Wallerian Degeneration: Definition
Wallerian degeneration (WD) is the stereotyped sequence of structural, molecular and cellular changes that occurs in the distal segment of an axon after interruption of axonal continuity. It begins distal to the site of axotomy, crush, severe traction injury or focal toxic/metabolic damage. Because the distal axon is separated from its neuronal cell body, it loses essential trophic and synthetic support and undergoes fragmentation. The associated myelin sheath also degenerates, while Schwann cells, macrophages and blood vessels participate in removal of the damaged tissue and preparation of the distal nerve for possible regeneration.
The term is used principally for degeneration of the axon and myelin distal to a nerve lesion. Degeneration proximal to the injury is usually limited to a short segment, although the extent depends on the severity and level of injury. The neuronal cell body may undergo a retrograde reaction, including chromatolysis and nuclear displacement, but this is a separate response from distal Wallerian degeneration.
Normal Axonal Dependence and Basis of the Process
An axon is metabolically dependent on its cell body. Proteins, membrane components, cytoskeletal elements and organelles are synthesized in the neuronal soma and transported along the axon by anterograde axonal transport. Conversely,…