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J Struct Biol in press Google Scholar. Lemke G Unwrapping myelination. Adjacent to the nodes of Ranvier are the paranodes, where the spiraled lamellae of myelin gradually terminate Fig. Between the paranodes is the extensive intersegmental myelin sheath. The myelin sheaths in the central nervous system are formed by a pair of layers of the plasma membrane of the myelin- forming oligodendrocyte. The paired plasma membranes wrap around a length of the enclosed axon in a spiral fashion, with the outer surfaces of the paired plasma membrane being apposed to each other to form the intraperiod line of the mature sheath.
During development the cytoplasmic surfaces of the paired plasma membrane are separated by oligodendrocyte cytoplasm, but as the sheath matures the cytoplasm is lost and the cytoplasmic surfaces of the plasma membrane become apposed to form the major dense line of the sheath.
However, in the mature sheath some cytoplasm is retained in the inner and outer ends of the spiraled plasma membranes and in transverse sections of mature myelinated nerve fibers this cytoplasm is seen to be contained in the inner and outer tongue processes of cytoplasm Fig. Obviously these tongues of cytoplasm are sections through ridges of cytoplasm that extend along the length of the segment of myelin and at some point the outer tongue is in continuity with a cell processes that connects the segmental length of myelin to the cell body of the parent oligodendrocyte.
At the ends of the myelin segments are the paranodes, where the myelin sheath gradually becomes thinner as it approaches the node of Ranvier and the spiraled turns of myelin terminate. The inner turns of the spiraled lamellae of myelin terminate first and as they terminate, the major dense line opens up to contain a spiraled tunnel of cytoplasm which interconnects the inner and outer tongues of cytoplasm along the main length of the sheath.
In longitudinal sections of the paranode the spiraled tunnel appears as a series of pockets of cytoplasm on each side of the axon Fig. Here the axolemma and the plasma membrane bounding the inner faces of the pockets of cytoplasm of the sheath come into close apposition so that they are separated by a gap of only 3nm Figs. This junctional complex between the two membranes is very distinctive and makes it very easy to recognize profiles of transverse sections through paranodes.
Transverse section of the anterior commissure of a 16 year old monkey. In younger monkeys the myelin sheaths are compact and do not display significant distortion. Most of the profiles are sections through the internodal lengths of myelin, where there is a gap between the axolemma and the membrane on the inner face of the myelin sheath.
It is important to recognize the generally benign nature of MRNF to avoid superfluous medical testing. It is helpful to document a MRNF with imaging as certain conditions can be associated with the loss of myelination, as described above. Special thanks to Giulio Barteselli and Gabriel Balea for the ophthalmic photography presented here. Create account Log in. Main page. Getting Started. Recent changes.
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Residents and Fellows contest rules International Ophthalmologists contest rules. Original article contributed by :. Hema L. All contributors:. Ramkumar, MD , K. David Epley, M. Assigned editor:. Allison R. Myelinated Retinal Nerve Fiber Layer. ICD - ICD - 9. Zur pathologischen anatomic der netzaut und des scherven. Virchows Arch Pathol Anat. Journal of the American Medical Association.
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