Editing Myelin (section)

== Development ==
{{Main|Myelination}}
{{Further|Critical period#Myelin}}

The process of generating myelin is called [[myelination]] or ''myelinogenesis''.  In the CNS, [[oligodendrocyte progenitor cells]] [[Cellular differentiation|differentiate]] into mature [[oligodendrocyte]]s, which form myelin. In humans, myelination begins early in the [[third trimester]] which starts at around week 26 of [[gestational age]].<ref>{{Cite web|url=http://library.med.utah.edu/pedineurologicexam/html/dev_anatomy.html|title=Pediatric Neurologic Examination Videos & Descriptions: Developmental Anatomy|website=library.med.utah.edu|access-date=2016-08-20}}</ref> The signal for myelination comes from the axon; axons larger than 1-2 [[micrometre| μms]] become myelinated.<ref name="Larsen2015">{{cite book |last1=Schoenwolf |first1=Gary C. |last2=Bleyl |first2=Steven B. |last3=Brauer |first3=Philip R. |last4=Francis-West |first4=P. H. |title=Larsen's human embryology |date=2015 |publisher=Churchill Livingstone |location=Philadelphia, PA |isbn=9781455706846 |page=242 |edition=Fifth}}</ref> The length of the internode is determined by the size of the axonal diameter.<ref name="Larsen2015"/> During infancy, myelination progresses rapidly, with increasing numbers of axons acquiring myelin sheaths. This corresponds with the development of cognitive and motor skills, including language comprehension, [[speech acquisition]], crawling and walking. Myelination continues through adolescence and early adulthood and although largely complete at this time, myelin sheaths can be added in [[grey matter]] regions such as the [[cerebral cortex]], throughout life.<ref>{{cite journal|vauthors=Swire M, French-Constant C|title=Seeing Is Believing: Myelin Dynamics in the Adult CNS|journal=Neuron|volume=98|issue=4|pages=684–86|date=May 2018|pmid=29772200|doi=10.1016/j.neuron.2018.05.005 |doi-access=free}}</ref><ref>{{cite journal|vauthors=Hill RA, Li AM, Grutzendler J|title=Lifelong cortical myelin plasticity and age-related degeneration in the live mammalian brain|journal=Nature Neuroscience|volume=21|issue=5|pages=683–95|date=May 2018|pmid=29556031|pmc=5920745|doi=10.1038/s41593-018-0120-6 }}</ref><ref>{{cite journal|vauthors=Hughes EG, Orthmann-Murphy JL, Langseth AJ, Bergles DE|title=Myelin remodeling through experience-dependent oligodendrogenesis in the adult somatosensory cortex|journal=Nature Neuroscience|volume=21|issue=5|pages=696–706|date=May 2018|pmid=29556025|pmc=5920726|doi=10.1038/s41593-018-0121-5 }}</ref>

Not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated. Instead, they are ensheathed by [[Nonmyelinating Schwann cell|non-myelinating Schwann cells]] known as Remak SCs and arranged in [[Group C nerve fiber#Remak bundles|Remak bundles]].<ref name="Monk">{{cite journal|vauthors=Monk KR, Feltri ML, Taveggia C|title=New insights on Schwann cell development|journal=Glia|volume=63|issue=8|pages=1376–93|date=August 2015|pmid=25921593|pmc=4470834|doi=10.1002/glia.22852 }}</ref> In the CNS, non-myelinated axons (or intermittently myelinated axons, meaning axons with long non-myelinated regions between myelinated segments) intermingle with myelinated ones and are entwined, at least partially, by the processes of another type of [[glial cell]]  the [[astrocyte]].{{Citation needed|date=July 2018}}<ref>{{Cite journal|last1=Wang|first1=Doris D.|last2=Bordey|first2=Angélique|date=11 December 2008|title=The astrocyte odyssey|journal=Progress in Neurobiology|volume=86|issue=4|pages=342–67|doi=10.1016/j.pneurobio.2008.09.015|pmid=18948166|via=Elsevier Science Direct|pmc=2613184}}</ref>