Regeneration of earthworm giant axons following transection or ablation.

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Abstract
The regenerative ability of the septate medial giant axon (MGA) and the septate lateral giant axons (LGA) of the earthworm ventral nerve cord (VNC) was investigated following simple transection, or following ablation of 1 or 3 segments from the VNC. Physiological evidence for regeneration was assessed via extracellular and intracellular recordings, while morphological evidence for regeneration was analyzed using standard paraffin histology and/or intracellular injection of lucifer yellow. The giant axons showed physiological evidence of regeneration following VNC transection to sever the axons, or following VNC ablations to remove the original cell bodies in the ablated segments. Action potentials of regenerated giant axons were conducted slowly through the lesion site compared to normal. Action potentials in transected axons were usually unidirectionally conducted at 1-3 wk postoperative and bidirectionally conducted as postoperative time increased from 1-8 mo. Following ablation, action potentials were often conducted bidirectionally at 1-2 mo. postoperatively, but were usually conducted unidirectionally as postoperative time increased from 2-9 mo. Morphological examination of regenerated axons injected with lucifer yellow showed that neuronal processes grew out from both the anterior and posterior stumps, neither of which was necessarily in direct contact with a cell body. These axonal processes usually associated with the appropriate homologous neuron on the opposite side of the lesion. Some processes grew beyond or away from the lesion site. Although processes of giant axons grew in inappropriate directions, functional misconnections were not detected in any regenerated giant axon. Ablated cell bodies of the giant axons were not replaced. Ablated cell bodies of some nongiant axons did regenerate. The mechanism of giant axon regeneration is rather similar following transection or cell body ablation, i.e., neuronal processes arising from a giant axon grow across the lesion site and make functional connections only with the appropriate giant axon on the opposite side of the lesion.

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