Directed regrowth of axons from a misrouted nerve to their correct muscles in the limb of the adult newt.

When the forearm flexor nerve (f.f.n.) of the newt forelimb is surgically rerouted to the ventral body wall, regrowth of axons occurs and these axons reinnervate the muscle targets of the f.f.n. This process of nerve regeneration has been studied in detail over a 12 week period by using light and electron microscopy, electrophysiology and nerve fibre tracking after filling with cobalt chloride. The regrowing axons were analysed by electron microscopy and it is shown that they derive from the rerouted nerve at the position at which the f.f.n. leaves its normal ventral limb pathway. Axons in the pathway do not originate from the cut end of the f.f.n. on the ventral body wall. The regrowing axons are identified within the body of the rerouted nerve and they leave the f.f.n. by growing through the perineurium. Schwann cells are invariably associated with the regrowing axons and the pathway through which the growth cones and neurites grow consists predominantly of extracellular matrix fibrils. The stages of maturation of the regenerated f.f.n. including fasiculation of neurites, myelination and reformation of a perineurium are also described. The results of the study are discussed in terms of current ideas as to how specific regeneration of a correct and functional peripheral nervous system is achieved in urodele amphibians.