Homing behaviour of regenerating axons in the amphibian limb.

Following peripheral nerve deviation in the limbs of urodele amphibians axons regrow distally toward their previous target muscles (Holder et al. 1984; Proc. Roy. Soc. Lond. B 222, 477-489). This study describes analysis of this axon regeneration over time following deviation of the forearm flexor nerve in Triturus cristatus and the extensor cranialis nerve in the axolotl. Using horseradish peroxidase (HRP) axonal tracing, electrophysiology and electron microscopy, we describe the sequence of events leading to reestablishment of functional innervation. HRP fills reveal axons leaving the deviated nerve via a number of possible routes and they invariably grow distally. Many axons take a path close to that of the original nerve but others fasciculate forming parallel paths. Electrophysiology and electron microscopy show that axons in the deviated region of the nerve degenerate extensively compared with cut, but undeviated, controls. The results are discussed in terms of the possible axon-growth-promoting mechanisms that result in directed growth.