissue and its associated inhibitory molecules. The implications of these studies for repair of spinal cord injuries are reviewed.