Manyneurodegenerativediseasesareassociatedwithspecificintracellular inclusions (e.g.,LewybodiesinParkinsondiseaseandtanglesinAlzheimerdisease).Pathogenicvirusesmayforminclusionsininfectedneurons,justasinothercells,andsuchinclusionsaidinthediagnosis.Insomeneurodegenerativediseases,neuronalprocessesbecomethickenedandtortuous;thesearetermeddystrophic neurites.
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Fig. 23.1 Patternsofneuronalinjury.(A)Acutehypoxic-ischemicinjuryinthecerebralcortex.Thecellbodiesareshrunkenandeosinophilic(“redneurons”),andthenucleiarepyknotic.(B)Reactiveastrocytes,witheosinophiliccytoplasmandmultipleradiatingprocesses.(C)Collectionofmicroglialcellsformingapoorlydefinednodule,acommonfindinginviralinfections.
Astrocyte Injury and Repair. Astrocytesaretheprincipalcellsresponsibleforrepairandscarformationinthebrain,aprocesstermedgliosis. Inresponsetoinjury,astrocytesundergobothhypertrophyandhyperplasia.Thenucleusenlargesandbecomesvesicular,andthenucleolusbecomesprominent.Thecytoplasmexpandsandtakesonabrightpinkhue,andthecellextendsmultiplestout,ramifyingprocesses(calledgemistocytic astrocyte; see
Fig.23.1B ).Unlikeelsewhereinthebody,fibroblastsparticipateinhealingafterbraininjurytoalimitedextentexceptinspecificsettings(penetratingbraintraumaoraroundabscesses).Inlong-standinggliosis,thecytoplasmofreactiveastrocytesshrinksinsize,andthecellularprocessesbecomemoretightlyinterwoven(fibrillary astrocytes). Rosenthal fibers arethick,elongated,brightlyeosinophilicproteinaggregatesfoundinastrocyticprocessesinchronicgliosisandinsomelow-gradegliomas.
Oligodendrocytes, whichproducemyelin,exhibitalimitedspectrumofspecificmorphologicchangesinresponsetovariousinjuries.Inprogressivemultifocalleukoencephalopathy,viralinclusionscanbeseeninoligodendrocytes,withasmudgy,homogeneous-appearingenlargednucleus.
Microglial cells arelong-livedcellsderivedfromtheembryonicyolksacthatfunctionastheresidentphagocytesoftheCNS.Whenactivatedbytissueinjury,infection,ortrauma,theyproliferateandbecomemoreprominenthistologically.Microglialcellstakeontheappearanceofactivatedmacrophagesinareasofdemyelination,organizinginfarct,orhemorrhage;inothersettingssuchasinfections,theydevelopelongatednuclei(rod cells). Aggregatesofelongatedmicroglialcellsatsitesoftissueinjuryaretermedmicroglial nodules (see
Fig.23.1C ).Similarcollectionscanbefoundcongregatingaroundandphagocytosinginjuredneurons(neuronophagia).
Ependymal cells linetheventricularsystemandthecentralcanalofthespinalcord.Certainpathogens,particularlycytomegalovirus(CMV),canproduceextensiveependymalinjury,withtypicalviralinclusions.Choroid plexus isincontinuitywiththeependyma,anditsspecializedepithelialcoveringisresponsibleforthesecretionofcerebrospinalfluid(CSF).
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