Axonal regeneration in the adult lamprey spinal cord

Larval sea lampreys recover from complete spinal transection by a process involving directionally specific axonal regeneration. In order to determine whether this is also true of adults, 14 adult lampreys were transected at the level of the 5th gill and allowed to recover for 10 weeks. Müller and Mauthner cells and their giant reticulospinal axons (GRAs) were impaled with microelectrodes and injected with horseradish peroxidase (HRP). The tissue was processed for HRP histochemistry and wholemounts of brain and spinal cord were prepared. All animals recovered coordinated swimming; 61 of 121 (50%) neurites emanating from 30 axons regenerated caudal to the scar into the distal stump. Of the neurites which had grown beyond the scar, 92% were correctly oriented, i.e., caudalward and ipsilateral to the parent axon. Retransection in two additional animals eliminated the recovered swimming. Thus, behavioral recovery in adult sea lampreys is accompanied by directionally specific axonal regeneration.