Injury to a peripheral nerve leads to degeneration of the segment distal to the site of lesion, a process referred to as Wallerian degeneration. During Wallerian degeneration, axons and myelin sheaths undergo degeneration and are phagocytosed by macrophages and Schwann cells. The Schwann cells proliferate and the endoneurial tubes persist, together the whole structure is known as the band of Büngner. Within few hours, the damaged axons in the proximal stump initiate a regeneration response, with formation of new growth cones. During Wallerian degeneration, neurotrophins, neural cell adhesion molecules, cytokines and other soluble factors are upregulated to facilitate regeneration. The recovery of the target in mammals is often variable, but almost never complete. In humans, scar tissue forms at the site of lesion and this often results in poor recovery of the target. The major events underlying this regenerative process is highlighted and discussed in this review.
Contact repulsion of growing axons is an essential mechanism for spinal nerve patterning. In birds and mammals the embryonic somites generate a linear series of impenetrable barriers, forcing axon growth cones to traverse one half of each somite as they extend towards their body targets. This study shows that protein disulphide isomerase provides a key component of these barriers, mediating contact repulsion at the cell surface in chick half-somites. Repulsion is reduced both in vivo and in vitro by a range of methods that inhibit enzyme activity. The activity is critical in initiating a nitric oxide/S-nitrosylation-dependent signal transduction pathway that regulates the growth cone cytoskeleton. Rat forebrain grey matter extracts contain a similar activity, and the enzyme is expressed at the surface of cultured human astrocytic cells and rat cortical astrocytes. We suggest this system is co-opted in the brain to counteract and regulate aberrant nerve terminal growth.