Proc.Natl.Acad.Sci.USAVol.96,pp.5268 - PDF document

Proc.Natl.Acad.Sci.USAVol.96,pp.5268±5273,April1999Neurobiology,AnthropologyAneuronalmorphologictypeuniquetohumansandgreatapesA.NM.AANIELP.PM.ERWINR.H*KastorNeurobiologyofAgingLaboratoriesandFishbergResearchCenterforNeurobiology,andDepartmentsofPathology(Neuropathology),**GeriatricsandAdultDevelopment,andOphthalmology,MountSinaiSchoolofMedicine,NewYork,NY10029;DivisionofBiology,CaliforniaInstituteofTechnology,Pasadena,CA91125;andDivisionofNeurobiologyandBehavior,BioqualInc.,Rockville,MD20850CommunicatedbyFrancisCrick,TheSalkInstituteforBiologicalStudies,SanDiego,CA,March1,1999(receivedforreviewNovember16,1998)ABSTRACTWereporttheexistenceanddistributionofanunusualtypeofprojectionneuron,alarge,spindle-shapedcell,inlayerVboftheanteriorcingulatecortexofpongidsandhominids.Thesespindlecellswerenotobservedinanyotherprimatespeciesoranyothermammaliantaxa,andtheirvolumewascorrelatedwithbrainvolumeresiduals,ameasureofencephalizationinhigherprimates.Theseobservationsareofparticularinterestwhenconsideringprimateneocorticalevolution,astheyrevealpossibleadaptivechangesandfunc-tionalmodificationsoverthelast15±20millionyearsintheanteriorcingulatecortex,aregionthatplaysamajorroleintheregulationofmanyaspectsofautonomicfunctionandofcertaincognitiveprocesses.ThatinhumanstheseuniqueneuronshavebeenshownpreviouslytobeseverelyaffectedinthedegenerativeprocessofAlzheimer'sdiseasesuggeststhatsomeofthedifferentialneuronalsusceptibilitythatoccursinthehumanbraininthecourseofage-relateddementingillnessesmayhaveappearedonlyrecentlyduringprimate Theevolutionoftheneocortexinprimateslonghasbeenrecognizedtobetheresultofgreatexpansionofcorticalareas,withaseveral-hundredfoldincreaseincorticalvolumebe-tweenprosimiansandhumans(1,2).However,theneuronaltypesthatpopulatetheneocortex,i.e.,pyramidalneuronsandnumerousclassesofnonpyramidalneurons,haveremainedremarkablyconstant,beingmorphologicallyrecognizableacrossprimatespeciesandevenotherorders.Anexceptionisthespindleneuron,foundintheanteriorcingulatecortex.Ithasbeendescribedinthehuman(3±10)andreportedinthecommonchimpanzee(6).Inthehuman,spindlecells,socalledfortheirdistinctivemorphology,arefoundinlayerVbinBrodmann'ssubareas24a,24b,and24candaremostabundantinthecortexformingthemedialwallofthecingulategyrus(area24b;ref.10).Theseneuronsarecharacterizedbyaveryelongate,graduallytapering,large-sizedsomathatisvirtuallysymmetricalaboutitsverticalandhorizontalaxesaswellasalightstainingpatternwiththeNisslstain(8,10).Morerecently,astudyinhumansdemonstratedthatthespindlecellsrepre-sentaclassofprojectionneuronsthatisespeciallyvulnerabletodegenerationinAlzheimer'sdisease,withalossofapprox-imately60%oftheseparticularneurons(10).Hereweshowthatthisneuronaltypeisafeatureoftheanteriorcingulatecortexofallgreatapes,butnotofanyotherprimatespecies.MATERIALSANDMETHODSSamplesoftheanteriorcingulatecortex(Brod-mann'sarea24)wereobtainedfrom28primatespeciesrepresentingallsuperfamiliesofprosimianandanthropoidprimates(Table1;ref.11).Materialsfromadditionalneocor-ticalregionswereavailableforcomparisonfromalloftheanthropoidspecies.Allspecimenswereobtainedpostmortemorfromterminallyilladultanimalssacrificedforhumanereasonsandwerefixedbyimmersionin10%neutralformalin.Specimensofmacaque,owl,squirrel,andcapuchinmonkeyswereobtainedfromanimalsperfusedtranscardiallywith4%paraformaldehydeinthecontextofunrelatedexperiments.Thegreatapebrainswerefromyoungandadultindividuals(agerange,4±34years).Humanbrainspecimenswereob-tainedatautopsyfromneurologicallynormalindividuals(65±78yearsold)andwerepreparedasdescribedpreviously(8,10).Inmanycases,onlyasinglespecimenwasobtainedbecauseofthescarcityofsuchtissue.Thespecimenswereobtainedfromthefollowinginstitutionsandcollections:Bio-qualInc.,theCaliforniaInstituteofTechnology,ClevelandMetrozoo,CoulstonFoundation,LincolnParkZoo,LosAn-gelesZoo,MountSinaiSchoolofMedicine,OregonRegionalPrimateResearchCenter,SouthwestFoundationforBiomed-icalResearch,UniversityofTexasM.D.AndersonCancerCenterSciencePark,NationalInstituteofMentalHealth,andUniversityofGenevaSchoolofMedicine(Switzerland).HistologicalProcessingandAnalysis.Allsampleswerecutinto40-m-thicksectionsonacryostat,Nissl-stained,andanalyzedbytwoindependentobservers(P.R.H.,E.A.N.).Thecriteriaforcategorizinganeuronasaspindlecellwere:anelongate,largesomainlayerVb,lighterstainingthansur-roundingpyramidalneurons,andsymmetricalmorphologyaboutthecell'shorizontalandverticalaxes(10).Whenpresent,theywereunambiguous.Arealboundarieswerebasedonpreviousparcellationsofthecingulatecortexinmacaquemonkeyandhuman(8,12).DistinctionsfromlayerVIatypicalpyramidalcellsandsmallverticalfusiformneurons(10,13)weremadebyanalyzingthelaminarboundariesatlowmag-nificationtodeterminethelaminarpositionoftheneurons.Althoughthenatureofmanyofthespecimensdidnotpermitaquantitativeanalysisbasedonrigorousstereologicmethods,theavailabletissuewaspreparedaccordingtoaserialsamplingparadigmsimilartothatusedinourpreviousanalysesofthehumanandmacaquemonkeyneocortex(8,10,14,15).Re-flectionsofthelocaldensityofspindlecellsingreatapesandhumanwereobtainedfromcountingthenumberofspindlecellsin10sections1mmapartatthelevelofthegenuofthecorpuscallosum,wheretheyaremorefrequentthanatmoreposteriorlevels(10).Inthesesections,thenumberofspindlecellsalsowasexpressedasapercentageofthetotalnumberofresidentneuronsinseriesof1-mm-widetraversesinlayerV.Itisthereforeunlikelythatisolatedspindleneuronswere 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Presentaddress:ColdSpringHarborLaboratory,ColdSpringHar-bor,NY11724.Presentaddress:DepartmentofAnatomicalSciences,MedicalSchool,UniversityoftheWitwatersrand,Parktown2193,SouthAfrica.Towhomreprintrequestsshouldbeaddressedat:NeurobiologyofAgingLaboratories,MountSinaiSchoolofMedicine,Box1639,OneGustaveL.LevyPlace,NewYork,NY10029.e-mail:hof@neuro. overlookedinprosimians,OldandNewWorldmonkeys,andlesserapes.Representativemapsofspindlecelllocationwerepreparedfromeachsamplebyusingacomputer-assistedimage-analysissystemconsistingofaZeissAxiophotphotomi-croscopeequippedwithaZeissMSP65computer-controlledmotorizedstage,aZeissZVS-47Evideocamera,aMacintosh840AVmicrocomputer,and,acustom-designedsoftwareformorphologyandstereology(16).Toproducethesemaps,thecoordinatesofeachlabeledelementwererecordedineachmicroscopicfield,typicallyafractionofacorticallayerata20magnification,relativetoanorigin,andthemapswereassembledautomatically.Thedegreeofran-domness()ofthelaminardistributionofspindlecellswasassessedfurtherbyrelatingtheirmeannearest-neighbordis-tancetotheirmeandensityasdescribedbyMorrisonetal.Withthismethod,if1thedistributionisrandom,if1itisclustered,andif1itisnonrandom(17).Stereologicestimatesofindividualneuronalvolumeswereperformedat100byusingtherotatorprotocolinsoftware(16,18).Theseneuronalvolumeswerecomparedwithrelativebrainvolumebycalculatingthelinearregressionofestimatedbrainvolumevs.bodyweightforadatasetincludinghumans,apes,andmonkeys.Thebrainresidualsareameasureofencephalizationwithinanthropoidprimates(19,20).Statisti-calanalysiswasdonebyusingaone-wayANOVAandpost-hoctestandtestsofcorrelation.RESULTSInthehumananteriorcingulatecortex,spindlecellsoccurmostofteninclustersofthreetosixneurons(Fig.1),arelocatedexclusivelyinlayerVb,andareconspicuousbecauseoftheotherwiselowcellulardensityofthislayer(10).Inthebonobo,thedistributionofspindlecellsmostcloselyresembledthatseeninhumans,withclearclustersfoundthroughoutlayerVbofarea24(Fig.1).Thecommonchimpanzeehadabundantspindlecells,buttheyweremorelikelytobefoundsinglyoringroupsoftwotothreeneurons(Fig.1).Inthegorilla(Fig.1),theoveralldistributionresembledthatinthecommonchimpanzee,butthespindlecellswereconsiderablylessabundant.Intheorangutan(Fig.1),spindlecellswereevidentbutobservedonlyoccasionally.Spindlecellswerenotablyabsentinthegibbon(Fig.1),aswellasinNewWorldmonkeys(e.g.,CallithrixSaimiri,and),OldWorldmonkeys(e.g.,Erythrocebus,and),andalloftheprosimians(Fig.1;Table1).Nearest-neighboranalysisrevealedthatspindlecellsaredistributedregularlyinlayerVbinorangutan,gorilla,andcommonchimpanzeewithvaluesof2.11,1.74,and1.28,respectively.Theirdistributionwasmarkedlyclusteredinbonobo(0.80)andhumans(0.40;Fig.2).Semiquantitativeesti-matesoflocalnumbersofspindlecellsrevealed7±10spindleneuronsinthesectionsfromtheorangutanspecimen,21.95.4(meannumberpersectionSD)ingorillas,37.19.4incommonchimpanzees,60±75inthebonobospecimen,and14.6inhumans.Spindlecellnumbersaccountedfor0.6%inorangutan,2.3%ingorilla,3.8%incommonchim-panzee,4.8%inbonobo,and5.6%inhumansofthenumberofpyramidalcellsinlayerVtraverses,furtherdemonstratingtheirrarity(Figs.2and3).Thesedatawereobtainedfrom4-to34-year-oldapes,sobrainagingisveryunlikelytohaveplayedaroleinthelownumbersoftheseneuronsinthesespecimens,andnormalbrainagingisknowntoinduceonlyaverymarginallossofneuronsinhuman(21).Qualitativeassessmentsofspindlecelldensitiesanddistributiononsec-tionsatlevelsrostralandcaudaltothegenuofthecorpuscallosumshowapatterncomparabletothatpreviouslyde-scribedinthehumananteriorcingulatecortex(10),indicatingthatitrepresentsavalidsamplingpointforcomparisonacrossspecies(Fig.3).Stereologicvolumeestimatesindicatethatspindlecellswere,onaverage,largerthanneighboringlayerVpyramidalcellsandconsiderablylargerthanthesmallfusiformneuronsoflayerVI(Table2).Thesevolumetricdataalsodemonstratethatspindlecellswerelargerinchimpanzeesandhumansthaningorillasandorangutans,whereasthetwoothercelltypeshadcomparablevolumesacrossspecies(Table2).Furthermore,whencomparingneuronalvolumeswithrelativebrainvolumes(19,20),weobservedthatthevolumeofspindlecellsiscorrelatedstronglytoencephalization,assessedbycalculatingbrainresiduals(0.001;Fig.4),whereasthevolumesoflayerVpyramidalneuronsandlayerVIsmallTable1.Summaryoftheprimatespeciesinvestigated TaxonomySpindlecellsEulemurfulvusNone1LemurcattaNone1PropithecusverreauxiNone1CheirogaleusmediusNone1MicrocebusmurinusNone1LoroideaGalagonidaeGalagosenegalensisNone1GalagoidesdemidoffNone1LorisidaeLoristardigradusNone1NycticebuscoucangNone1PerodicticuspottoNone1TarsiussyrichtaNone1AnthropoideaCallithrixjacchusNone2AotustrivirgatusNone2CebusapellaNone4SaimirisciureusNone1CercopithecoideaCercopithecidaeMacacafascicularisNone8MacacafuscataNone4MacacamulattaNone4MacacanemestrinaNone4MacacanigraNone4ErythrocebuspatasNone4PapiohamadryascynocephalusNone1HylobateslarNone4PongidaePongopygmaeusRare1GorillagorillagorillaFrequent5PantroglodytesAbundant8Panpaniscusclusters1Homosapiensclusters6Taxonomicpositionoftheprimatespeciesinvestigatedwithintheirfamilies,superfamilies,andsuborders(11).SpindlecellsinlayerVbofanteriorcingulatecortexarea24areobservedwithcertaintyonlyamonghominoids,inallextantpongidandhominidspecies(shownin,numberofavailablespecimensineachspecies.Neurobiology,Anthropology:Nimchinskyetal.Proc.Natl.Acad.Sci.USA96(1999) fusiformcellsarenot(0.196and0.33,respectively).Thepresentseriesspansalloftheprimatesuperfamilies,althoughitlacksrepresentativesofseveralfamiliessuchaslargeceboids,colobinemonkeys,guenons,andthesiamang.However,giventhelackofspindlecellsin23primatefamilies,aswellinalloftheothermammalswestudied(22),theparallelemergenceofthiscelltypeinanotherprimateseemsunlikely.Infact,samplesfromtheanteriorcingulate(oranteriormedial)cortexandseveralotherneocorticalregionsofmore .2.High-magnificationcomputer-generatedmapsofthelocalizationofspindleneuronsinlayersVoftheanteriorcingulatecortexinorangutan(),gorilla(),commonchimpanzee(),bonobo(),andhuman().Spindlecellsarerepresentedbysolidmarks,andtheneighboringpyramidalneuronsarerepresentedbyopentriangles.Notethedifferencesinthedensitiesoftheseneuronsamongspeciesandtheclusteredpatterninbonoboandhuman.are1-mmwideandare1.5-mmwide. .1.MorphologyofspindlecellsinlayerVboftheanteriorcingulatecortexinhuman(),bonobo(),commonchimpanzee(),gorilla),andorangutan().Inallofthesespeciesthespindlecellsdisplaysimilarmorphologyandapparentsomaticsize.Notetheclustersofspindlecellsinthethrough-focusphotomontagefromthehumanandinthebonobo,whereasisolatedneuronsareobservedinthethreeothergreatapes.)Nospindlecellsarepresentintheanteriorcingulatecortexofthewhite-handedgibbon(),Patasmonkey(),orring-tailedlemur((550mm(A),80),and1205270Neurobiology,Anthropology:Nimchinskyetal.Proc.Natl.Acad.Sci.USA96(1999) than30othermammalianspecies,includingmonotremes,marsupials,insectivores,micro-andmegachiropterans,ro-dents,carnivores,artiodactyls,andcetaceans,wereanalyzed(22),andspindlecellswerenotfoundinanyofthem.Spindlecellsarealsofound,albeitinmuchlowernumbers,intheanterior,agranular,anddysgranularportionsoftheinsularcortexofthehuman,butnotthemacaquemonkey(5,8±10,23).Unfortunately,samplesofinsularcortexwerenotavail-ableforthepresentstudyfrommostofthespeciesstudied,althoughfromthelimitednumberofsampleswedidobtain,itappearsthatsomespindlecellsalsoarefoundintheagranularinsulaofgreatapes.Spindlecellswereneverobservedinanyotherneocorticalareasinalloftheavailableprimatespecies.Theforegoingsuggeststheveryrare,ifnotunprecedented,emergenceofaunique,morphologictypeofprojectionneu-ron,madeallthemoreremarkablebyitsrestrictiontoaverydiscretecorticalregionandaverysmallbuthighlysignificantgroupofspecies.Fromaphylogeneticstandpoint,theobser-vationthatamonghumansandgreatapes,chimpanzeeshavespindlecelldensitiescomparabletohumansÐanditisthebonobowhosespindlecelldistributionmostcloselymatchesTable2.VolumesoflayerVspindleandpyramidalcellsandsmall-layerVIfusiformcells SpeciesPyramidalcellsSpindlecellsFusiformcellsP.pygmaeus9136,648G.g.gorilla1,2955,684P.troglodytes1,1988,796P.paniscus1,8597,743H.sapiens2,29520,822Volumetricdata()wereestimatedinasampleof50neuronsineachlayerfromeachcasebyusingtherotator(18)andareexpressedasmeansSD.Notethat,inspiteofhighvariability,thevolumeofspindlecellstendstobegenerallyhigherthanthatoflayerVpyramidalcells(0.05)andsmall-layerVIfusiformneurons(²,andthatspindlecellsarelargerinand,particularly,incomparedwithGorilla .3.Computer-generatedmapsofthedistributionofspindleneuronsintheanteriorcingulatecortex.Allmapsshowalevelsituatedatthegenuofthecorpuscallosuminthehuman(),bonobo(),commonchimpanzee(),gorilla(),andorangutan().ThespindlecellsarerestrictedtolayerVbandshowmuchhighernumbersinhumanandthetwochimpanzeespecies,whereasfewerareseeningorillaandorangutan.Nospindlecellsareobservedinthewhite-handedgibbon(),long-tailedmacaquemonkey(),andowlmonkey().Themapsareapproximatelytoscale.Thedarklyshadedareasinreflectlocaldamagetothesuperficiallayersofthespecimen.Thecaudatenucleusisshadeddarklyin.CC,corpuscallosum;CS,cingulatesulcus.Neurobiology,Anthropology:Nimchinskyetal.Proc.Natl.Acad.Sci.USA96(1999) thatofthehumanÐunderscorestherelatednessofandoffersaneuroanatomicalcorrelatetothegroupingofthesetaxainthesameclade,acontentionthatissupportedbymolecularbiologicalandmorphologicaldata(24±27).More-over,inviewofdataindicatingthatneuronsformingdefinedprojectionstendtobeclusteredinthecortex(28),themorepronouncedgroupingofspindlecellsinsuggestiveoftheemergenceandprogressiveanatomicalre-finementofahighlyspecificpathwayinhominids,whichbegantoemergeinthecommonancestorofpongidsandhominidsandisparticularlywelldevelopedinThenatureoftheavailablematerialprecludesthephysio-logicalandconnectionalstudiesthatcouldhelpelucidatetheroleofspindlecells.However,existingdataonthecorticalareatowhichtheyarerestrictedmightprovidecluestotheirfunction.Itisalsoworthremarkingthatthedensityofspindlecellsismarkedlyreducedpreciselyinthetransitionareabetweenanteriorandposteriorcingulatecortex(aregiontermedarea24inhumans;refs.8and29),whichcorrespondstotheregionsthatcontainthecingulatemotorareasinmacaquemonkeys(30±33),suggestingthatthedistributionofspindlecellsdoesnotoverlapwithregionsofthecingulatecortexinvolvedinsomaticmotorfunction.Infact,thecingu-latemotorareasinhumanshavewelldefinedcyto-andchemoarchitecturalpatterns(8,29,34)andarelocatedcau-dallyanddorsallytotheregioninwhichspindlecellsaredistributed.Althoughexhaustivesamplesseldomareavailable,preventingproperstereologicestimatesofthenumberoftheseneurons,theyconstituteasmallfractionofthecorticalpyra-midalneuronpopulation(10).Theycould,however,representadiscreteprojection,reminiscentoftheMeynertneuronsoftheprimaryvisualcortex(35,36)ortheBetzcellsintheprimarymotorcortex(37),ofprimates.Alternatively,theemergenceofamorphologictypecouldbemerelyaninciden-talconsequenceofvigorousadaptivechanges.Eitherinter-pretation,however,wouldsuggestthattheanteriorcingulatecortexmayhavebeensubjectedtounusualadaptivepressureoverthelast15±20millionyears.Theanteriorcingulatecortexisregardedasaphylogeneti-callyancientarea(38).Itsubservesmanyfunctionsthatmayvaryacrossspecies,althoughitsroleincorticalcontrolofautonomicfunctions,suchasheartrate,bloodpressure,anddigestivefunctions(39±43),appearstobewellconserved.Alongtheselines,area24inmacaquesisinterconnectedwiththeamygdalaandhasbeenshowntohavelayerVprojectionstothehypothalamusandtheperiaqueductalgray(43±45).Fromthisperspective,thespindlecellsoftheanteriorcingu-latecortexmightrepresentapopulationofspecializedneuronsthatcouldintegrateinputswithemotionalovertonesandprojecttohighlyspecificmotorcenterscontrollingvocaliza-tion,facialexpression,orautonomicfunction.Inaddition,thepresenceofatleastsomespindlecellsintheanteriorportionoftheinsulasupportsthenotionoftheirroleinautonomiccontrol,becausethisregionalsoisknowntobeinvolvedintheregulationofvisceral,olfactory,andgustatoryfunctions,aswellascomplexalimentarybehaviors(46±49).However,inhumanstheanteriorcingulatecortexalsoappearstobeinvolvedinhigher-levelprocessesthatareresponsibleformorethanmerelysensoryinputormotoroutput.Recentfunctionalimagingstudieshavedemonstratedanimportantroleforthedorsalhumananteriorcingulatecortexinattention,withitsdegreeofactivationincreasingwithtaskdifficulty,andthemoreventralportionintheexperienceofthe``unpleasantness''ofpainandintherecognitionoftheemotionalcontentoffaces(43,50,51).Thus,inhumans,atleast,theanteriorcingulatecortexisinvolvedincomplexprocessesthatassistinintegra-tionandinterpretationofsensoryinformation.Inthiscontext,theuniquecorrelationamongtheresidentneuronsoflayersVandVIofthecellularvolumeofspindlecellswithenceph-alizationinhumanandgreatapeslendsfurthersupporttothepossibleassociationofspindlecellswithhighercorticalfunc-tioning.Interestingly,lesionsintheanteriorcingulatecortexinhumanareassociatedwithaformofmutism(43,52,53),andtheareatowhichthespindlecellsarerestrictedisoneoftheonlycorticalareasknowntoelicitmeaningfulvocalizations(andnotmerelysounds)insquirrelmonkeyswhenstimulated(54,55).Ithasalsobeenshowntoparticipateinvoluntaryphonationinmacaquemonkeys(56).Thus,thisregionmaybeinvolvedinsomeaspectsofcommunicationinprimates,anditispossiblethattheappearanceofthesemodifiedpyramidalneuronsmightsignalthefurtheranatomicandpossiblyfunc-tionalelaborationofthiscorticalareaintheonlymammalianlineageknowntohaveevolvedspeechanditsemotionalimplications.Itisalsoworthnotingthattheemergenceofthisuniqueneuronaltypeinaneocorticalareainvolvedinvocal-izationinprimatescoincideswiththeevolutionasadefinableanatomicstructureoftheplanumtemporale,aregionthatisimportantforlanguagecomprehension(57,58).Inviewofthelanguagecomprehensionabilitiesofgreatapes(59),itisthereforepossiblethatseveralcorticalstructuresinvolvedintheproductionofspecificvocalizationsandincommunicativeskillssustainedsimultaneous,considerable,adaptivemodifi-cationsduringbrainevolutioninhominoids.Finally,takentogetherwithourearlierfindingthatspindlecellsinthehumanapparentlyaremorevulnerabletoneuro-degenerationinAlzheimer'sdiseasethanotherpyramidalneurons,possiblyowingtotheirhighcontentofneurofilamentprotein(10,21),thepresentstudypointsoutsomeofthepossiblelimitationsofmakingcomparisonsbetweenhumansandmoredistantlyrelatednonhumanprimatesandempha-sizestheimportanceofthestudyofgreatapesinthecontextofagingandofage-relateddiseasesaffectingthecerebralcortex.Suchknowledgemightassistinourunderstandingofthepossiblyphylogeneticbasisofdifferentialneuronalvul-nerability(60)andofsomeofthemostdevastatingneurologicandpsychiatricillnessesfromwhichourownspeciessuffers.WethankDrs.C.Bouras,I.I.Glezer,S.G.Kohama,J.Marcus,E.J.Mufson,andL.G.Ungerleiderforprovidingsomeofthespecimens;Dr.T.InselforprovidingadatasetonbonobobrainvolumeandA.Hakeemforhelpwithdataanalysis;Drs.P.J.Gannon,C.V.Mobbs,J.H.Morrison,P.R.Rapp,andB.A.Vogtforhelpfuldiscussions;Dr.W.G.Youngforsoftwaredevelopment;andS.Bruns,A.P.Leonard,E.Lugo,F.Robenzadeh,andR.Vertesifortechnicalassistance.MostofthegreatapebrainsweresuppliedbytheGreatApeAgingProject,acomparativeneurobiologyofagingresourceatBioqualInc.(J.M.E.).ThisworkwassupportedbyNationalInstitutesofHealthGrantsAG14308(J.M.E.),CAMH08944,andEY11759(J.M.A.),theDelWebbFoundation(E.G.),andMountSinaiSchoolofMedicine .4.Comparisonofspindlecellvolumewithrelativebrainvolume(seerefs.19and20).Thereisastrongcorrelationbetweenthevolumesoftheseneuronsandbrainvolumeresiduals.NosuchcorrelationexistedforpyramidalandsmallfusiformneuronsoflayersVandVI.5272Neurobiology,Anthropology:Nimchinskyetal.Proc.Natl.Acad.Sci.USA96(1999) 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