Editing Myelin (section)

==Composition==
[[Image:Myelinated neuron.jpg|thumb|[[Transmission electron micrograph]] of a cross-section of a myelinated PNS axon, generated at the Electron Microscopy Facility at [[Trinity College (Connecticut)|Trinity College]], [[Hartford, Connecticut]]]]
[[File:Myelin sheath (1).svg|thumb|upright|Diagram of a myelinated axon in cross-section{{ordered list
|[[Axon]]
|Nucleus of Schwann cell
|[[Schwann cell]]
|[[Myelin sheath]]
|[[Neurilemma]]}}]]

Myelin is found in all [[vertebrate]]s except the [[jawless fish]].<ref name="Daniel" /><ref name="Salzer" /> Myelin in the central nervous system (CNS)  differs slightly in composition and configuration from myelin in the peripheral nervous system (PNS), but both perform the same functions of insulation and nutritional support. Being rich in [[lipid]], myelin appears white, hence its earlier name of [[white matter]] of the CNS. Both CNS white matter [[nerve tract|tracts]] such as the [[corpus callosum]], and [[corticospinal tract]], and PNS [[nerve]]s such as the [[sciatic nerve]], and the [[auditory nerve]], which also appear white, comprise thousands to millions of axons, largely aligned in parallel. In the corpus callosum there are more than 200 million axons.<ref name="Luders">{{cite journal |vauthors=Luders E, Thompson PM, Toga AW |title=The development of the corpus callosum in the healthy human brain |journal=J Neurosci |volume=30 |issue=33 |pages=10985–90 |date=August 2010 |pmid=20720105 |pmc=3197828 |doi=10.1523/JNEUROSCI.5122-09.2010 |url=}}</ref> Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which also contain other cell types including [[astrocytes]] and [[microglia]] in the CNS and [[macrophages]] in the PNS.

In terms of total mass, myelin comprises approximately 40% water; the dry mass comprises between 60% and 75% [[lipid]] and between 15% and 25% [[protein]].<!-- https://www.ncbi.nlm.nih.gov/books/NBK28221/ --> Protein content includes [[myelin basic protein]] (MBP),<ref>{{cite journal|vauthors=Steinman L|title=Multiple sclerosis: a coordinated immunological attack against myelin in the central nervous system|journal=Cell|volume=85|issue=3|pages=299–302|date=May 1996|pmid=8616884|doi=10.1016/S0092-8674(00)81107-1 |s2cid=18442078|doi-access=free}}</ref> which is abundant in the CNS where it plays a critical, non-redundant role in formation of compact myelin; [[myelin oligodendrocyte glycoprotein]] (MOG),<ref>{{cite journal|vauthors=Mallucci G, Peruzzotti-Jametti L, Bernstock JD, Pluchino S|title=The role of immune cells, glia and neurons in white and gray matter pathology in multiple sclerosis|journal=Progress in Neurobiology|volume=127–128|pages=1–22|date=April 2015|pmid=25802011|pmc=4578232|doi=10.1016/j.pneurobio.2015.02.003 }}</ref> which is specific to the CNS; and [[proteolipid protein]] (PLP),<ref>{{cite journal|vauthors=Greer JM, Lees MB|title=Myelin proteolipid protein – the first 50 years|journal=The International Journal of Biochemistry & Cell Biology|volume=34|issue=3|pages=211–15|date=March 2002|pmid=11849988|doi=10.1016/S1357-2725(01)00136-4 }}</ref> which is the most abundant protein in CNS myelin, but only a minor component of PNS myelin. In the PNS, [[myelin protein zero]] (MPZ or P0) has a similar role to that of PLP in the CNS in that it is involved in holding together the multiple concentric layers of glial cell membrane that constitute the myelin sheath. The primary lipid of myelin is a [[glycolipid]] called [[galactocerebroside]]. The intertwining hydrocarbon chains of [[sphingomyelin]] strengthen the myelin sheath. [[Cholesterol]] is an essential lipid component of myelin, without which myelin fails to form.<ref>{{cite journal|vauthors=Saher G, Brügger B, Lappe-Siefke C, Möbius W, Tozawa R, Wehr MC, Wieland F, Ishibashi S, Nave KA|title=High cholesterol level is essential for myelin membrane growth|journal=Nature Neuroscience|volume=8|issue=4|pages=468–75|date=April 2005|pmid=15793579|doi=10.1038/nn1426 |s2cid=9762771}}</ref>

[[Myelin-associated glycoprotein]] (MAG) is a critical protein in the formation and maintenance of myelin sheaths. MAG is localized on the inner membrane of the myelin sheath and interacts with axonal membrane proteins to attach the myelin sheath to the axon.<ref>{{cite book | vauthors = Lopez PH | chapter = Role of Myelin-Associated Glycoprotein (Siglec-4a) in the Nervous System | series = Advances in Neurobiology | title = Glycobiology of the Nervous System | year = 2014 | volume = 9 | pages = 245–62 | pmid = 25151382 | doi = 10.1007/978-1-4939-1154-7_11 | isbn = 978-1-4939-1153-0 }}</ref> Mutations to the MAG gene are implicated in demyelination diseases such as multiple sclerosis.<ref>{{cite journal | vauthors = Pronker MF, Lemstra S, Snijder J, Heck AJ, Thies-Weesie DM, Pasterkamp RJ, Janssen BJ | title = Structural basis of myelin-associated glycoprotein adhesion and signalling | journal = Nature Communications | volume = 7 | pages = 13584 | date = December 2016 | pmid = 27922006 | doi = 10.1038/ncomms13584 | pmc = 5150538 | bibcode = 2016NatCo...713584P }}</ref>