Changes occur in the ability to promote axonal regeneration as the post-injury period increases.

This study tested whether adult rat brain stem neurons could respond to growth or trophic factors provided after an extended post-injury period. The number of neurons that regenerated their axon into a peripheral nerve graft following exposure to ciliary neurotrophic factor (CNTF) 8 weeks after a cervical lesion was comparable to the number regenerating after exposure to CNTF 4 weeks after injury. In contrast, there was a significant decrease of 50% in the number of regenerating neurons following exposure to basic fibroblast growth factor (bFGF) 8 weeks after injury compared with the number regenerating after treatment with bFGF 4 weeks after injury. These results indicate that some factors are effective promoters of regeneration only if provided within a defined post-injury period.