发育生物学与再生医学：Fertilization

FertilizationFertilization:

Fertilizationistheprocesswherebytwosexcells(gametes)fusetogethertocreateanewindividualwithgeneticpotentialsderivedfrombothparents.

Theeffectsof

fertilization:

(1)theeggisactivatedtobeginitsdevelopmentalprogram;(2)the

haploid

nucleiofthetwogametescometogethertoformthe

genome

ofanew

diploid

organismContactand

recognition

between

sperm

and

egg.Inmostcases,thisensuresthatthe

sperm

and

egg

areofthesamespecies.Regulationof

sperm

entryintothe

egg.Onlyone

sperm

canultimatelyfertilizethe

egg.Thisisusuallyaccomplishedbyallowingonlyone

sperm

toenterthe

egg

andinhibitinganyothersfromentering.Fusionofthegeneticmaterialof

sperm

and

egg.Activationof

egg

metabolismtostartdevelopment.ThemajoreventsofFertilization:

StructureoftheGametesSpermAreHighlyAdaptedforDeliveringTheirDNAtoanEggheadandtailheadinclude

acrosome(顶体)

andhaploidnucleus（单倍体核）

acrosomeisderivedfromtheGolgiapparatusandcontainsenzymesthatdigestproteinsandcomplexsugars

thehaploidnucleusbecomesverystreamlined,anditsDNAbecomestightlycompressedspermisabletotravellongdistancesbywhippingitsflagellum（鞭毛）Themajormotorportionoftheflagellumiscalledtheaxoneme（轴丝）Themotileapparatusofthesperm——the“9+2”arrangementofthemicrotubules

theproteinsforflagellafunction

tubulin：structuralprotein

dynein：providetheforceforspermpropulsion

histoneH1：stabilizestheflagellamicrotubulessothattheydonotdisassembleThemotileapparatusofthespermtheeggnucleusisstilldiploid,andthespermentersbeforethemeioticdivisionsarecompleted.theeggplasmamembrane——regulatetheflowofcertainionsduringfertilizationandmustbecapableoffusingwiththespermplasmamembrane.vitellineenvelope(zona

pellucida(透明带)

inmammalian)——isessentialforthespecies-specificbindingofsperm.cortex：corticalgranules

（皮质颗粒）

contain

digestiveenzymes,mucopolysaccharides,adhesiveglycoproteins,andhyalinprotein,whichhelppreventmorethanonespermfromfusingwiththeegg.AnEggIsHighlySpecializedforIndependentDevelopmentEggstructureJellycoat(辐射冠)Receptorforbindingspermzona

pellucida

(透明带)

corticalgranules（皮质颗粒)EggcytoplasmplasmamembraneThebio-chemicalcomponentsofegg

Proteins

RibosomesandtRNA

MessengerRNA

Morphogeneticfactors

ProtectivechemicalsRecognitionofEggandSperm1.Thechemoattractionofthespermtotheeggbysolublemoleculessecretedbytheegg2.Theexocytosisoftheacrosomalvesicletoreleaseitsenzymes3.Thebindingofthespermtotheextracellularenvelope(zona

pellucida)oftheegg4.Thepassingofthespermthroughthisextracellularenvelope5.FusionofeggandspermcellplasmamembranesTheinteractionofspermandegg

Spermcapacitation(精子获能):

Thechangesundergonebysperminthefemalereproductivetractthatenablesthemtopenetrateandfertilizeanegg.extensivebiochemicalandfunctionalchangesinthespermplasmamembrane

Capacitationalterstwocrucialaspectsofspermbehavior:(1)itgreatlyincreasesthemotilityoftheflagellum(2)itmakesthespermcapableofundergoingtheacrosomereaction（顶体反应）SpermBecomeCapacitatedintheFemaleGenital

Tractacrosomereaction:Reactionthatoccurswhenaspermstartstoenteranegg,inwhichthecontentsoftheacrosomalvesiclearereleased,helpingthespermtopenetratethezona

pellucida.Inmostmarineinvertebrates,theacrosomalreactionhastwocomponents:thefusionoftheacrosomalvesiclewiththe

sperm

plasmamembrane(anexocytosis)andtheextensionoftheacrosomalprocess(thepolymerizationofglobularactinmoleculesintoactinfilament)Acrosomereactioncandigestapaththroughthejellycoattotheeggsurface.AcrosomeReactionCapacitatedspermbindtothezona

pellucidaandundergoanacrosomereactionAcrosomeReactionAfteraspermhasundergonetheacrosomereactionandpenetratedthezona

pellucida,itbindstotheeggplasmamembraneoverlyingthetipsofmicrovilliontheeggsurface.Theacrosomalproteinmediatingtheirrecognition

iscalled

bindin

(结合素）Sperm–eggbinding:species-specificrecognitionLocalizationofbindinontheacrosomalprocess.二、精子和卵子的接触和识别Sperm–eggbinding:species-specificrecognitionSpecies-specificbindingofacrosomalprocessto

egg

cellsurfaceinseaurchins.GametebindingandrecognitioninmammalsThezona

pellucidainmammalsglycoproteinmatrixplaystwomajorrolesduringfertilization:

itbindsthesperm

itinitiatestheacrosomalreactionafterthespermisboundThemajorstrandsofthezonaarecomposedofrepeatingdimersofproteinsZP2

and

ZP3.ThesestrandsareoccasionallycrosslinkedtogetherbyZP1,formingameshlikenetwork.ZP3issynthesizedandsecretedbythegrowingoocyteandaspecificglycoproteininzona

pellucida.O-linkedoligosaccharidesonZP3arepartlyresponsibleforthespecies-specificbindingofspermtothezona

pellucida.ZP3inducesaninfluxofCa2+

intothespermcytosol;whichinitiatestheacrosomalreaction.TheeffectsofZP3inZona

PellucidaSeveralproteins(sp56,galactosytransferaseandP95)onthespermsurfacethatbindtospecificO-linkedoligosaccharidesonZP3havebeenimplicatedasZP3receptors,

whicharecapableofrecognizingspecificcarbohydrateregionsofZP3.1.sp56（56kDa，半乳糖结合蛋白）2.Galactosytransferase(半乳糖基转移酶

)3.P95（95kDa，透明带受体激酶）ZP3receptorsonthespermsurface半乳糖转移酶半乳糖结合蛋白透明带受体激酶SpermZP3-bindingproteinsatthezona

pellucida.(A)ZP3-bindingproteinsonthemousespermarelocatedintheplasmamembrane,overlyingtheacrosome.Inthisconfocalimage,aZP3-bindingproteinisstainedredbyantibodyimmunofluorescence.(B)RadioactivelylabeledZP3bindstocapacitatedmousesperm.Secondarybindingofspermtothezona

pellucidaSecondarybindingtothezonaisaccomplishedbyproteinsintheinneracrosomalmembranethatbindspecificallytoZP2

ZP2proteinhelpthespermmaintainitstightbindingtothezonawhileburrowingthroughit.GameteFusionandthePreventionofPolyspermy

Scanningelectronmicrographsoftheentryofspermintoseaurchineggs.(A)Contactofspermheadwitheggmicrovillusthroughtheacrosomalprocess.(B)Formationoffertilizationcone.(C)Internalizationofspermwithintheegg.(D)TransmissionelectronmicrographofsperminternalizationthroughthefertilizationconeFusionoftheeggandspermplasmamembranesDiagramofthefusionofthespermacrosomeandplasmamembraneswiththeeggmicrovilli.spermplasmamembrane——fertilin

proteinseggplasmamembrane——α6β1integrinproteinThepreventionofpolyspermyThemostimportantthingistopreventtheentryofmorethanonespermintotheegg.Ifmorethanonespermfuses（polyspermy),thefaultysegregationofchromosomesareformedduringthefirstmitoticcelldivisions,leadingtodevelopmentover.

Twomechanismsoperatetoensurethatonlyonespermfertilizestheegg:A

fastreaction,accomplishedbyanelectricchangeintheeggplasmamembrane;Aslowerreaction,causedbytheexocytosisofthecorticalgranules.

ThefastblocktopolyspermyThefastblocktopolyspermyisachievedbychangingtheelectricpotentialoftheeggplasmamembrane.asmallinfluxofsodiumionsTheslowblocktopolyspermyTheslowblocktopolyspermyismediatedbytheeggcorticalgranulereaction,whichhelpstoensurethatonlyonespermfertilizestheegg.releasingboundspermfromthezona

preventingtheattachmentofotherspermthecorticalgranulereactionisinitiatedbyawaveofcalciumions

ReleasedenzymesclipsofftheterminalsugarresiduesofZP3andZP2thecorticalgranulesfusewiththeeggplasmamembranereleasetheircontentsCorticalGranuleReactionawaveofcalciumions

Thecalciumionsresponsibleforthecorticalgranulereactionarestoredintheendoplasmicreticulumoftheegg.HowthecorticalreactioninamouseeggisthoughttopreventadditionalspermfromenteringtheeggThereleasedcontentsofthecorticalgranulesbothremove

carbohydrate

fromZP3soitnolongercanbindtothesperm

plasmamembrane

andpartlycleaveZP2,hardeningthe

zona

pellucida.Togetherthesechangesprovideablocktopolyspermy.anunfertilizedeggafertilizedeggFormationofthefertilizationenvelopeandremovalofexcesssperm.Themechanismofblockstopolyspermy

Thereareoftentwoblockstopolyspermy:Thefastblockiselectricalandmediatedbysodiumions:theeggmembranerestingpotentialrises,andspermcannolongerfusewiththeegg.Theslowblockisphysicalandmediatedbycalciumions.Awaveofcalciumionspropagatesfromthepointofspermentry,causingthecorticalgranulestorelease.Thereleasedcontentsofthegranulescausethezona

pellucidatoriseandtoharden.

TheActivationofEggMetabolismThematureeggisactivatedbythesperm.Thisactivationsetsintoaction—apreprogrammedsetofmetabolicevents.Theresponsesoftheeggtothespermcanbedividedinto“early”responses,whichoccurwithinsecondsofthecorticalreaction,and“late”responses,whichtakeplaceseveralminutesafterfertilizationbegins

Modelofpossiblepathwayofeggactivation.AburstofproteinsynthesisatfertilizationusesmRNAsstoredintheoocytecytoplasm.

Thebindinreceptorinthe

egg

plasmamembranehastyrosinekinaseactivity.ThetyrosinekinaseactivatesPLC.Thebindinreceptoractivatesacytoplasmictyrosinekinase.Anactivatedtyrosinekinaseinthespermplasmamembraneactivatesthe

egg

pathways.Solubleactivator

pathways

activatePLCordirectlyreleasecalciumfromtheendoplasmicreticulum.Possiblemechanismsof

egg

activation

Thespermnucleus(haploid)——malepronucleusTheeggnucleus(haploid)——

femalepronucleus

Themitochondrialgenomeistransmittedbythematernalparent.Thecentrosomeisderivedfromthemaleparent.FusionofgeneticmaterialinmammalsThecomingtogetherofthespermandeggpronucleiaftermammalianferti