Electron microscopy of severed motor fibers in the crayfish.

Abstract

Cut and crushed crayfish claw nerves were examined with the electron microscope at intervals up to 6 months after lesion. In sections 1 centimeter distal to the lesion there were no signs of degeneration among the giant motor axons even after many months. Swelling of glial wrappings was observed within 48 hours of nerve severance and was particularly notable in the innermost glial layer, the adaxonal layer. Golgi elements, rough endoplasmic reticulum, and mitochondria accumulated in the glia. These changes were perhaps indicative of a greater supportive role required by the severed axons. Regeneration from the proximal stumps of the giant axons began within one week and had proceeded across the lesion gap by 4 weeks. Axon sprouts appeared to travel toward the terminals within the glial sheaths of the distal giant axon segments. Before regeneration was complete, as determined by a simple behaviour test, the regenerating axons occupied increasing proportions of the sheath space. After regeneration was complete occasional degenerations were seen among the sprouts. These degenerations may have occurred in regenerating axons which had grown to the incorrect muscles. The original distal giant axons probably degenerated, as well, after regeneration was complete. There was no evidence of rehealing of proximal and distal segments of the axons.