Introduction to demyelinating diseases د.هشام عون

8/3/2019 Introduction to demyelinating diseases .

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Introduction to

demyelinating diseasesDR : Hesham Awn

TAIZ 2010


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introductionGlial cells constitutes the large majority of cells in the nervous system.

As an overview the number of glial cellsincreases during evolution ,glial cellsconstitute 25% of total cells in the drosophila

,65% in Rodents , and 90 % in human brain.


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Virchow (1846) first described that there were cells otherthan neurons. He thought that it was the connective tissueof the brain ,which he called Nervenkitt (nerve glue) ,neuroglia.The name survived , although the original concept radicallychanged.

introduction


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introduction

The characterization of the major glial celltype was the result of microscopic studies ,

and specially the techniques of metallicimpregnation developed by Ramon Cajal and

Rio Hortege (1913) using gold impregnation,identified the Astrocyte Among neuronalcells ,as well as third element which was notimpregnated by this technique.


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Copyright 2001 American Physiological Society

Baumann, N. et al. Physiol. Rev. 81: 871-927 2001

Fig. 1.


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introduction

In (1921) ,using silver carbonate impregnationthey found tow other cell types ,the

oligodendrocyte (interfasicular glia andmicroglia).The progress of morphological techniques

and the discovery of cellular markers byimmunocytochemical techniques indicate theof multiple functional macroglial subclasses.


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introduction

Glial cells are necessary for correct neuronaldevelopment and for the functions of mature

neurons.It now seems likely that oligodendrocyteshave functions other than those related to

Myelin formation and maintenance.


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Oligodendrocytes:The termoligodendroglia wasintroduced by Rio

Hortega (1921), todescribe thoseneurological cells thatshow few processes in

material stained bymetallic impregnationtechniques.


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Oligodendrocytes:The oligodendrocyte is mainly a myelinforming cell, but there are also satellite

oligodendrocytes, who are perineuronal andmay serve to regulate the microenvironmentaround neurons.


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Oligodendrocytes:An oligodendrocyte extends many processes ,each of which contacts and repeatedly envelopes a stretch of axons with subsequent condensation of thismultispiral membrane forming myelin .On the same axon, adjusted myelin segments belongto different oligodendrocytes, the number of

processes that from myelin sheaths from a singleoligodendrocyte varies according to the area of CNSand possibly the species.


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myelinThe myelin sheath around most axons constitutesthe most abundant membrane structure in thevertebrate nervous system.

Its unique composition (richness in lipids and lowwater content allowing the electrical insullotion of axons ) and its unique segmental structureresponsible for the saltatory conduction of nerveimpulses allow the myelin sheath to support the fastnerve conduction in the thin fibers in the vertebratenervous system.


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High- speed conduction ,fidelity of transfersignaling on long distances ,and space

economy are the three major advantagesconferred to the vertebrate nervous system bythe myelin sheath, in contrast to theinvertebrate nervous system where rapidconduction is accompanied by increase axonalcalibers.

Cont.


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The importance of myelin in humandevelopment is highlighted by its involvement

in different neurological diseases such asleukodystrophies and multiple sclerosis in theCNS and peripheral neuropathies in PNS.

Cont.


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Cont.Myelin is the essential constituent of white matter inthe CNS which contains a 40-50% myelin on a dryweight basis.Myelin is a poorly hydrated structure containing40% water in contrast to gray matter 80%.Myelin dry weight consist of 70% lipid and 30%

proteins this lipid to proteins ratio is very peculiar tothe myelin membrane ,it is generally the reverse inother cellular membrane.


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The insulating properties of the myelin sheath,which favor rapid nerve conduction velocity

,are largely due to its structure , its thickness ,its low water content , and its richness inlipid.

Cont.


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Disease of myelinDiseases affecting central nervous system (CNS)

myelin can be classified on the basis of

whether a primary biochemical abnormalityof myelin exist (dysmyelinating) or whether some other process damages the myelin or oligodendroglia cell (demyelinating )


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Dysmyelinating diseases: Include several hereditary disorders:

adrenoleukodystrophy. metachromic leukodystrophy. krabbes disease . Alexanders disease. Canavan-Von Baaert-Bertrand disease.

Pelizaeus-Merzbacher disease. Phenylketonuria.


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Demyelinating disease :Autoimmune: Acute dissemenated encephalomyelitis

(ADEM) Acute hemorrhagic leukoencephalopayh. Multiple sclerosis.

Infectious: progressive multifocal leukoencephalopathy(PML).


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Cont.Toxic / metabolic: Carbon monoxide. Vitamin B12 deficiency. Mercury intoxication. Alcohol /tobacco amblyopia. Central pontine myelination. Marchiafava Biqnami syndrome. Hypoxia. Radiation .


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Cont.Vascular: Binswangers disease. CADASIL.


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Introduction to demyelinating diseases د.هشام عون