Developmental regulation of regenerative specificity in the bullfrog

The accuracy with which motor axons reinnervate the hindlimb following ventral rhizotomy was studied in the bullfrog (Rana catesbeiana) by mapping the locations within the lumbar lateral motor column of motoneurons retrogradely labeled with horseradish peroxidase. At early stages of development, axons regenerate to the correct hindlimb region. In older tadpoles and frogs, axons appear to regenerate to targets determined by basal lamina tubes, which persist in the distal nerve stump after the severed nerve fiber degenerates. Basal lamina tubes develop at the stages when regenerative specificity is no longer expressed. These results led to the hypothesis that guidance cues may be present in older animals, and axons may be capable of responding to these cues, but are prevented from doing so because they are confined in basal lamina tubes. Results obtained from crosstage hindlimb transplants are consistent with this hypothesis.