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The evolution of complex nervous systems in vertebrates has been accompanied by the acquisition of the myelin sheath. Acting as insulation, myelin is crucial for rapid, saltatory nerve conduction. In the past few years, tremendous progress has been made in our understanding of the myelinforming glia — Schwann cells in the PNS and oligodendrocytes in the CNS — and this has shed new light on their embryonic origins and differentiation, and on the molecular basis of myelination.

During the development of peripheral nerves, neural crest cells generate myelinating and non-myelinating Schwann cells through a protracted process. The molecular regulators responsible for this transition were identified 10 years ago, and since then our knowledge of Schwann cell development has been transformed. As discussed by Jessen and Mirsky (page 671), each developmental stage can now be unambiguously defined by molecular profiles, signalling responses and tissue characteristics. One surprising finding is that Schwann cell precursors, in addition to being the main source of Schwann cells, can give rise to peripheral nerve fibroblasts, which were previously thought to arise from other cell lineages.

Despite the burgeoning knowledge of glial development, until recently little was known about how the myelin sheath is extended and stabilized around axons. However, this has changed, and we now know that these processes are the result of an intricate interaction between axons and myelin-forming glia. In a second article, Sherman and Brophy (page 683) highlight crucial stages of myelination, which include the selection of axons and initiation of axon–glia interactions, the establishment of stable intercellular contact and assembly of the nodes of Ranvier, regulation of myelin thickness and, finally, longitudinal extension of myelin segments in response to lengthening of axons during postnatal growth.

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In This Issue. Nat Rev Neurosci 6, 661 (2005).

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