Chapter 5 What cells can do（第1页）

Chapter5Whatdo

Inanimalsandplants，cellsaregroupedtogetherintissuesandans。Eaisformedofamixtureoftissueswhichthemselvesseveraldiffereypesthatwethertoperformthetaskshesurvivalaionanism。ectivetissueischaracterizedbylargeamouracellularmatrixsecretedbywell-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）thatabsorberialdiretothebloodstream。Patrollingthissingledaryandguardingagairyireambygutdwellihegut-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。Gutepithelium。（a）Surfaceviewofvillisurroundingthreedome-likePeyerspatches，（b）afracturedvillus，showingitssihiess，（throughamicrovillus，showiwolayersofthemembraernalafilaments，（d）surfadedgeviewofmicrovilli

Iomaximizethesurfaceareaavailableforabsorptiolilyaofiionscalledvilli，eachmadeofaround2000cells（Figure　14）。Eachvilluscellhasitsluminalsurfaizedintomicrovilli（Figure　14c，d）supportedbyatralcoreofafilaments，fa‘brushborder’。Thesespecializatioheareaofuptakeinthesmallihiirefootballfield。Guttsareabsorbedacrossthebrushborderoftheehehecapillarywallsandiream。Allthisfreabolicactivity，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）butinadultsitmovestothemarrowofthepelvisandsternum。Humansmakearound150billionnewbloodcellsperhour。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。Thisallowsthebodytoseitselfofiionahmosterrapreseheanism。

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-derivedcellsgiverisetoalltheadultdinthebodyanism。

Thenervoussystem

Manismshaveameenttofindfoodoresdahesimpleflagellumonabacteriumallowsittoswim，whereasis，fish，andmammalshavesophistidatedsetsofmusdomovetheehamazingspeedandagility。Feelioexternalstimulibeginswiththeworkofheodewormhasnobrainbutdoeshaveaedofresponsiveperformsabasicallysimilarfunassivelyplexanizationofthehumanbrainandnervoussystem。

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，andhaveanism。Otherrarermutatioiationsmayarisebythealterationofsingleaminoacidsofaparticularprotein。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，disablingmutationsaccumulateandgwiththeiruchfanism。Iftheresultofthesefaultsistissueahisacceleratesthedemiseanism。Oyofapopulationfailstoadaptdoesaspeciesdieout。‘thecell’asadrivingforevolutionreallyoure—diofthesamebuildingblocksasourselves，yettheydisappeareddramatically，haviinfinitelyloihanourownmiserable10，000years。Itwouldseemthattheinglyunsteadyedifiergeiesfrombiotissues，tissuesintans，andansiselfmakestheendproduitelymoreseosuddenealgethanitsindividualbasigblocks。Afterall，weknowthatsinglecells（spermandeggs）besuccessfullyfrozenfordecades，whereasthoseindividualswhohadtheircorpses（orjustheads）frozeniurerevivalaremerelywastingtheirmoney。