ABSTRACT
Although regeneration in the peripheral nervous system (PNS) after injury is robust, regeneration in the central nervous system (CNS) is abortive. The results from differences in the balance of regeneration inhibiting and promoting factors, which in the CNS is skewed toward inhibition while in the PNS it is skewed towards promotion of nerve growth. In addition to lacking regeneration promoting factor the CNS has the ubiquitous distribution of factors that inhibit regeneration. PNS Schwann cells release a number of characterized and uncharacterized neurotrophic factors that exert powerful regeneration promoting influences on axons in the PNS. Thus it has been hypothesize that implantation of Schwann cells, or infusion of factors they release into the lesioned spinal cord should lead to CNS regeneration. However, Schwann cell implants alone are not very successful in promoting CNS regeneration Although still limited, improved regeneration takes place when there is the simultaneously inhibition of CNS regeneration blocking factors and the presence of Schwann cell-released factors. To further improve the extent of CNS regeneration we must determine the best combination of neurotrophic factors to infuse into the site of a CNS lesion, as well as be able to characterize and block all CNS regeneration inhibiting factors. This review examines what is known about promoting and inhibiting regeneration in both the PNS and CNS, and the approaches that may allow us to change the cellular environment of the CNS to one that is permissive to and promotes regeneration.