The origin and development of glial cells in peripheral nerves

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Abstract
The Schwann cell precursor (SCP) represents the first step in the process of gliogenesis in growing nerves. In addition to generating Schwann cells, these cells are likely to provide essential trophic support to sensory and motor neurons and are necessary for the structural cohesion of peripheral nerves. They also have the potential to generate neurons and may be the source of the fibroblast population that is found in peripheral nerves. In many ways our ideas about CNS and PNS glial cells have changed along a similar trajectory during recent years. In both cases, novel and unexpected glial functions have been determined and glia are increasingly recognized as sources of signals that are essential for the survival and function of neurons and other cells. Furthermore, the emerging idea that glial cells can act as multipotent progenitors seems to be true not only in the CNS, but also in the PNS, as SCPs, like radial glia, can give rise to unexpected lineages that were previously thought to arise from different sources. Although a large number of molecules have now been implicated in the regulation of Schwann cell development, it is notable that our knowledge about postnatal events greatly exceeds what we know about the control of the embryonic phase of the lineage. Neuregulin 1, in particular the axon-associated type III isoform, has emerged as a signalling molecule of fundamental importance and considerable versatility, as it is likely to carry out different functions at different stages of the lineage. Another key regulator is SOX10, which is required for the gereration of Schwann cell precursors from the neural crest. Understanding the signals that regulate the appearance of glial differentiation in neural crest cells, and defining the role of positive and/or negative inductive signals or default mechanisms in this key event remain challenging areas. Another important step will be the clarification of the molecular regulators of myelination.