Role of Schwann Cells
Schwann cells are active in Wallerian degeneration. They not only have a role in phagocytosis of myelin, but they also have a role in recruitment of macrophages to continue the phagocytosis of myelin. The phagocytic role of Schwann cells has been investigated by studying the expression of molecules in Schwann cells that are typically specific to inflammatory macrophages. Expression of one such molecule MAC-2, a galactose-specific lectin, is observed in not only degenerating nerves that are macrophage-rich but also degenerating nerves that are macrophage-scarce and Schwann cell-rich. Furthermore the effects of MAC-2 in degenerating nerves are associated with myelin phagocytosis. There was a positive correlation between the amount of MAC-2 expression and the extent of myelin phagocytosis. A deficiency in MAC-2 expression can even cause inhibition of myelin removal from injury sites.
Schwann cell are active in demylenation of injured nerves before macrophages are even present at the site of nerve injury. Electron microscopy and immunohistochemical staining analysis of teased nerve fibers shows that before macrophages arrive at the injury site, myelin is fragmented and myelin debris and lipid droplets are found in the cytoplasm of Schwann cells, indicating phagocytic activitiy before macrophages arrive.
Schwann cell activity includes recruitment of macrophages to the injury site. Monocyte chemoatrtactant protein (MCP-1) plays a role in recruiting monocytes/macrophages. In tellurium-induced demylenation with no axon degeneration, nerve crush with axon degeneration, and nerve transection with axon degeneration an increase in MCP-1 mRNA expression followed by an increase in macrophage recruitment occurred. In addition varying levels of MCP-1 mRNA expression also had an effect. Increased MCP-1 mRNA levels correlated positively with an increase in macrophage recruitment. Furthermore in situ hybridation determined that the cellular source of MCP-1 was Schwann cells.
Schwann cells play an important role in not only producing neurotrophic factors such as nerve growth factor (NGF) and ciliary neurotrophic factor (CNTF), which promote growth, of both the damaged nerve and supporting Schwann cells, but also producing neurite promoting factors, which guide the growing axon, both of which are discussed below.
Read more about this topic: Peripheral Nerve Injury
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