Tiam Mana Saffari

14 CHAPTER 1 PERIPHERAL NERVE REGENERATION AFTER INJURY Following traumatic peripheral nerve injury, the nerve undergoes a number of processes. At the proximal stump, the neuronal cell body undergoes chromatolysis. This is characterized by swelling within 48 hours, causing displacement of the nucleus to one side of the cytoplasm (an eccentric nucleus) and fragmentation of the Nissl granule (Figure 2) 12 . The distal nerve segment undergoes a process known asWallerian degeneration to create a microenvironment conducive for axonal reinnervation 13 . Schwann cells clear cell debris through phagocytosis and by recruiting macrophages, and release important neurotrophic factors and cytokines crucial for successful nerve regeneration 12,14 . Within two-three weeks, the axons and myelin degenerate along the entire length of the nerve distal to the lesion, creating an empty tube for axons. Schwann cells in the nerve segment proliferate by aligning longitudinally and form bands of Bünger to guide the regenerating axons from the proximal nerve stump, thus establishing reinnervation of the target muscle 15-17 . Axonal regeneration proceeds at a rate of approximately 1 mm per day 18 . Figure 2. Schematic illustration of Wallerian degeneration. A typical motor neuron with its axon surrounded by myelin sheath, formed by Schwann cells, is depicted. Traumatic injury (e.g. axoto- my) results in immediate tissue damage at the lesion site (marked by square). The proximal nerve segment undergoes chromatolysis and swelling of the nucleus. Schwann cells that surround the distal portion of the axon shed their myelinated lipids (droplets). Phagocytic activity is stimulated by Schwann cells and macrophages to remove debris and subsequently release trophic factors in the preparation of nerve regeneration. With permission of the Mayo Foundation, Copyright Mayo Foundation. All rights reserved.