Ultrastructural studies of severed medial giant and other CNS axons in crayfish.

The distal stumps of severed medial giant axons (MGAs) and of nongiant axons (NGAs) in the CNS of the crayfish Procambarus clarkii show long-term (5--9 months) survival associated with disorientation of mitochondria and thickening of the glial sheath. However, the morphological responses of the two axonal types differ in that neither the proximal nor the distal stump of severed MGAs ever fills with mitochondria as is observed in some severed NGAs. Furthermore, the adaxonal glial layer never completely encircles portions of MGA axoplasm as occurs in many severed NGAs; in fact, ultrastructural changes in the adaxonal layer around severed MGAs are often difficult to detect. No multiple axonal profiles are ever seen within the glial sheath of the proximal or distal stumps of severed MGAs whereas these structures are easily located within severed NGAs.