Regeneration of subnormally innervated axolotl arms.

Forearms of juvenile axolotls contain about 5000 axons, of which only 25% are myelinated and visible by light microscopy. Virtually all the axons degenerate after transection of the brachial plexus, but repeated operations fail to keep the arm completely denervated. Regrown nerve fibres were detected by electron microscopy after 6 weeks of attempted denervation and related to the quantity usually considered necessary for limb regeneration. Such arms regenerated quite normally, provided their innervation had been depleted for several weeks before amputation. Among other ways of reconciling these observations to the neurotrophic theory of limb regeneration, it is suggested that tissues can adapt to deprivation of their nerve supply.