Why does the central nervous system not regenerate after injury?

Spinal cord injuries in humans and in other mammals ale never followed by r egrowth. In recent years, considerable progress has been made in analyzing mechanisms that promote and inhibit regeneration. The focus of this review is changes that occur in the transition period in development when the cent ral nervous system (CNS) changes from being able to regenerate to the adult state of failure. In our experiments we have used the neonatal opossum (Mo nodelphis domestica), which corresponds to a 14-day embryonic rat or mouse. The CNS isolated from an opossum pup and maintained in culture shows drama tic regeneration. Fibers grow through and beyond lesions and reform synapti c connections with their targets. Similarly, anesthetized neonatal. pups at tached to tilt: mother recover the ability to walk after complete spinal co rd transection. Although the CNS isolated from a 9-day-old animal will rege nerate in vitro, CNS from a 12-day-old will not. This is the stage wt which glial cells in the CNS develop. Present research is devoted toward molecul ar screening to determine which growth-promoting molecules decrease during development, which inhibitory molecules increase, and which receptors on gr owing axons become altered. Despite progress in many laboratories, major hu rdles must be overcome before patients can hope to be treated. Nevertheless , the picture today is not as discouraging as it was: one can think of stra tegies for research on spinal cord injury so as to promote regeneration and restore function. (C) 1999 by Elsevier Science Inc. All rights reserved.