PERIPHERAL-NERVE REGENERATION

Peripheral nerve regeneration comprises the formation of axonal sprout s, their outgrowth as regenerating axons and the reinnervation of orig inal targets. This review focuses on the morphological features of axo nal sprouts at the node of Ranvier and their subsequent outgrowth guid ed by Schwann cells or by Schwann cell basal laminae. Adhesion molecul es such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon a nd axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment betw een axons and Schwann cell basal laminae. The presence of synaptic ves icle-associated proteins such as synaptophysin, synaptotagmin and syna psin I in the growth cones of regenerating axons indicates the possibi lity that exocytotic fusion of vesicles with the surface axolemma supp lies the membranous components for the extension of regenerating axons . Almost all the subtypes of protein kinase C have been localized in g rowth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of gr owth cones to the substrate and their growth activity. The significanc e of tyrosine kinase in growth cones is emphasized. Tyrosine kinase pl ays an important role in intracellular signal transduction of the grow th of regenerating axons mediated by both nerve trophic factors and ad hesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on re generating axons.