Wallerian degeneration

If a nerve fiber is cut or crushed, the part distal to the injury (i.e. the part of the axon separated from the neuron's cell nucleus) will degenerate, in a process known as Wallerian degeneration.

Process
The axons and surrounding myelin sheath are digested by macrophages and Schwann cells (only found in the peripheral nervous system). The target tissue of the nerve (typically an innervated muscle) atrophies as a result.

The nerve fiber's neurolemma does not degenerate and remains as a hollow tube. Within 96 hours of the injury, the proximal end of the nerve fiber sends out sprouts towards those tubes and these sprouts are attracted by growth factors produced by Schwann cells in the tubes. If a sprout reaches the tube, it grows into it and advances about 1-3 mm per day, eventually reaching and reinnervating the target tissue. If the sprouts cannot reach the tube, for instance because the gap is too wide or scar tissue has formed, surgery can help to guide the sprouts into the tubes. This regeneration however happens only in peripheral nerves, not in the spinal cord. The crucial difference is that in the central nervous system, including in the spinal chord, myelin sheaths are produced by oligodendrocytes and not by Schwann cells. Oligodendrocytes secrete growth inhibiting myelin-associated glycoprotein.

History
Wallerian degeneration is named after Augustus Volney Waller who first described it in 1850, in the frog. The degenerated axon forms droplets that can be stained, and this allowed the detailed study of the course of individual nerve fibers.

Delayed Wallerian degeneration
Mice belonging to the strain C57BL/Ola have delayed Wallerian degeneration, and thus allow to study the roles of various cell types and the underlying cellular and molecular processes.