Regeneration of motor axons in crayfish limbs: distal stump activation followed by synaptic reformation.

Severed distal stumps of limb motor axons in the crayfish Procambarus clarkii remain ultrastructurally intact for at least 2-3 ms after being severed from their cell body. Initial regeneration of a motor axon is associated with the appearance of up to 200 small profiles (satellite axons) having no glial sheath adjacent to the large surviving stump for about 1 cm distal to the lesion at 4-5 wks postoperatively. These satellite axons are seen 2-4 cm distally at the target muscles 3-4 ms postoperatively. By 14-15 ms postoperative, the motor sheaths from the lesion site to the target muscles contain small axonal processes having thick glial sheaths. Behavioral tests show that some axons that are reconnected to the CNS at 4-5 wks may not be connected at 14-15 ms, whereas other axons not connected by 3-4 ms may be connected at 14-15 ms when the original distal stumps have degenerated. We suggest that all these data can best be explained by the view that motor axons in crayfish limbs initially regenerate via activation of the surviving distal stump by satellite axons which grow out from proximal stump. In most cases, these satellite axons continue to activate the surviving distal stump as they slowly grow to the target muscle. Eventually the satellite axons reform synapses on the target muscle and the original distal stump degenerates.