神經(jīng)干細(xì)胞-(完整版)實(shí)用資料

神經(jīng)干細(xì)胞（完整版）實(shí)用資料(可以直接使用，可編輯完整版實(shí)用資料，歡迎下載）

神經(jīng)干細(xì)胞（完整版）實(shí)用資料(可以直接使用，可編輯完整版實(shí)用資料，歡迎下載）NeuralStemCellsAnOverviewRossellaGalli,AngelaGritti,LucaBonfanti,AngeloLuigiVescoviAbstract—Thisreviewfocusesonthenatureandfunctionalpropertiesofstemcellsoftheadultmammaliancentralnervoussystem(CNS.Ithasrecentlybeenshownthatcellturnover,includingneurons,doesoccurinthematureCNS,thankstothepersistenceofprecursorcellsthatpossessthefunctionalcharacteristicsofbona-fideneuralstemcells(NSCswithinrestrictedbrainareas.Wediscusshowthesubventricularzoneoftheforebrain(SVZisthemostactiveneurogeneticareaandtherichestsourceofNSCs.TheseNSCsensurealife-longcontributionofnewneuronstotheolfactorybulband,whenplacedinculture,canbegrownandextensivelyexpandedformonths,allowingthegenerationofstemcelllines,whichmaintainstableandconstantfunctionalproperties.AsurveyofthedifferentiationpotentialoftheseNSCs,bothinvitroandinvivo,outlinestheirextremeplasticitythatseemstooutstretchthebrainboundaries,sothattheseneuroectodermalstemcellsmaygiverisetocellsthatderivefromdevelopmentallydistincttissues.Acriticaldiscussionofthelatest,controversialfindingsregardingthissurprisingphenomenonisprovided.(CircRes.2003;92:598-608.KeyWords:stemcellsⅢtransdifferentiationⅢneurogenesisThenervoustissue,particularlythatofthecentralnervoussystem(CNS,isendowedwithseveralpeculiarcharac-teristicsthatmakeituniqueamongtheotherbodilytissues.Ultimately,thesepropertiesrelatetotheCNSheterogeneouscellcompositionthatunderliestheelaborationofanenor-mousamountofinformationintoacomplexoutput.Histori-cally,thiscomplexityhasbeenseeninextricablylinkedtothelackofanycellturnoverintheadultbrain.Thedogmaticviewofanever-immutableneuraltissueinmammalsisnowbeenreplacedbythenotionthatcellreplacementoccurswithinspecificbrainregionsthroughoutadulthood.Thiscontinuousneurogeneticprocessissustainedbythelife-longpersistenceofneuralstemcells(NSCswithinrestrictedCNSareas.Intheadultmammalianbrain,thegenesisofnewneuronshasbeenconsistentlydocumentedinthesubgranularlayerofthedentategyrusofthehippocampus1andthesubventricularzone(SVZofthelateralventricles.1–3FromtheSVZ,newlygeneratedneuronsreachtheirfinaldestinationintheolfac-torybulbafterlong-distancemigrationthroughawell-definedpathcalledtherostralmigratorystream(RMS2,3(Figure1.TheSVZistheadultbrainregionwiththehighestneurogeneticrate,fromwhichNSCshavebeenfirstlyisolat-ed4andcharacterizedfortheirabilitytogiverisetonon-OriginalreceivedMarch4,2002;resubmissionreceivedFebruary4,2003;revisedresubmissionreceivedFebruary25,2003;acceptedFebruary25,2003.FromtheStemCellResearchInstitute(R.G.,A.G.,A.L.V.,DIBIT,HospitalSanRaffaele,Milan,Italy;andtheDepartmentofVeterinaryMorphophysiology(L.B.,RitaLeviMontalciniCenterforBrainRepair,UniversityofTurin,Grugliasco,Italy.?2003AmericanHeartAssociation,Inc.598Reviewneuralcells.5–7Thus,inthisreviewweshallfocusourattentiononSVZ-derivedNSCs.Lately,the“antidogmatic”natureofNSCshasemergedtwice.Althoughthescenarioisgraduallychanging,8–10thelackofspecificmarkershaslonghamperedtheunivocalidentificationandcharacterizationofNSCs.However,theuseofspecificsystemshaspermittedtheisolationandimpressiveexpansionofNSCsexvivo—quiteastrikingphenomenonforcellsofthemostquiescentbodilytissue.ThishasallowedtheestablishmentofNSClinesfromvariousspecies,includ-inghumans,thatarebeingusedtoexplorenoveltherapeuticstrategiestotreatneurologicaldisorders(seereviews1,11,12aswellastriggedanimpressiveplethoraofstudiesinvestigatingthedifferentiationpotentialofNSCs,whichmaybebroaderthanexpected.Infact,animpressiveexampleoftheextremedevelopmentalpotentialofNSCsmayhavebeenprovidedbystudiesinwhichNSCsappeartogeneratecelltypesthatbelongtodevelopmentallyunrelatedtissues—aconceptthatmayalsoapplytostemcellsfromthehemopoieticsystemandfromotheradulttissues.TheForebrainSubventricularZoneasaSourceofAdultNSCs:A“BrainMarrow”Case?Inthemammalianadultbrain,thegenesisofnewneuronscontinuesthroughoutlifewithintwo3-layeredcorticalre-gions,thehippocampusandolfactorybulb(OB,whereitissustainedbyendogenousstemcells.ThemostactiveNSCcompartmentisfoundinSVZ.Thisarearepresentsaremnantoftheembryonicgerminalneuroepithelium,whichpersiststhroughoutlifeasanactivelymitoticlayerinthewallofthetelencephaliclateralventriclesandalongitsrostralextensiontowardtheolfactorybulb(Figure1.Acompleteturnoveroftheresidentproliferatingcellpopulationoccursevery12to28daysintheSVZ,13withabout30000newneuronalprecursors(neuroblastsbeingproducedeverydayandmi-gratingtotheOB3(Figure1.TwomaincelltypesarefoundintheSVZ:migratory,proliferatingneuroblastsandastrocytes(Figure1.ThelatterformameshworkthroughoutthewholeSVZandareorga-nizedintochannelsthatareorientedalongtheanteroposterioraxisandarecalledglialtubes.14–16Withintheseglialtubes,NSC-generatedneuroblastsundergorostraward,tangentialmigrationascellsgroupedintotightlypackedchains.14,17Eventually,neuroblastsreachthedistaltipoftheOB,separatefromeachotherandleavethetubes,shiftingtheirmigrationpatternfromtangentialtoradial.Inthisway,theyreachthemoresuperficialOBlayersandterminallydiffer-entiateintogranuleandperiglomerularneurons.Importantly,tangentialmigrationtowardtheOBhasalsobeendocu-mentedintheadultbrainofprimates18,19andevenintheinfanthumanbrain.20Also,thepossiblemigrationofcellsfromtheSVZtotheneocortexhasbeenproposedinmouse21,22aswellasinadultmacaquemonkeys,23–25al-thoughthisviewhasrecentlybeenquestioned.18GlialtubesarecomposedofaspecialtypeofastrogliathatexpressesthemarkerofmatureCNSastrocytes,namelyglialfibrillaryacidicprotein(GFAP,butalsocontainthecy-toskeletalproteinsvimentinandnestin.16,17Thepresenceinglialtubesofsuchmolecules,knowntoaboundinthoseimmatureandradialglialcellsthatguideyoungneuronsduringCNSdevelopment,suggestedaroleforglialtubesininfluencingthemigration/guidanceofneuroblastsintheSVZ.However,theactualroleofastroglialcellsinthisprocessremainsunclear.26TheymaysimplyactasabarriertopreventFigure1.A,Globalviewoftheadultrodentbrainthatdepicts(ingraytheregionswithinwhichcontinuousneuro-genesisoccursthroughoutadulthood,namelytheolfactorybulb(OBandthehippocampus(H.AreasthatareknowntocontainNSCsareoutlinedinblack:thesubventricularzone(SVZwithitsrostralextensionreachingtheOBandthedentategyrus(DGofthehippocam-pus.B,SchematicsagittalsectionoftheadultrodentbrainthroughtheSVZ-REsystem;thedottedlineoutlinesthecon-tourofthelateralventricle(LV.TheSVZiscomposedofaposteriorpart(darkgrayliningthelateralventricle(SVZ-LVandananteriorpart(lightgrayformingtherostralextension(SVZ-RE.CCindi-catescorpuscallosum;CX,cerebralcor-tex;andOB,olfactorybulb.C,Trans-versesectionsoftheSVZatthelevels1and2asindicatedinAandB;1,SVZ-RE,2,SVZofthelateralventricle(LV;seealsoFigure2A.Glialtubesareinblack;CH,chainsofneuronalprecur-sors;CC,corpuscallosum;e,ependymalmonolayer.D,Schematicrepresentationofthechainsofmigratingneuroblastswithintheastroglialtubes(inblack.TubesarenotpresentinthedistaltipoftheOB(grayshad-owedarea,withinwhichtheneuroblastsdepartfromtheterminalendofthetubes,leavethechainsbydetachingfromeachother,andmigrateoutwardly(radiallyasisolatedcells(nthroughtheOBparenchyma.Tindicatestangentialchainmigration;R,radialmigration;N,differentiatedneuronsintheolfactorybulb;SC,areainwhichthestemcellsarefound;andMP,areacontainingmigratingprecur-sorsbutnotNSCs(modi?edfromPerettoP,MerighiA,FasoloA,BonfantiL.Thesubependymallayerinrodents:asiteofstructuralplasticityandcellmigrationintheadultmammalianbrain.BrainResBull.1999;49:221–243.GallietalNeuralStemCells:AnOverview599neuroblastsfromescapingthepropermigratoryroutebeforetheyreachtheOBs.17Althoughthepresenceofastrocytesisnotessentialforchainmigration,27theydosecretefactorsthatfacilitatethisprocess.28Notably,astroglialtubesandNSCsdonotcoexistsolelywithintheperiventricularaspectoftheSVZbutalsowithintherostralmigratorystreamthatextendsintotheOB,29withtheformerperhapscontributingtocreateanappropriatestemcell“niche”forthemaintenanceofNSCsallalongthepathway.Inanappealingfunctional/structuralparallelismwiththehemopoieticsystem,ithasbeenproposedtousetermslike“neuropoiesis”toportraythepersistenceofneurogenesisinadulthood,and“brainmarrow”todescribetheanatomicalregionsthatcontaintheNSCssupportingneurogenesis.30,31Thus,whereashematopoiesistakesplacewithinthebound-ariesofthemarrowinthedeepadultbone,adultneurogenesisisaspatiallyconfinedprocess,constrainedwithinthebound-ariesofthebrain-deepSVZ.Inboth“marrows”,wecanfind(1proliferatingstem/progenitorcells,(2celltypes,whichcancarryoutstromalfunction(s,and(3extracellularmatrixthatcanmodulatecellproliferation,adhesion,andmigration.Stromalcellsofthebonemarrowareknowntoactasaninterfacebetweenperipheralbloodandthestem/progenitorpool,thusmodulatingthefateofhematopoieticstemcells(HSCs.Inasimilarfashion,astrocytesandependymalcellsoftheSVZmayactas“stromal”elementsoftheCNSbyproducingmoleculesasbonemorphogeneticproteinsthataffecttheneuronalversusglialfateofthestem/progenitorcells.32Furthermore,theextracellularmatrixofboththeSVZandbonemarrowcontainstenascinandproteoglycans,mol-eculesthatareimportantintheformationofdevelopmentalcompartmentsandinthecontrolofcelladhesion,migrationanddifferentiation.15,30,31,33Finally,similarlytothebonemarrow,theproximityoftheSVZwiththecerebrospinalfluid,theenlargedintercellularspaces,thereducedcell-cellcontacts,15,16,19andthepresenceofmoleculeslinkedtowatercotransport34,35contributetocreateintheSVZacytoarchi-tectural/biochemicalniche,whichisverydifferentfromtheenvironmentofthematureCNSparenchyma.TheNatureandOriginoftheAdultNeuralStemCellInVivoAseriesofobservationsindicatethataspecificsubtypeofSVZastroglialcellsistheactualNSC36(seereview26.Infact,afterthekillingofalltheproliferatingSVZneuroblastsbyadministrationofsubdural,cytotoxicdosesofantimitoticdrugs,onlyastroglialcellswereleft,whichsurprisingly,wereshowntobeabletoregeneratetheentireSVZcellsystem.Accordingtothisview,someoftheSVZastrocytes(namedtypeBcellsarethoughttorepresentrelativelyquiescentstemcellsthatnormallyproliferateatalowrateandgeneratetheneuronalprecursors(typeAcells,throughthegenerationofathird,intermediatecelltype:theCcell(orDcellinthehippocampus37,whichhasthecharacteristicsoftheclassic,fast-proliferating,transit-amplifyingprogenitorcellsfoundinmanyself-renewingtissues.36,38,39TakingintoaccounttheimmaturefeaturesofSVZastrocytes16,17,36andthefactthatradialgliaofthedevelopingcerebralcortexcanactasabipotentstemcell,40itismostlikelythatNSCsoriginatefromtypeBcellsthatretainradialglia-likefeatureswithinspecificregionsoftheadultbrain(seereview38.Itshouldbeemphasized,however,thatNSCscanalsobeisolatedandgrowninvitrofromnonneurogenicperiventric-ularregions,inwhichthematureparenchymaisdirectlyincontactwiththeependymalmonolayer,suchasthefourthventricleorthespinalcord.41Thesefindingsmayindicatethatcellsendowedwithsomeextentofstem-likepotentialmayexistthroughoutthewholeadultCNStissue.However,onlythoseNSCsthatoccurwithinadultbrainareasthatretainabrainmarrow-likeenvironment,astheSVZ,takeupanactualstem-cellbehavior,self-renew,andgenerateamatureprogenyinvivo,whereasthoseresidingwithinnonneuroge-neticregionsmayremaindormant.42–44ExVivoBehaviorofNSCsTocarryoutthevitalfunctionofmaintainingcellhomeosta-sisandintegritythroughoutlife,stemcellsareendowedwithpeculiarfunctionalcharacteristicsthatprovidesuitablecrite-riatoidentifyandclassifythem.Infact,stemcellsarehighlyundifferentiatedcellspossessingasignificantproliferationpotential,abroaddevelopmentalfate,andmostandforemost,anintrinsiccapacityforself-perpetuation—betterdefinedasself-maintenanceorself-renewal—throughoutadulthood.39Historically,hemopoieticstemcells(HSCshavebeenthefirsttoprovideausefulprototypicfunctionalmodelinordertodeviseconceptualandpracticalstrategiestotackleinves-tigationsonotherstemcelltypes.45WhencomparingHSCsandNSCswithregardtotheirfunctions,thetwosystemsappeartositatoppositesidesofthefunctionalspectrum.Becauseenormousamountsofnewbloodcellsaregeneratedeverydayfromstemcellsofthehemopoietictissue(s,ithasbeenpossibletodevelopaninvivofunctionalassaybywhichbonafidestemcells(evenasinglecandidatecell46havebeenidentifiedfortheircapacitytoreconstitutethehemopoieticstemcompartmentafteritsdeadlyexperimentalablation.Moreover,areliableantigenicfingerprintbasedontheexpressionofmultiple,lineage-specificmarkersisnowavail-abletodistinguishvarioushemopoieticprecursors.However,oneofthemostsignificanthurdlesinthefieldofHSCsistheirresiliencetoundergoextensiveproliferationinculture.Fascinatingly,amirror-likesituationhasemergedinthefieldofNSCs.Giventherelativeoverallquiescenceandstructural/cytoarchitectural“rigidity”ofthebrainparen-chyma,theclassicalhemopoieticexperimentalparadigmsofreconstitutionofablatedstemcellcompartmentsarerarelyusedtoinvestigateandcharacterizeNSCs.Thus,thestudyofNSCsreceivedmuchofitsinitialimpulsefrominvitrofindingsthatfirstlydemonstratedandexploitedtheamazingproliferationcapacitythatNSCsdisplayinculture.Underappropriateconditionsandinthepresenceofmito-gens(epidermalgrowthfactor(EGFand/orfibroblastgrowthfactor2[FGF2],itispossibletoinducetheprolif-erationofrapidlydividingprecursorsfromtheSVZ.Atleastinvitro,theseprecursorsfulfillmostofthecriteriaofbonafidestemcells.47–49Lately,theseculturedNSCshavebeensuggestedtoderivefromtheCtypecellsoftheSVZ.50SinceinvivotypeCcellsrepresenttransit-amplifyingprogenitorsratherthanbonafidestemcells,theconcepthasbeenput600CirculationResearchApril4,2003forthforthepossibilitythattissueculturingand/ormitogenscanre-awakealatentstemcellprograminwhatcanbeconsidered“potentialstemcells.”39Onremovalofthemito-gens,theprogenyofNSCspromptlydifferentiateintothethreemaincelltypesoftheCNS(astrocytes,oligodendro-cytes,andneurons(Figure2.36,42,51–54Itshouldbenotedthatsimilarresultscouldbeobtainedwithadulthippocampalcells.55Themostimportantconceptregardingthismethodisthatitrepresentsaselectivesystembywhich,inaheterogeneousprimaryculture,themorecommittedprogenitorsand/ordifferentiatedmaturecellsrapidlydieandthusareeliminated,whereastheundifferentiatedNSCsarepositivelyselectedandforcedtoaccessastateofactiveproliferation.12NSCsstartproliferatinginitiallyasadherentcellsandattachtoeachother,eventuallygivingrisetosphericalclustersthatfloatinsuspensionandformtheso-called“neurospheres.”Ingivingrisetoneurospheres,NSCsundergomultiplesymmetriccelldivisionsbywhichtwonewNSCsaregeneratedateachcycle(Figure3.ItshouldbenotedthatnotalltheNSCprogenyfoundinaneurospherearestemcells.Indeed,only10%to50%oftheseprogenyretainstemcellfeatures,whereastheFigure2.Neuralstemcellsoftheadultrodentbrain.A,Transversesectionthroughtheperiven-tricularSVZisshown.Thissectioncorrespondstotheplanesection2asshowninFigure1.Vindi-catesventricle;SVZ,subventricularzone.ArrowspointtosomeoftheproliferatingcellsintheSVZthatwerelabeledafterintraperitonealinjectionofthethymidineanalogue,5-bromodeoxyuridine.SVZcontainsfastandslowproliferatingprecursorpop-ulationswithacellcycletimecomprisedbetween12hoursand28days.BthroughD,StemcellsisolatedfromtheSVZareshowninserum-freecul-turesinthepresenceofbothEGFandFGF2.Undertheseconditions,stemcellsundergoprolif-eration,eventuallygivingrisetoneurospheres.Oneday(B,4days(C,7days(Dafterplating.E,Onremovalofgrowthfactorsandplatingontoalaminin-coatedsubstrate,cellsinthespherediffer-entiateintomaturebraincellsthatexpressthetyp-icalmarkersofneurons(red,?-tubulin,astrocytes(green,glial?brillaryacidicprotein,andoligoden-drocytes(blue,O4.Bars:A?100?m;BandC?20?m(barinB;E?30?m.Figure3.Schematicoutlineofhowstemcellexpansionisachievedinculturedneuralstemcells.A,Underserum-freeconditionsandinthesustainedpresenceofmitogenssuchasEGFand/orFGF2,NSCs(red,Sarethoughttoundergosymmetricdivisionsinwhicheither(itwomoredifferentiatedcells(likelytran-sientamplifyingprecursors,green,T(symmetricdifferentiativedivisionsor(iitwoNSCsaregenerated(symmetricpro-liferativedivisions;(iiiasymmetriccyclesgivingrisetooneNSCandtoonemoredifferentiated(notshownorapoptoticcell(yellowcanalsotakeplaceatthesametime.IfwithinaNSCpopulationthefrequencyofproliferativecyclesequalsthatofdifferentiativedivisions,theoverallnumberofNSCswithinthepoolismaintainedatasteadylevel39;(ivundertheconditionsusedforNSCsexpansion,thenumberofproliferativedivisionsnormallyexceedsthatofdiffer-entiativecycles,sothatanincreasingnumberofNSCsisgeneratedwithinthestemcellpoolateachcellgeneration.B,StartingfromasingleNSC(redallthetypesofcelldivisiondescribedinAcon-curtotheformationaclonalclusterofcellscalledneurosphere.Therefore,eachneurospherecontainsamixtureofmanyNSCs,aswellastransientamplifyingprecursors(greenanddead/dyingcells(yellow.Whenneurospheresaredissociated,allbutthestemcellsdie(lightblueshadowedarea,duetothestringentcultureconditions,whereasNSCscontinuetoproliferate,givingrisetomore,sec-ondaryneurospheres.Thisprocedurecanberepeatedsequentiallyovermanymonths,resultinginanexponentialincreaseintheover-allnumberofNSCsandintheestablishmentofNSCstablecelllines.GallietalNeuralStemCells:AnOverview601remaindersarecellsthatundergospontaneousdifferentiation.Consequently,aneurosphereisamixtureofNSCs,differen-tiatingprogenitors,andevendifferentiatedneuronsandglia,dependingontheneurospheresizeandtimeinculture.Thisisthereasonwhyneurospheresaresubculturedbyharvesting,followedbymechanicaldissociationandbyre-platingunderthesamegrowthconditions.Asintheprimaryculture,differentiating/differentiatedcellsrapidlydiewhiletheNSCscontinuetoproliferate,givingrisetomanysecondaryspheresandexponentialgrowthinvitro.Inthisway,stableNSCscelllinescanbeobtained.12Duetotheseproperties,ithasbeenpossibletoestablishcontinuousmousetransgenic/geneticallymodified56orhu-manNSClines57,58thatexpandmerelybygrowthfactorstimulationandundercompletelychemicallydefinedcondi-tions.Inparticular,humanNSClinesrepresentarenewablesourceofnormalnervouscellsthatmightfacilitatebasicstudiesonhumanneurogenesisanddrugdiscoveryand,remarkably,mayvirtuallyeliminatetheneedoffetalhumantissuefortherapeuticneuraltransplantation.ItiswellknownthatsomeCNSdisordersmayrequiregraftingofspecificneuronalsubtypes.Notably,humanNSCs,whichareavail-ableinhugeamountascomparedwithfetaltissues,canbeeasilyinducedtodifferentiateintospecificcelltypesand/orneurotransmitterphenotypesbysimplymodifyingtheculturemedium.Forexample,NSC-derivedGABAergicneuronscouldbeusedtoreplaceGABAergicspinyneuronslostinHuntington’sdisease.Similarly,wehavebeenabletoinduceasignificantfractionoftheprogenyofvarioushumanNSClinestoexpresscatecholaminergicneuronalfeatures59thatmayproveusefulforcellreplacementtherapyinParkinson’sdisease.Veryrecently,humanNSCshavebeenshowntoefficientlyintegrateintheadultratbrainwheretheydiffer-entiatedacquiringacholinergicphenotype.60Thus,inaddi-tiontomarkedlyreducingtheuseofhumanfetalbraintissue,stemcelllinesmayeventuallyeliminatetheneedfortissueexclusivelyfromselectedembryonicregions.TheissuesdiscussedsofarpromptsomefinalremarksastotheclassificationofNSCswithrespecttotheirexvivobehavior.Recently,theidentificationofputativeNSCmark-ersasAC133,PNAlow/HSAlow,andLeX/ssea-18–10haspro-videdresearcherswithavaluabletoolforidentifyingandenrichingbonafideNSCs.ThisantigeniccharacterizationwilleventuallyhelptoresolvethestateofconfusionintheNSCareainwhichclassificationbasedonfunctionalap-proachespresentsseverelimitationsduetothedifferenttechniques,thevarietyofthebrainregionsinvestigated,andthevariableagesofthedonorsyieldingtheisolationofdifferentprecursorcelltypesthatallpromiscuouslygobythenameofNSCs.Onceagain,itistemptingtoborrowfromthehemopoieticfield,inwhichalineageofstemcellsendowedwithprogres-sivelynarrowingself-renewalcapacitieshasclearlybeendefined.61Althoughlimitedtotheinvitrostudiesandmainlyforpracticalpurposes,wespeculatethatasimilarcriterionofclassificationmaybeinitiallyassumedforNSCs,thusiden-tifyingthemaslong-termorshort-termself-renewingNSCsbasedontheextentoftheirself-maintenanceabilityinculture.Atthispoint,weoughttorememberthat(1challengingacellinvitroonlyunveilsitsdevelopmentalpotentialandnotitsactualinvivofate,and(2theextentbywhichacellcanself-renewinvitromaydependonthecultureconditionsadopted.Nevertheless,ithasalsoemergedthatsomeNSCsfromdistinctregionsdisplayclearlydistinctself-renewalcapacitiesunderidenticalcultureconditions,thuslendingtotheattempttoclassifyculturedNSCsbasedontheself-renewalcapacitythattheydisplayinvitro.62,63PlasticityofNSCsWithintheCNSAlthoughinvivoNSCsappeartogeneratealmostexclusivelyneuronalcells,theiractualdevelopmentalpotentialasob-servedexvivoismuchbroaderthanexpected.Infact,culturedNSCsdogiverisetoneurons,astrocytes,andoligodendrocytesandarethereforenormallyclassifiedasbeingmultipotentinnature.TheconceptofNSCplasticityandoftheirdependenceonenvironmentalcuesisstrength-enedbytransplantationandmanipulation/recruitmentstudiesinvivo.Forexample,intrahippocampustransplantationofhippocampalstemcellsresultsinthegenerationoftheneuronaltypesnormallyfoundwithinthisregion.55However,ifthesamehippocampalstemcellsaretransplantedhetero-topically,thatisintotheSVZ,theyproducecelltypesthatarenormallyfoundinthelatter,ratherthanhippocampalones.55Further,whentransplantedinadultbrainregionsinwhichnoneurogenesistakesplace,NSCsproduceexclusivelyglialcells.Afterintracerebraladministrationofgrowthfactors,namelyEGF,64FGF2,65ortransforminggrowthfactor-?(TGF-?,66theproliferationofcellsintheSVZisdramati-callyincreasedandthefateoftheprogenycanchangedependingonthetypeoffactor(sused.Inparticular,EGFinfusionresultsinanincreasedproductionofcellsintheSVZ,inadiversionoftheirmigrationpatternfromtangentialtoradial,65andintheeventualgenerationofcellsofthegliallineageratherthanneurons.64,65Thesefindingsaltogetherre-emphasizetheconceptexpressedearlierinthisreviewthatNSCsappeartopossessaratherbroaddevelopmentalpoten-tialandfunctionalrepertoirewhoseexpressionisstronglyinfluencedbyextracellularcues.PlasticityofNSCsOutsidetheCNSSomeimportantconsiderationshaveemergedsofar.First,thenervoustissuecontainsbonafidestemcellsthatsupportneuronalcellturnoverthroughoutlife.Second,despitetheiroriginfromoneofthemostquiescenttissuesinthebody,NSCscanundergoeffectivelong-termculturing,proliferationandexpansionwhileretainingstablefunctionalcharacteris-tics.Third,whenproperlychallenged,theoveralldevelop-mentalpotentialappearstobebroaderthanthatobservedunderphysiologicalconditionsinvivo.Thissectionreviewsrecentobservationssuggestingthatthisplasticitymayperhapsoutstretchthebrainboundaries,sothatNSCs(neuroectodermalinorigincangiverisetocellsthatnormallyderivefromgermlayersotherthantheneuro-ectoderm.Thisisdiscussedlater,inthemoregeneralper-spectivethatmanysomaticstemcellsmaygiverisetocellsofanembryonicorigindifferentfromtheirown.Oneofthemostintriguingcasesisthevirtualpluripotencyofbonemarrow–derivedcells67;however,multipleexamplesofSCs602CirculationResearchApril4,2003givingrisetocellsnormallyfoundinothertissueshavebecomeavailable.Insomecases,boththeoriginalSCsandthecellstowhichtheygiverisederivefromthesameembryonicgermlayer(intragermlayerconversion.Forinstance,intramesodermconversionhasbeendocumentedbyshowingthegenesisofskeletal68orcardiacmusclecellsfrombonemarrowcells.69Similarly,muscleprecursorscangiverisetohematopoieticcells,70–72althoughithasnowbeenshownthattheoriginalmusclepopulationundergoingcon-versionareSca-1andCD45-positivecells,whicharehemo-poieticinorigin.71,73Finally,musclesatellitecellsretainanosteogenicandadipogenicdifferentiationpotentialthatisnormallyretrievedinmesoderm-derivedstromalcells.74Morestrikingexamplesoftransgermlayerconversion—herebyalsodefinedastransdifferentiation—inwhichSCsandtheirprogenybelongtodevelopmentallyunrelatedcelllineageshavebeenreported(Table.Thus,mesoderm-derivedmesenchymalstemcellsappeartogenerateastro