Nerve fibre regeneration after traumatic lesions of the CNS; progress and problems

Abstract

Four distinct phases can be distinguished in the regenerative response of a lesioned CNS axon: sprouting of the proximal axon stump, elongation, target recognition, and formation of appropriate synapses. These processes can be observed in such a way only in lower vertebrates, in particular, in the optic system. In these species and systems, both, the guidance mechanisms leading regenerating fibres to their former target areas, and the mechanisms responsible for specific synapse formation are retained throughout life. As during development, guidance crucially depends on the presence of favourable substrate molecules, and on chemotropic signals (Dodd & Jessel 1989; Tessier-Lavigne et al .1988; Harris 1989). The cell biological mechanisms responsible for target recognition including the arrest of long-distance growth, the initiation of side branch formation and terminal arborization, and the selection of specific post-synaptic partners (cell type; soma, proximal or distal dendrite, spines, axons) remain unknown up to now.