Morphological evidence that regenerating axons can fuse with severed axon segments.

Regenerating axons of sensory neurons in the leech nerve cord usually reconnect with their normal targets by growing the entire distance from the site of lesion to the target. However, in less than 1% to nearly 10% of cases a rapid restoration of the normal arborization occurs when the regenerating axon connects with the severed distal segment of the same cell or another cell of the same modality. The passage of horseradish peroxidase (mol. wt approximately 40,000 daltons) from the regenerating axon selectively into the axon or cell with which it has connected indicates that the two have joined or fused, rather than become linked by an electrical synapse, as sometimes occurs for other neurons in the leech. These results support the conclusions, based largely on physiological data from regenerating motor axons in crayfish, that unusually rapid and complete regeneration can occur when a growing axon fuses with its severed distal segment.