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Chapter5Whatdo
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Inanimalsandplants，cellsaregroupedtogetherintissuesandans.Eaisformedofamixtureoftissueswhichthemselvesseveraldiffereypesthatwethertoperformthetaskshesurvivalaionanisectivetissueischaracterizedbylargeamouracellularmatrixsecretedbywell-separatedcells，providiissuessue，cartilage，tendons，amakeupthestructuralframeworkofthebody.Themoresophisticatedtheanism，themoreplexandnumerousareitscelltypes.Iiohisallowsfortheofspecialistcellsthatdtoandsurviveawiderangeofges.Inthischapterweexaminevariedexamplesofcellspecialismsthatallanismtoprotectitselfaoitse.
&surfaces
Inbothplantsandanimalsthereareoohowcellsgroupandfuntogether.Startingfromtheoutside，thereisaprotectivelayerofplantsthisiscalledtheepidermallayer，whichsecretesawaxygorcuticlethathelpstheplaer.Inwoodyspeciesthatundergrowth，theperiderm，onlycalledbark，replacestheepidermisandsistsivifurtherprotepathogensandthermalinsulatiosaredifferent，produexoskeletonmadeupoflayersof，adensehornyroofsubstanceprovidiivecoverwhichdoublesupastheirskeletasuitofarmour，theexoskeletonisposedofjoihemembrahesegiveflexibilitytotheibody.Theansaachedtotheinnersurfacesoftheexoskeletoes（animalshavingabae），bodystructureisformedbyaninteriscarefullylaiddocells（osteoblasts）startingearlyi.
Skininanimalsfollolestotheprotectivelayerofplantsbuthasmuchmorefunplexityay.Allsurfacesofourbodiesarecoveredbyepithelia，arasttheday-to-dayexistesoldiersoftheepithelialcellsatthesesurfatheoutsideaharereplaadailybasis，butiways，determitheyfulfll.
&erheliumiscalledtheepidermis，edfromthesurfaprisedoffiatteesofkeratincalledsquames（Figure 13）.Squamesbeginlifeashelowerlayersoftheepidermisbut，astheytraveltowardsthesurface，theyprogressivelyls，beingplatesofmaiinprotein，basedressivedepositioheiefilameoskeleton.Becauseourskiniswarmandmoist，itprovidesanattractivesurfacethatistlyizedbybadfungi.Ourrespohisistoshedtheouterlayerofsquamesonadailybasis，togetherwiththeattachedhitch-hikers.Sheddingtakesplaecellatatime，notasainthemannerofasnake.Ascellsattheskinsurfaedetached，theyarereplacedbycelldivisioninthebasallayeroftheepidermis.Betweenthebasallayerandthesurfad14layersinhumans），thedifferentiatingcellsarearrains.Basalepidermalcellshaveioplasmmanybundlesofkeratiefilameinandmicrotubuleworks，whichmediateshapefromcuboidaltofiatteravelupwards，untilthecellreachestheupperlayers.Bythistimethewholeofthehasbeeatratedworkofkeratierspersedackedfulloflipids.Abouthalf，thenucleusisbrokendownandreabsorbed.Althoughtheepidermalsurfaceappearsquitedisahe‘loose’cellsareremoved，thenastrikirigementisrevealed，wherethesquameshavearegularhexagonalshape（Figure 13b，c，d）.Thesquamesare，astheyhavefadtheiredgeswheretheyoverlapwiththeirneighbours.Tweflatsurfadtwelveedgefacetsmakeeachsquameafourteensidedsolidortetrakaide.Thisisexactlythesameminimumareasurfafigurationadoptedbybubblesihisdemoheshapeofcellsfollohysisuringthemaximumsurfaceceforthemierialforeachsquame.Thisarraalsoguarahecellsatthetopfalloffasindividualcells，becauseeachlyfreetodetaallitssixneighbosownsixedgesarefree（Figure 13d）.Lossofsurfaceeatatimemaintainsatoverallthiess，withnwhichmightalloeemoredeeply.Sharpedgesandsolidgeometryarehieswithcells，butarethereasthemosteffitsolutionformaintaiernalsurfadasaresultofthemeetingofionwiththelawsofphysics.
Whileskinprovidesaremarkablyeffitwatertightandmeicalbarriertotheexternaleheseparametersareexactlytheoppositeoftherequirementsofgutepithelium，whereweimizeouruptakeofs，whileatthesametimeinhibitiakeofanythiiallyharmful.Anythingissubjecttoajourneyofarouresoverthecourseestedfoodissubjectedfirsttlyavirohestomach，followedbypowerfulehesmallihenfluidabsorptioninthelargeihefiioerial.Wewilltrateonthesmalliirientuptakeoccurs.Here，intotaltrasttothemultiplelayersofcellsmakingthebarrierofskin，thealayerthatisjustohick，directlyabovetheworkoftinybloodvessels（capillaries）thatabsorberialdiretothebloodstreaPatrollingthissingledaryandguardingagairyireambygutdwellihegut-associatedlymphoidtissue.Thispartoftheimmuemproduunecells（seelaterinthischapter）thanahebodyatspecializedareascalledPeyerspatches，reagtoahreatients（Figure 14a）.Thisisacrucialpartestivesystem，becausearound1000differeeriaresideinut，whichtogetheroutalnumberofourbodybya1000fold.Mostgutbacteriaareharmlessorevenbenefidaretoleratedbythepowerfulimmuheiine.Wheeharmfulanisms，thesearedealtwithbythelargenumbersofmonoimulatedbyantigehegutahegutinastateofdefensivereadiness，whichiscalled‘physiologifiammation’.Inmostcases，afterafewdaysofuoms，thionormal.Thus，thereisadelitolerandimmuionbyourimmunologicesleadstiesandfoodintoleranoreseriousssuchasirritablebowelorcoeliacdisease.
13.Surfaceviewsofskiingmagnifis（a–gthestacked，hexaganization，andreleaseofasinglesquame（d），（e）sethroughthestackofcells，withthelaststageofacellwithanuuc）beforedifferentiationintosquames（squ）above
&majorityoftheingthesmalliineareterocytes，althoughthereareothercelltypeswithimportantfuns.Gobletcellssecretethemucusthatcoversthesurfaceoftheentiresmallihcellsresideihelialorelater）ayofanti-mizymes.Withouttheuptakefunalliinewewouldveryquicklystarve，andwithouttheanti-microbialbarrier，weidlydieofiion.
14.Gutepitheliu（a）Surfaceviewofvillisurroundingthreedome-likePeyerspatches，（b）afracturedvillus，showingitssihiess，（throughamicrovillus，showiwolayersofthemembraernalafilaments，（d）surfadedgeviewofmicrovilli
Iomaximizethesurfaceareaavailableforabsorptiolilyaofiionscalledvilli，eachmadeofaround2000cells（Figure 14）.Eachvilluscellhasitsluminalsurfaizedintomicrovilli（Figure 14c，d）supportedbyatralcoreofafilaments，fa‘brushborder’.Thesespecializatioheareaofuptakeinthesmallihiirefootballfield.GuttsareabsorbedacrossthebrushborderoftheehehecapillarywallsandireaAllthisfreabolicactivity，asosuretotheualthreatofbavasioedinthelifeofaebeingashortoheachcelllastiwoorthreedaysbeforereplat.Thesupplyofesinvaginationsratherlikesmallpodatthebaseofthevilli，calledcrypts.Eachvillushasfivetotencryptswhereprecursorcellsdivideataratethatprodud1400cellseachday.Theseesmigratetowardsthetipofthevillus，where，havingusefullyabsorbedforacoupleofdays，theyarethearateofoe.Thisual（lifetime）replatofcellsaddsuptoastaggeringproduthatisequivalentonayearlybasistoaboutthreetimesthebodyweight（asworkedoutiheexaismsofthismassmigratioillunknown，althoughthereareseveraltheories，ingtheideaeedbycelldivisiosliterallyfcellsupwards.Alterhecellsmaymigrateupthebasementmembraneinmuchthesamewayasmigrationelsee（butuseful）lifeatthetopofthevillus，theetattothebasementmembrawhitthedetachedcellsgetejectedbygcellsratherlikegraspingandsqueezingabarofsoapuntilitshootsoutofyourhand.
Bloodcells
Bloodisclassifiedasaectivetissue（likebowithaliquidratherthansolidmatrix.Thisfluidtransportssaearoundtheanimal.Isandsoftenhaveyellreen‘blood’astheyabseotheirtissuesthroughsmalltubesthroughouttheirbodyandtheirbloodlacksen-gproteincalledhaemoglobin.
Bloodisformedfromaofvariouscelltypes，whitinuouslyproduallemcells（cellscapableofdevelopingintomaypes）.ofhowbloodcellsdevelophasactedasamodelformaissuesystems.Inadultmammals，bloodstemcellsresideinthebure 15）aerizedbya‘selfrenewing’division，inwhichhalfhtercellsremainasstemcells，withtheotherhalfbeiorcells.Theseprogenitorcellsdividemanythousandsoftimes，progressivelyundergoingaprocessofdifferentiationthatgesbiochemistry，shape，ail，finally，amaturebloodcellisproduced.Thesiteofhumaiohroughoutlife.Inembryos，itregatesofbloodtheyolksac（calledbloodislands）.Asdevelopmentues，bloodisproduthespleen，liver，andlymphhebonesdevelop，bloodprodubeginsinthemarrowofjuvenilelohighandshin）butinadultsitmovestothemarrowofthepelvisandsternuHumansmakearound150billionnewbloodcellsperhour.Mostaretheredcellsthatcarryoxygenaroundthebody.Immatureredcellsdevelopintotheirfihrodertheinfiuencerowthfactor（erythropoietigtheirproteinproduovertothetwoglobilobin（Figure 15b，c）.Redcellsliveforalittleover100daysinthebody.Ifyouliveortrainfhaltitudewhereoxygenlevelsarelower，theproduofEpoisstimulatedandyourbloodoreredeofthebiggestgestokeepingsport‘’hasbees’abilitytodiscoverwhethersportsmenhavegainedunfairadvaheartifiistratiooboosttheirperformance.
15.Bloodcells.（a）ewlyformedinthebonemarrthebloodstreamatthetralvenoussinus，（b）adevelopingredgitsspheriucleus，（c）agroupofdevelopihesurfaacrophage，（d）plateletsmakelooformad（e）whiteculturedbonemarrow
Battlinganddefendingcells
&cellshaveevolvedtoprotectmultisfromattackbybacteria，viruses，andparasites.Whitebloodcellsperformthesedefensiveandg-uproleswithinthebinallyproduthebonemarrow，someofthesewhitecellstraveltothespleen，thymus，andlymphglandsfortheirfiioimesforwhitecellproduarestaggering.Forexample，ifwecatchflu，ourwhitecellprodubetripledwithinhoursoftheiihowthesebloodimmueiiooavibrantareaofbiomedicescalledimmunology.
&oheimmuemarelymphocytesorTedbecausethesecellsmatureihymus）andBcells（Bfrombursa，theaninwhichtheymatureinbirds;inotheranimalstheBcelldevelopsinthebure 15a）.Tcellsarefurtherdividedintovariousclasses—memory，helper，cytotoxidregulatoryorsuppressor.T-memorycellsationaboutharmfulsubstandbiologifeownasantigens），lohebodyhasresolvedtheiheyalseredbyinos.Thesecellsoftensurviveforalifetimeandonitorforthepreseigeotypesofiion.Upoothisa-memorycellsrapidlydivideandsignaltotheimmuhereisaproblemviaproteinsontheircellsurfathemassiveentialantigenices，whibeassmallastwoorthreesecutiveaminoaaprotein，achemicallymodifiedsugarmolecule，oreventheshapeoftheprotein，therearemillionsofdifferentT-memorycells.
&oryTcellsareimportantintheprounologicaltolerance，sdownorsuppressesreactiveTcellsiftheymistakeyourownproteinsasfnfollowingaiocellsstimulatethegrowthofotherimmuheotherT-celltypesarethediplomatsaheimmuhencytotoxiaturalkillerTcellsaretheinfantry.Theyreizevirallyiumourcelltargetsandkillthembyihetargetcellwithproteinsthatih.
MatureBplexproteinscalledimmunoglobinswhibioformantibodies，eachcapableofbindingtoaspecificmolecularstructure.Thenecessarilymassiverepertoireofantibodiesiscreatedbyspligseparatepieceseimmunoglobihertomakeimmunoglobulihdifferees，eachcapableofbindingtojustoigen（onesmallpartofaprarstructure）.
&hermajroupofwhitecellsarethemyeloidcells，morevariedinstruTandBdprisedofgranulocytes，megakaryodmacrophages.AllareinvolvedinimmunityandtheremovalnbiologicalmaterialbydifferentmeismstothatoftheTandBcells.Granulocytes，whigrahersubdividedirophils，eosinophils，andbasophils.Oreofhumanbloodsaboutfivebilliorophils（aroundhalfofallwhitebloodcells）.Ifrophilsreceiveasignalfromasiteofinjury，ittakesaround30mihemtoleavethebloodahesiteofpoteiorophilsareseriouskillingcells，ferogulfinginvadihavebeeedbyanantibodyai.Oheirjobisfiurnihelessoneosinophils（fromtheLatinfortheir‘ag’reaicaldyes）areresporoyingparasitesbyihemwithhydrogenperoxide（onlyusedasahairbleaddisi）.Fortunately，eosinophilsiionlyliveforafewhourswhenactivated.Eosinophilsaremediatorsisesaiveiofasthma.Theyarealsoinvolvedinmanyicalproggraftrejedcer.Basophils（theyreactwithalkalinechemicaldyes）arefouhesiteofparasiteiioicks.MastcellswerefirstobservedbyPaulEhrlithelate19thtury.ThelargegraaiheideathattheyexistedtofeedthesurroundingdthusErliamedthemMastzellen（fromtheGermanMast，meaningforaainhistaminewhireleased，triggersallergissuchashayfever.
Megakaryocytesaretentimeslargerthanredblooddeachprehesmallplateletsthatareresponsibleforbloodg.Theynormallyatfor0.001%ofbonemarrowcells.Astheymature，theDedseveraltimesbutthecelldoesnotdivide，aknoloidywhichallowscellstoinsize.Somecellshaveasmanyas64copiesoftheirDNA（aainsjusttwocopies）.Atthisstage，thecellmaturesaotheproteihrombopoietin，produces‘pro-platelet’bodies.Thewholemegakaryodergoesatrolled‘explihousas，whipersistasribbonsinthebloodvessels.Humansproduceabilliosperdayhavingalifespanoffourtofivehours.Platelets（theonlycellsbesidesredbloodcellsthatlaucleus）aggregateasclumpsattheendothelialgtopreventbloedbloodvessels.Astheyareactivatedtatetheygeshape，extendinglongfinger-likeprocessesthatiheachure 15d）.Theremainingmyeloidcellsarelargelumberingamoeba-likecellscalledmacrophages.Theirroleisrubbish.Theyengulfcellulardebrisandpathogensandshole（aproasphagocytosis）beftheirposdownforfurtheruse.Theaveragemacrophagedigestahueriabefasitsuccumbstoitsownexaterialisphagocytosedwholeanditsmembranesarebiochemicallydegraded，produgsmall‘fn’prments（ahesearetheheoutersurfaacrophagewheretheyare‘presecetheTcellhasmemorizedthisproteirapidlydivides，stimulatedbyagroroducedbythemacrophage.
Whathasbeeclearbyexamihefunolecularmeunecells，espemammals，isthatallareplex，interlinked，aimesredundantinfun.Thisallowsthebodytoseitselfofiionahmosterrapreseheanis
Respondingtothephysicalworld
Howhavecellsevolvedtoreadrespoernalphysical，chemidbiologicaleffely，cellsbothrespondto，aheehtandgravity.Photosynthesiswasestablishedsomebilliobyaerialpreodernobacteria.Thesephotosynthesizinganismswereengulfedbyearlycellstoproducechloroplasts，leadiionofplants.Photosynthesisisatrovolvirappingoflightenergyanditsplex–dedsubstancessuchassugars.Thesesugarmolebeusedbytheplantfrowthbutalso，eitherdiredirectly，actasafoodsrowthofallthenon-photosynthesizinganismso.Bacteriaofthedeepseaaerrestrialcavesystemsaretheosurviveialabsenlight，astheyusevolierearthheatastheirenergysourdhaveabiochemistrybasedonsulphurratherthanoxygen.AswedisoreabouttheprocessesthatothefirstbillionyearsofEarth’shistory，itseemsthatallbacteriamusthavesustaiheabsenceofoxygen.Asthingscooleddowhreeandfourbillio，aquaticbacteriawereabletousedioxideiostoproduandplexcarbohydrates.Theriseinatmeinseveralstages，beginningwithasignifitriseabout2.5billioknowOxygehoughitwasoherrisesfollowiingoftheso-laciersthatoxygenlevelsweresuffittosustainmultiimals.Thechemiditioyofplexlifeformswereestablishedbyabout600millio，perhapsthemosttrulyearth-gevent.
Alllivingaolight（phototropism）andgravity（gravitropism）.Theupperpartsofplantsusuallygrravityandtht.Theeffectravitationalpullonplalyshownwhenmossesgrowioiongreiralfashioheirnhtstructure.Thegrowthehigherplahepreseiarch-filledparticles（amyloplasts）fouheofspecializedcellscalledstatocytes.Thesecellsarefoundintheandthejoiheshoot.Gravitynormallypullsthesepartiwardsiinmicrogravitysofspacethey‘float’，andfailtoproducetheusualgrowthpattern.Theionofamyloplasts，withinacytoskeletalworkoffiihoughttoactivateamnallingpathwaythatleadstotheredistributiohormoneauxin.gthelevelsofauxinwithincellsisplex，asitotegatiobutinhibititi.
Cellswether
Wemuchabouthoitheastudyingsimpleanimalssuorhabditiselegans，aroundwormthesizeofaaonthispage，lessthareloalbiologistLewisWolpertmayhavecalledit‘themanimalimagihephylumofowhich sbelongsprobablymakesup80%（numerically）ofallanimalsintheworld.Althoughitpossessesnobrainitdoeshaveasimplenervoussystem，awigglingmeism，adigestivetradegg-layingcapacity.Asmehepreviouschapter，itbegih1090cells，but131areeliminatedbyapoptdevelopmeodesenjoyasoil-boringbadreprodugusuallywiththemselvesashermaphroditesbysequentiallyprodugspermandtheneggs（only1in2000wormsaretrulymale）.Foritssize， shasasurprisinglylargenumberofgenes，around20，000（humanshavearound24，000）.Manyofitsgenesareinvolvedincelldivisionand50%ofthemaresharedwiththebahirdofitsgehuma.Howdoweexplainthishighnumberofgenesinsuchasimpleaionisthatthereareaninfiatednumberofchemicalrees，allowioeffitlydeteydifferentsmellswhenhuntingitsbaatodewormsarefoundineveryimagiypeaosurviveinthesewidelydifferentecologiiematodeshaveevolvedbygmoreaiveoradaptivegeoallowthemtosurvivethegesmadebyallthevariedspeciesofsi，andothermiwhichtheydiofsischaracterizedbetterthai，thankstotheNobelPrizewinningworkofSydneyBrennerandhisteam，andweandpreciselywhichcelldevelopsiherghowallthe302nerveecttoeachother.
Movingupthroughtheevolutiohecelldevelopmefly，Drosophilamelanogaster，hasalsobeencarefullymapped.Withinthreehoursoffertilization，theflyembryocellsalreadystarttoshowthefirstvisiblesignsofdifferentiatioicularanortissuethatdevelopsdepepositionofeachcell.Justhansandtissues，ingbrain，blood，fattissue，thorax，aina，developfromtheembryohasbeeablished.Withitsextremelywelldefihismanismhashelpedustoualyhowtheentireeageoftheiislaiddowchapter，weexamiheseembryo-derivedcellsgiverisetoalltheadultdinthebodyanis
Thenervoussystem
Manismshaveameenttofindfoodoresdahesimpleflagellumonabacteriumallowsittoswim，whereasis，fish，andmammalshavesophistidatedsetsofmusdomovetheehamazingspeedandagility.Feelioexternalstimulibeginswiththeworkofheodewormhasnobrainbutdoeshaveaedofresponsiveperformsabasicallysimilarfunassivelyplexanizationofthehumanbrainandnervoussyste
Thehumanbrainsaroundtenbillionneurons.Easswiththousandsofothers，allowiostoreandtransmitinformationbypassirialswithinthedchemials（raweenaplexendingthroughoutthebody.Millionsofsensoryneurohatvertstimulifromthee，touell）irialsthatfeedbacktothebraiorneuronssendinformatioothemusdhlands.Interheinformatiohesensoryandmotorneurons.Neuronshavespecializedextensioesandaxoesbringinformationtothecellbodyandaxoionawayfromthecellbody.Extendedaxedistandaresurroundedbyaspecializedstructurecalledamyelih，madeupofcellsthattheaxoninmultiplelayersofmembrahemyelisasinsulationtopromotethepassageofnerveimpulsesahenerveaxoernalinterferenapsesarethejuhecellswherechemicalransferred.Neuronsareestandmostlong-livedcells.Cortieurons（whiectthemotorcortextothespinaldprimaryafferentneurons（whidfranssuandgutintothespinalduptothebrainstem）beseveralfeetlong.Neuronslastalifetimebutnumbersdodecreasewithage.Gliacellsarecloselyassociatedwithallneurons，haviiveandnurturingrole.Wederstandtheplexfunsofvision，ess，ahesinglecelllevelbutonlyintermsofiioweenmanybillionsofdperhapsthemeyinbiology.Importantinsightshavebeehegstudyofthenervoussystemiflies，andothersimpleanismswhichwill，intime，shedlightonhowourownbrainfuns.
Cellularge
&ime，allcellsaccumulatemultiplegeigestotheirDhesegesareofteofradiatioravioletlight，icrays，radioactivity）orexposuretolowlevelsoftoxicchemicals.Astheyocdom，themajoritywillmissthe2%ofg（geicallyaA，andhaveanisOtherrarermutatioiationsmayarisebythealterationofsingleaminoacidsofaparticularprotein.Thismodifythethree-dimensionalstructureoftheproteinwhiraise，lateitsnormalactivity.esresultieorpartialdeletionofgenes（inggerolsequences）orasubtraordupliiiallylheramountsofaparticularprotein.Inrarecases，amajemehenucleusmayresultielynehisisthemeismofevolutionatitssimplestandifsuetigesadverselyaffegle-itdies.Shouldanadvantagebeferred，itwillspreadthroughoutapopulatiouallybeeestablished.
MultisaresubjeeevolutiohroughDeachtissueanhasbillionsofdataimeonlyaproportiotogeisulteresultinginadisease.Forexample，ifsugeresultsieofcelldivisioime，thesecellswilleventuallyrowtheirnormalterparts（asseenintumrotationreliesoleparticularcellstorespondtoealges.Withintheplexgeanismstherearealteriplepathroteinstheindividualcellsurviveavarietyofinsults，forexampleradiation，toxicchemicals，heat，excessiveorredu.Oftenthisisperfthedamagetheactivityoftheaery，waitiogoaway.Manyoftheseresponsivebiochemicalpathwayshaveevolvedfromthosefoundinsingleeswhosegeicmake-upadistantpastwheh’sclimatewasfarmoreextreme.Iaheeahroughapoptosis（asoutlichapter）.Withtime，disablingmutationsaccumulateandgwiththeiruchfanisIftheresultofthesefaultsistissueahisacceleratesthedemiseanisOyofapopulationfailstoadaptdoesaspeciesdieout.‘thecell’asadrivingforevolutionreallyoure—diofthesamebuildingblocksasourselves，yettheydisappeareddramatically，haviinfinitelyloihanourownmiserable10，000years.Itwouldseemthattheinglyunsteadyedifiergeiesfrombiotissues，tissuesintans，andansiselfmakestheendproduitelymoreseosuddenealgethanitsindividualbasigblocks.Afterall，weknowthatsinglecells（spermandeggs）besuccessfullyfrozenfordecades，whereasthoseindividualswhohadtheircorpses（orjustheads）frozeniurerevivalaremerelywastingtheirmoney.
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