Myelin is a biomembrane that wraps the axons of most vertebrate nerve cells . The myelin sheath thus formed increases the speed of the conduction of excitation considerably beyond that of a bare axon. Compared to other biomembranes, myelin has a particularly high lipid content (70%) and a relatively low protein content (30%). Therefore myelin appears white in the macroscopic view, which is why strongly myelinated regions in the central nervous system are also referred to as “ white matter ”, in contrast to the slightly myelinated “ gray matter”". The rapidly conducting sensory and motor axons of the peripheral nervous system are also myelinated.
Myelin is made by cells : in the central nervous system by oligodendrocytes , in the peripheral nervous system by Schwann cells . The fact that central nervous myelin is formed by cellular processes of oligodendrocytes has long been controversial and was first shown in 1962 by Mary Bartlett Bunge (* 1931) and Richard P. Bunge (1932–1996) in electron microscope images. Myelin is often viewed as a special characteristic of vertebrates . However, some groups of invertebrates have functional and structural analogies .
2019 demonstrated in a study that by glyphosate-containing herbicides such as Roundup in cell cultures of Altweltmäusen degraded the myelin sheath, and its formation is prevented. The cause for this does not seem to be glyphosate itself, but the auxiliary substances it contains.
Proteins specific for myelin are:
- Myelin Basic Proteins or Myelin Basic Protein (MBP)
- Proteolipid Protein (PLP / DM20)
- Myelin Associated Glycoprotein (MAG)
- Connexin -32 (Cx-32)
- Cyclic nucleotide phosphodiesterase (CNP)
- Protein zero (P0, MPZ), periaxin (PRX), peripheral myelin protein 22 kDa (PMP-22)
Diseases in which the myelin sheath is damaged are called demyelinating diseases and are often autoimmune . Genetically inherited diseases that primarily affect the myelin in the central nervous system are known as leukodystrophies . These include Pelizaeus-Merzbacher's disease , Krabbe's disease and X-linked hereditary adrenoleukodystrophy . A subgroup of hereditary neuropathies is caused by mutations in genes whose protein products are relevant for the myelin of the peripheral nervous system (MPZ, PRX, PMP22) . A role in the development of mental illnesses such as schizophrenia is also being discussed . In the case of pernicious anemia , which can be traced back to a deficiency in vitamin B 12 , degeneration of the myelin sheaths and the resulting deficits occur. As in other fatty body tissues and organs as well as breast milk, the high fat content of the myelin results in high adsorption rates and cumulative accumulations with neurotoxic chemicals that have lipophilic or fat-soluble properties, for example with halogenated hydrocarbons such as dioxin / polychlorinated biphenyls.
Mice with specific defects in the myelination are used for the scientific investigation of this complex process. This enables a better understanding of the corresponding human hereditary diseases , the leukodystrophies .
|Mouse mutant||Myelin gene||human myelin disease|
|jimpy||PLP / DM20||Pelizaeus-Merzbacher disease|
|rumpshaker||PLP / DM20||Spastic paraplegia , X-chrome. (SPG-2)|
|Twitcher||Galactocerebrosidase||Krabbe disease , Krabbe syndrome|
|Aryl sulfatase A (- / -)||Aryl sulfatase A.||Metachromatic Leukodystrophy (MLD)|
|ALDP (- / Y)||ALDP||Adrenoleukodystrophy (X-ALD)|
|Sphingomyelinase (- / -)||Sphingomyelinase||Niemann-Pick Syndrome|
|Hexosaminidase A (- / -)||Hexosaminidase A||Tay-Sachs syndrome|
|Hexosaminidase B (- / -)||Hexosaminidase B||Sandhoff disease|
|Asparto acylase (- / -)||ASPA||Canavan disease|
Myelin was discovered in 1854 by the pathologist Rudolf Virchow (1821–1902) using light microscopy on tissue sections. He found a medullary sheath in nerve fibers and suggested calling it myelin (Greek myelòs = marrow, brain). The current concept of myelin in biology and medicine goes back to detailed structural descriptions by the Parisian pathologist Louis-Antoine Ranvier in 1878. The ring-shaped recesses in the myelin sheath, which are essential for the function of the saltatory conduction of excitation, bear his name ( Ranvier cord rings ).
A typical swelling of the myelin in water with the formation of worm-like shapes is of historical interest. In 1858 , the Frankfurt physician Carl von Mettenheim (1824–1898) discovered the optical birefringence of myelin as another important feature . From these properties and on the basis of his own experiments, the Karlsruhe physicist Otto Lehmann (1855–1922), the “father of liquid crystals”, concluded that myelin is “liquid crystals”, more precisely: lyotropic liquid crystals that are combined with a Solvent, here water, form. Indeed, Virchow was the first to observe a liquid crystal .
- Renate Lüllmann-Rauch: Pocket textbook histology. 3. Edition. Thieme, Stuttgart 2009, ISBN 978-3-13-129243-8 , p. 189.
- Fabian Szepanowski, Leon-Phillip Szepanowski, Anne K. Mausberg, Philipp Albrecht, Christoph Kleinschnitz, Bernd C. Kieseier, Mark Stettner: Differential impact of pure glyphosate and glyphosate-based herbicide in a model of peripheral nervous system myelination . In: Acta Neuropathologica . tape 136 , no. 6 , November 16, 2018, ISSN 0001-6322 , p. 979-982 , doi : 10.1007 / s00401-018-1938-4 .
- Daniela Albat: Does glyphosate damage the nerves? Glyphosate-based crop protection products promote the breakdown of cells in the nervous system. In: scinexx.de. December 3, 2018, accessed June 9, 2019 .
- Klaus-Armin Nave, Hauke B. Werner: Myelination of the Nervous System: Mechanisms and Functions . In: Annual Review of Cell and Developmental Biology . tape 30 , no. 1 , January 1, 2014, p. 503-533 , doi : 10.1146 / annurev-cellbio-100913-013101 .
- Federal Environment Agency: Dioxins
- R. Virchow: About the widespread occurrence of a substance analogous to the nerve marrow in animal tissues. In: Virchows Arch. Pathol. Anat. 6, 1854, pp. 562-572.
- H.-R. Stegemeyer, H. Stegemeyer: Finally, I propose the medullary substance to be named Myelin. In: Dtsch. Med. Wochenschr. 129, 2004, pp. 2784-2787.
- Horst Stegemeyer, Hans-Roland Stegemeyer: The apparently living liquid crystals. In: Nachr. Chem. 52, 2004, pp. 903-908, doi: 10.1002 / nadc.20040520907 .