Abstract
The remarkable plasticity of Schwann cells allows them to adopt the Remak (non-myelin) and myelin phenotypes, which are specialized to meet the needs of small and large diameter axons, and differ markedly from each other. It also enables Schwann cells initially to mount a strikingly adaptive response to nerve injury and to promote regeneration by converting to a repair-promoting phenotype. These repair cells activate a sequence of supportive functions that engineer myelin clearance, prevent neuronal death, and help axon growth and guidance. Eventually, this response runs out of steam, however, because in the long run the phenotype of repair cells is unstable and their survival is compromised. The re-programming of Remak and myelin cells to repair cells, together with the injury-induced switch of peripheral neurons to a growth mode, gives peripheral nerves their strong regenerative potential. But it remains a challenge to harness this potential and devise effective treatments that maintain the initial repair capacity of peripheral nerves for the extended periods typically required for nerve repair in humans.
Highlights
OVERVIEW OF SCHWANN CELLS AND NERVE INJURYExamination of the Schwann cells in uninjured nerves shows two surprisingly different cell types
An innate immune response is activated. This involves the upregulation of cytokines including tumor necrosis factor α (TNFα), interleukin-1α (Il-1α), Il-1β, leukemia inhibitory factor (LIF), monocyte chemotactic protein-1 (MCP-1) and toll-like receptors by the Schwann cells in the distal stump
Protein levels of cytokines such as Il-1β and TNFα, peak within 1 day of injury, but are sharply lower at 3 days (Rotshenker, 2011), autophagy is high at 5 days and reduced thereafter (GomezSanchez et al, 2015), glial cell line-derived neurotrophic factor (GDNF) protein levels peak at about 1 week after injury and brainderived neurotrophic factor (BDNF) is maximally expressed after 2–3 weeks (Eggers et al, 2010)
Summary
Examination of the Schwann cells in uninjured nerves shows two surprisingly different cell types. The larger axons, including some sensory axons and the axons of motor neurons, are wrapped by myelin Schwann cells They are 2–3 times longer that Remak cells and much bulkier, containing the myelin sheath, which is formed by the Schwann cell membrane wrapping multiple times around the axon and condensing to form a compact myelin cuff around the axon. Both Remak and myelin cells are coated by a basal lamina, outside of which lies the connective tissue, the endoneurium, which contains fibroblasts, blood vessels and a few macrophages and is surrounded by a multi-layered cellular tube, the perineurium (Figure 1). Small nerves are uni-fascicular, while large nerves contain many fascicles bound together by connective tissue, the epineurium
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