Axon-Schwann Cell Interactions during Peripheral Nerve Degeneration and Regeneration

Abstract Wallerian degeneration refers to all of the events that occur distal to the site of axotomy (for recent reviews, see Ide, 1996; Fu and Gordon, 1997; Stoll and Muller, 1999; Terenghi, 1999). Although usually induced by mechanical injury in experimental studies, Wallerian degeneration is a feature of any insult that causes axonal degeneration. During the first week post-axotomy, axons fragment and disappear, and the myelin sheaths separate at incisures, breaking up into so-called ovoids. Over the next few weeks, these myelin ovoids are phagocytosed in part by Schwann cells but mainly by macrophages that invade the degenerating nerve. The clearance of myelin debris by macrophages promotes axonal regeneration (Dahlin, 1995), perhaps owing to the ability of myelin proteins to inhibit axonal regeneration (see Chapter 14). Schwann cells undergo extensive proliferation between 3 and 5 days post-axotomy (Oaklander and Spencer, 1988). The basal lamina persists and surrounds the column of ‘denervated’ Schwann cells, traditionally called a band of Büngner..