Two types of peripheral nerve growth can restore a functional innervation to deprived target tissues

We are interested in the functional reinnervation of target tissues that can occur spontaneously, or be promoted surgically, after nerves are compromised by damage or disease. To learn more of the mechanisms involved in such recoveries–mechanisms which we believe are also involved in the intrinsic plasticity of the adult nervous system–we have studied the sensory and sympathetic nerves supplying the skin of the adult rat. We have focused on their collateral sprouting, an arborizing growth emanating from undamaged nerve axons, and axonal regeneration, an elongating growth induced by axotomy and originating at the region of injury. In vivo, collateral sprouting almost invariably occurs at or within the target tissue of the nerves concerned; experimentally it is usually evoked by partially denervating the target tissue (in our investigations, the skin), following which the remaining undamaged fibres begin to sprout into the deprived areas (Diamond et al., 1976). Axons regenerating after a nerve crush, the preferred injury in experiments to study this phenomenon, almost invariably grow within and along the degenerating nerve trunk to reach their former target regions, where they undergo branching much as in collateral sprouting. Despite an apparently identical morphological and functional end result, the surprising finding which emerged from our animal studies was that collateral sprouting and axonal regeneration of the same nerves are quite differently regulated (Diamond et al., 1987, 1992a, b; Gloster & Diamond, 1992).