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DevelopmentalBiology,Chapter3:Thegermcelldevelopmentandfertilization,Thegermcelldevelopmentandfertilization,Howarethegermcellsspecifiedanddetermined?Howaretheprimordialgermcellsdifferentiatedintoeggsandsperm?(includingmeiosis)Fertilizationandactivationoftheeggbythespermthatinitiatestheembryonicdevelopment.,Thegermcelldevelopmentandfertilization,1Thegermcellfatedeterminationanddevelopmentofgonads1.1Specificationanddeterminationoftheprimordialgermcells(PGCs)1.2Thegermcellmigrationanddevelopmentofgonads2Gametogenesis2.1Spermatogenesis2.2Oogenesis3Fertilization3.1Therecognitionofeggandsperm3.2Thepreventionofpolyspermy3.3Theactivationofeggmetabolism,Thegermcelldevelopmentandfertilization,1Thegermcellfatedeterminationanddevelopmentofgonads1.1Specificationanddeterminationoftheprimordialgermcells(PGCs)1.2Thegermcellmigrationanddevelopmentofgonads2Gametogenesis2.1Spermatogenesis2.2Oogenesis3Fertilization3.1Therecognitionofeggandsperm3.2Thepreventionofpolyspermy3.3Theactivationofeggmetabolism,,SpecificationanddeterminationofthePGCs,Gametesinallsexuallyreproducingorganismsarisefromtheprimordialgermcells(PGCs)Inmanyinstances(includingnematodes,flies,andfrogs),thePGCsarespecifiedanddeterminedautonomouslybythecytoplasmicdeterminantsintheeggthatarecomprisedofspecificproteinsandmRNAs.Thesecytoplasmicdeterminantsarecollectivelyreferredtoasthegermplasm（生殖質(zhì)）Germplasmcanbeidentifiedmorphologicallybythepresenceofconspicuousmembrane-unboundorganelleswithanelectron-densegranulofibrillarappearancecalledgermgranulesPgranulesinC.elegansPolargranulesinDrosophilaGerminalgranulesinXenopus,ThegermcellfateofC.elegansisdeterminedatthe16-cellembryo,TheadultgonadsandearlydevelopmentofthefertilizedegginCelegans,ThecelllineagechartinthedevelopmentofCelegans,558cellspresentinthenewlyhatchedlarva,959somaticcellsintheadult,,OriginofthePGCsinDrosophila,InDrosophila,thePGCsformasagroupofpolecellsattheposteriorpoleofthecellularizingblastoderm.Thepoleplasm（極質(zhì)）attheposteriorpoleiscriticalfordeterminationofthePGCsinflies.ThepoleplasmincludesseveralcomponentsthatarecrucialforthePGCdetermination.ThosearemRNAofgcl(germcellless)gene,Oskar,Nanos,Vasa,mitochondrialribosomalRNA(mtrRNA)etc.,OriginofthePGCsinDrosophila,ThePGCsknownaspolecellsbecomedistinctattheposteriorpoleoftheeggabout90minutesafterfertilization.Thecytoplasmattheposteriorpoleiscalledpoleplasmandisdistinguishedbylargeorganelles,thepolargranules,whichcontainsbothproteinsandmRNAs,,ThemolecularmechanismsunderlyingthespecificationanddeterminationofthePGCsinDrosophila(I),InDrosophila,thePGCsformasagroupofpolecellsattheposteriorpoleofthecellularizingblastoderm.Thepoleplasm（極質(zhì)）attheposteriorpoleiscriticalfordeterminationofthePGCsinflies.ThepoleplasmincludesseveralcomponentsthatarecrucialforthePGCdetermination.ThosearemRNAofgcl(germcellless)gene,Oskar,Nanos,Vasa,mitochondrialribosomalRNA(mtrRNA)etc.,,,InDrosophila,severalmaternalgene(atleast8genes)arefunctionallynecessaryandsufficientforthepoleplasmformation,andthegermcellspecification/determinationMutationsinanyofthosegenes,e.goskar,resultintheaffectedhomozygousindividualbeing“grandchildless”.Itsoffsprings(F1)lackaproperpoleplasm,andalthoughtheymaydevelopnormallyinotherways,theylackgermcellsandthereforearesterile(F2).Expressionofoskaraloneissufficientforectopicallyspecifyingthegermcells,ThemolecularmechanismsunderlyingthespecificationanddeterminationofthePGCsinDrosophila(II),ThegeneoskarissufficienttospecifythegermcellsinDrosophila,FunctionallyconservedgenesinGerm-CellDevelopment,,,OriginofthePGCsinmammals,Thereisnoobviousgermplasminmammals,andmammaliangermcellsarenotmorphologicallydistinctduringearlydevelopment.Rather,germcellsareinducedingastrulatingembryosInmice,thegermcellsformattheposteriorregionoftheepiblast,atthejunctionoftheextraembryonicectoderm,epiblast,primitivestreak,andallantois.,ThemammalianPGCsappeartobeinducedintheearlygastrulatingembryos,,ThemolecularmechanismsunderlyingthespecificationanddeterminationofthePGCs,Thereisnoevidenceforgermplasmbeinginvolvedingermcellformationinthemouseorothermammals.Instead,germcellspecificationinthemouseinvolvescell-cellinteractions.Inmice,Atday6.5ofembryonicdevelopment,BMP4andBMP8bfromtheextraembryonicectodermgivecertaincellsinthisareatheabilitytoproducegermcells.,Germcellspecificationinthemouseinvolvescell-cellinteractions,Fig11.3Germ-cellformationinthemouse.Theprecursors(white)ofprimordialgermcells(PGCs)andextra-embryonicmesodermareinducedintheproximalepiblastbysignalsfromtheextra-embryonicectodermthatincludeBMP-4.Duringgastrulation,thesecellsmovetotheposteriorendoftheembryoabovetheprimitivestreak.HeretheyformaclusterinwhichthecentralcellsbecomesspecifiedasPGCsandtheperipheralcellsasextra-embryonicmesoderm.Aftertheirformation,thePGCsmigratetothegonads.,BMP4isrequiredforthegenerationofprimordialgermcellsinthemouseembryos,,A:Wildtypeembryo;B:HighmagnificationofpartofAshowingindividualPGCsinthehindgut;C:EmbryosheterozygousforBMP4mutant.TherearefewerPGCscomparedwiththewildtype;D:Homozygousembryo.PGCsareentirelyabsentinthehindgut,Thegermcelldevelopmentandfertilization,1Thegermcellfatedeterminationanddevelopmentofgonads1.1Specificationanddeterminationoftheprimordialgermcells(PGCs)1.2Thegermcellmigrationanddevelopmentofgonads2Gametogenesis2.1Spermatogenesis2.2Oogenesis3Fertilization3.1Therecognitionofeggandsperm3.2Thepreventionofpolyspermy3.3Theactivationofeggmetabolism,Inmanyanimals,germcellsdevelopatsomedistancefromthegonads,andonlylatermigratetothem,wheretheydifferentiateintoeggsandsperm.Inthemousegastrula,germcellsfirstbecomedetectableattheposteriorregionoftheepiblast（上胚層）.Theybecomeincorporatedintothehindgutandthenmovefromguttubeintothegenitalridge.ThemechanismsbywhichthePGCsknowtherouteofmigratingjourneyisstillunknown.FibronectinislikelytobeanimportantsubstrateforPGCmigration.InvitroevidencesuggeststhatTGF-betalikeproteiniscapableofattractingmousePGCs.2genes,Whitespotting(W)andSteel,areinvolvedincontrollingproliferationofmigratinggermcells.,Germcellmigrationandtheunderlyingmechanisms(I),InDrosophila,thePGCsmovefromtheposteriorpoletothegonadsinamannersimilartothatofmammaliangermcells.ThewunengeneappearstoberesponsiblefordirectingthemigrationofthePGCsfromtheendodermtothemesoderm;2genes,columbusandhedgehog,appeartobecriticalfortheattractingtheDrosophilaPGCstothegonads.,Germcellmigrationandtheunderlyingmechanisms(II),PathwayforthemigrationofmammalianPGCs,Gonadsinmouseembryosatday13.5stainedwithantibodiestoE-cadherin(red)andlaminin(green),ProliferationofPGCsfromaninitialpopulationof10-100cellsto2500-5000cellsinthegonadsbyday12ofmouseembryosNote:ThePGCsofbothsexesexpresshighlevelsofE-cadherinandarearrangedincorticalclustersintheovaryandininternalcordinthetestes,columbusgeneisnecessaryforPGCstomigratetogonads,a-d:anti-VasalabelsthePGCs:e,f:anti-VasalabelsthePGCs(purple);anti-Cliftlabelsgonadalmesodermalcells(brown),Incolumbusmutantembryos,somePGCsfailtomigratefromtheendodermtothemesoderm,andinsteadremainassociatedwiththeendoderm(a).Inaddition,manyPGCsthatdomigratetothemesodermfailtoassociatewiththegonadalmesoderm(e),andinsteadscatterwidelyintheembryo(c),EctopicexpressionofcolumbusgeneissufficienttoattractPGCstonewlocations,a:theembryowasimmunostainedwithanti-b-galactosidase.b-e:theembryoswereimmunostainedwithanti-Vasa,TheGal4/Uasbinarysystemwasemployedtoectopicallyexpresscolumbusgeneinspecifictissuesincludingabdominalsegments(a,b),epidermis(c),thenervoussystem(d)andthePGCs(e),