發育生物學體軸與胚層

VertebrateembryosshowdifferencesinformbeforegastrulationAftergastrulation,rathersimilaratPhylotypicstage(種系型期):headstructure,neuraltube,notochord,somitesLifecycleofthemouse

9weeksfertilizationintheoviduct,meioisIIcompletedclevagemorula卵裂蛙的早期卵裂。A第一次卵裂，B第二次卵裂，C第四次卵裂，動物極和植物極細胞出現差異。卵裂期是指受精卵開始有絲分裂并產生由較小的細胞構成的囊胚(blastula)的過程。主要特點包括：分裂周期短;分裂球的體積下降：海膽胚胎的質/核比由550降至6;早期卵裂中合子基因大多處于休眠狀態；卵裂常經歷由均等裂向不均等裂變化。3.hatching1.compaction2.blastocystcavityformationgeneexpressionstarts128cellDay7-humanDay5.5-mouse哺乳動物的早期卵裂發生在輸卵管中Mousepre-implantationstages:cleavage,compaction,blastocystcavityformationCompactionatthelate8-cellstage,Withapical-basalpolarizationofcells128-cellblastocystcompaction128-cellblastocyst

緊密化(Compaction):8細胞分裂后不久，卵裂球突然擠在一起，卵裂球之間的接觸面增大，形成一個緊密的細胞球體。哺乳動物卵裂球的compaction:發生第三次卵裂后不久小鼠8細胞胚胎compaction前后小鼠桑椹胚的壓縮現象8細胞時期，小鼠細胞表面光滑，微絨毛均勻分布，壓縮后微絨毛僅分布于細胞的外表面，細胞之間的聯系加強了。未壓縮的和壓縮的8細胞小鼠胚胎的比較Compaction的機制:

8細胞胚胎的外層胞間形成致密連接(tightjunctions)，可將球內部的細胞與外環境隔絕，起穩定細胞球的作用。而內層胞間形成縫隙連接

(gapjunction)，可以交換小分子和離子。

注射E-Cadherin抗體的胚胎不能夠致密化。在4細胞期激活蛋白激酶C導致compaction發生。因此，Compaction可能始于蛋白激酶C的活化，它引起細胞骨架的重排，在膜上均勻分布的E-Cadherin重新定位在胞間相交處。哺乳動物囊胚細胞命運的早期分化－位置決定論16細胞期桑椹胚(morula):位于內部的少數細胞產生的子細胞將組成內細胞團(innercellmass)，而位于外部的細胞產生的子細胞大多構成滋養外胚層(trophoblast)。32細胞期胚泡(blastocyst):位于囊胚腔一端的內細胞團(約10個細胞)將發育胚胎的本體及與其相連的卵黃囊、尿囊和羊膜；外層的滋胚層生成絨毛膜，為胎兒從母體攝取營養物質和氧，并產生激素以避免母體的排斥反應。內細胞團的產生是哺乳動物早期發育的關鍵步驟之一。通過對活體胚胎的觀察研究，發現這種重要決定作用僅僅依賴于細胞于某一正確時間出現在某一正確的位置。壓縮后位于外層的細胞將形成滋養層細胞，而內部的細胞將發育成胚胎。一個細胞是否成為胚胎或滋養層細胞，完全取決于壓縮作用后細胞所處的位置是位于外周還是內部。哺乳動物胚泡的著床胚泡在向子宮移動過程中體積增大，是由定位于滋胚層細胞膜囊胚腔一側的Na－K泵將外部Na+泵入腔中，最后通過滲透作用吸水使囊胚腔增大。隨著囊胚腔內離子濃度的增加，水分就通過滲透作用進入進入囊胚腔內。囊胚腔內液體的積累對囊胚腔壁造成一個向外的壓力，這種靜水壓是參與形成和維持囊胚為球形的一個重要作用力。哺乳動物胚泡的著床胚胎外的透明帶阻止了胚泡與輸卵管壁的粘連-異位妊娠。胚泡到達子宮時，胚胎細胞分泌Strypsin（一種類胰蛋白酶)，它使透明帶穿孔，胚泡從孔中“孵化”(hatch)出與子宮壁接觸，通過一系列反應而著床。人類的同卵雙生的發生(占出生總數的0.25%)分離發生在滋胚層形成前(約受精后5天前)的分割，有獨立的絨毛膜和羊膜。占同卵雙生的33%。發生在滋胚層形成后但羊膜形成前(約受精后5－9天)的分割，共用絨毛膜，有獨立的羊膜。占同卵雙生的66%發生在羊膜形成后(約受精9天后)的分割，共用絨毛膜和羊膜，易出現連體兒。人類的連體嬰兒嵌合胚：早期卵裂球有同等的發育潛力。自然人群中也出現過同時有XX型和XY型細胞的人，異卵雙生的兩個胚胎融合形成的單一混合個體。。胚胎干細胞：保持了分化為胚胎本體的潛能的、可在體外增殖的胚胎細胞。在基因功能研究和疾病治療方面有重要的作用。哺乳動物中的嵌合胚、胚胎干細胞人工受精：如主要用于治療不育癥、保存和運輸優良個體。胚胎切割：在2－4細胞期分割胚胎細胞，用于繁殖優良家畜個體。哺乳動物中的人工受精和胚胎切割原腸作用Gastrulation原腸作用是指囊胚細胞有規則的移動，使細胞重新排列，用來形成內胚層和中胚層器官的細胞遷入胚胎內部，而要形成外胚層的細胞鋪展在胚胎表面。原腸作用期的胚胎叫原腸胚(gastrula)。通過原腸作用，胚胎首先建立起三個胚層，即外胚層、中胚層和內胚層；其次為重新占有新位置的胚胎細胞之間的相互作用奠定了基礎。因此，原腸作用是從尚未分化到分化為三個胚層和器官原基決定的關鍵時期。

動物身體的主軸（包括前后軸、背腹軸和左右軸）也會在卵裂和原腸作用期間建立起來，胚胎各部分的細胞會獲得各自的發育潛能。人類的原腸作用上胚層和下胚層的形成：在原腸作用開始時，內細胞團分裂為兩層。與囊胚腔接觸的一層為下胚層，將用于形成yolksac；另一層為上胚層。上胚層細胞間的縫隙將合并、擴大成為羊膜腔，腔中的液體可防止胚胎脫水和保護胚胎受振蕩。上胚層將發育為胚胎的本體。原腸作用前的胚泡，示內細胞團和滋養層細胞。下胚層細胞從內細胞團中分離出來形成卵黃囊，滋養層細胞分裂形成細胞滋養層和合胞體滋養層；細胞滋養層形成絨毛，合胞體滋養層融入子宮組織。上胚層分裂形成羊膜外胚層（包繞羊膜腔）和胚胎上胚層，哺乳類胚胎由胚胎上胚層形成。胚外內胚層形成卵黃囊人體胚胎三胚層的建立及其命運人類原腸作用過程中，羊膜結構的形成和細胞的運動。Note:thehumanepiblastisaflatdisc.Themouseepiblastiscup-shaped--hardertovisualize.40天妊娠后人類的胚胎被羊膜包裹，外面再被絨毛膜包被。血管已深入到絨毛膜內，絨毛膜上的微絨毛最大限度地與母體血液接觸。人類胚胎與母體間的物質交流人體胚胎細胞的分化路線Earlypost-implantationdevelopmentofthemouseembryoInnercellmass:primitiveendoderm(extra-embryonicmembrane)andprimitiveectodermorepiblast(embryoproperandsomeextra-embryoniccomponents)

Polartrophectoderm:theectoplacentalconeandtheplacentaMuraltrophectoderm:trophoblastgiantcellsEarlypost-implantationdevelopmentofthemouseembryoEggcylinder:aninternalcavity(proaminoticcavity)forms,U-shaped

Onsetofgastrulation:theappearanceofprimitivestreak小鼠胚胎上胚層細胞的程序性死亡導致囊胚腔的形成。Gastrulationinthemouseembryo6.5d:

beginningofgastrulation,theappearanceofprimitivestreak;antero-posterioraxis

7.5d:primitivestreakelongatesuntilthebottomofthecup,formingnotochord(headprocess)Gastrulationandneurulationinthemouseembryo7d:thenodeformation,anteriorectoderm→neurectoderm(brainandspinalcord)

Theanteriorpartoftheembryogrowsinsizeandheadfoldappears,definitiveendodermreplacevisceralendodermtoformanouterlayerontheventralsurfaceoftheembryo.Thenotochordbeginstoform.Gastrulationandneurulationinthemouseembryo8d:headisdistinct,neuralfoldshaveformed,guthaveclosed,somitesbeginstoformoneithersideofthenotochord.10.5d:thecompleteofgastrulationandneurulationafterturning.TuninginthemouseembryoTurning:adevelopmentalquirkpeculiartorodents:humanembryosaresurroundedbytheirextra-embryonicmembranesfromthebeginning.8.5-9.5d:themouseembryobecomesentirelyenclosedintheprotectiveamnionandamnioticfluid.Thevisceralyolksac,amajorsourceofnutrition,surroundstheaminon,theumbilicalcordconnectstotheplacenta.胚軸形成胚胎細胞形成不同組織、器官，構成有序空間結構的過程稱為圖式形成（patternformation）。在動物胚胎發育中，最初的圖式形成主要涉及胚軸（embryonicaxes）形成及其一系列相關的細胞分化過程。胚軸指胚胎的前－后軸（anterior-posterioraxis）和背–腹軸（dorsal-ventralaxis）。胚軸的形成是在一系列基因的多層次、網絡性調控下完成的。Bodyaxesformation：小鼠胚胎發育的早期無法辨認體軸Thespecificationoftheinnercellmassofamouseembryodependsontherelativepositionofthecellswithrespecttotheinsideandoutsideoftheembryos.Transcriptionfactors:Cdx2(滋養層細胞的分化)andOct/4(內細胞團的分化)TheaxesoftheearlymouseembryoBlastocyststage:4d:innercellmassisconfinedtoembryonicregionandthisdefinesanembryonic-abembryonicaxis(geometrically,notcellfate)5.5d:theantero-posterioraxisvisible,withtheformationofprimitivestreakattheposteriorend.Thesymmetry-breakingeventisthespecificationoftheanteriorvisceralendoderm(前端內臟內胚層,AVE)

5.5d:

Asmallregionofvisceralendodermatthedistal-mostendofthecupisspecifiesasAVEinducedbyNodalsignalingfromtheepiblastandbeginstoproliferateandextendtoonesideofepiblast.Theanteriorfateisrestrictedtothedistalregionbyinhibitorysignalsfromtheextra-embryonicectodermtotheproximalvisceralendoderm;inducesanteriorectodermThesymmetry-breakingeventisthespecificationoftheanteriorvisceralendoderm(前端內臟內胚層,AVE)

6.5d:

primitivestreakbeginstoformontheoppositeside,markingtheposteriorendoftheaxisRoleoftheAVEinestablishingtheanterior-posterioraxisofthemouseembryoCylindricalSymmetryBilateralSymmetryAPCerberus(anti-Nodal)Dkk(anti-Wnt)Lefty(anti-Nodal)Otx2AnteriorneuralectodermNodal,Cripto(co-receptorForNodal)Endo-mesoderminductionDay5.25Day5.5Day6.25orhypoblastEndo-mesoderminduction,Primitivestreakwnt,brachyury,fgfexpressedEpiblastectodermTheleft-rightasymmetryWhilethevertebratebodyisoutwardlysymmetric,mostinternalorgansareinfactasymmetricwithrespecttotheleftandrightsides,calledleft-rightasymmetry,e.g.,theheart,therightlunghasmorelobesthantheleft.1/10,000inhumans,situsinversusmutationintheivgenecauses50%ofmicehaveheartsthatlooptotheright.Keyproteinsinestablishingleftness:extracellelarsignalproteinNodalandtranscriptionfactorPitx2Determinationofleft-rightasymmetryinthechickleft-rightsymmetryisbrokenatanearlystagebytheasymmetricactivityofaproton-potassiumpump(H+/K+-ATPase)ThereleaseofCa2+fromcellsintheleftsideaspartofthesymmetry-breakingprocess.Pitx2:leftness;Lefty(Nodalantagonist)providemidlinebarriertopreventNodalcrossingtotheright-handside;ActivininhibitShhactivityLeft-rightasymmetryinthemouseembryoCilia(纖毛)-directedflowgeneratesleft-rightasymmetryDirectedleftwardflowofextracellularfluidatthenodeataroundthelatehead-foldstageLeftwardflowoffluidpropagatesthereleaseofintracellularCa2+toupregulateNodalexpressionontheleftside胚層的來源和特化(specification)

Fatemap:telluswhichtissuesthedifferentregionsoftheembryogiverisetoThefatemapsandthegenesinvolvedingerm-layerspecificationaremoresimilarinallvertebratemodelsthanareinearlyaxisspecification

根據被標記細胞的命運，構建兩棲類早期囊胚的命運圖：labeledbyinjectionofhigh-molecular-weightmoleculessuchasrhodamine(cannotpassthroughcellmembranes),asrhodaminefluorescesredinUVlight,sodetectedunderaUVmicroscope.Thefluorescentproteingreenfluorescentprotein(GFP)isnowwidelyused.胚層的來源和特化(specification)FatemappingoftheearlyXenopusembryo:C3islabeledwithfluorescein(green)---tailbudstage,thecellsgiverisetomesodermcells胚層的來源和特化(specification)FatemapofalateXenopusblastula

Ectoderm:epidermis,nervoussystemAlongthedorso-ventralaxisthemesoderm:notochord,somites,heart,kidneyandbloodEndodermoverlyingthemesoderminthemarginalzoneFatemapofamouseatthelategastrulastageNode:exclusivelynotochordThemiddlepartofthestreak:mainlylateral-platemesodermTheposteriorpartofthestreak:tailbud,extra-embryonicmesodermthatformstheextra-embryonicmembranes-theamnion,visceralyolksac,allantois.不同脊椎動物的fatemap基于一個圖式的改變Similarities:thefuturenotochordmesodermoccupiesacentraldorsalposition;theneuralectodermliesadjacenttothenotochord,withtherestoftheectodermanteriortoit.Differences:mainlytotheyolkinessofthedifferenteggs,whichdeterminesthepatternsofcleavageandinfluencestheshapeoftheearlyembryo.EndodermoverlyingthemesoderminthemarginalzoneFatemapsofvertebrateembryosatalateblastulaorearlygastrula早期脊椎動物的胚胎細胞的命運并未決定，是可以調整的Fusionofmouseembryosgiverisetoachimera(micethataremosaicsofcellswith2differentgeneticconstitutions-byfusing2embryos):8-cellstageembryoofanunpigmentedstrainofmouseisfusedwithapigmentedstrainwillgiverisetoachimericanimalGraftingexperiments:transplantingthecellsorregionstoahostembryo,ifdetermined,theywilldevelopaccordingtotheiroriginalposition;ifnotyetdetermined,developinlinewiththenewpositionEarlymouseembryoscanregulatetoachievethecorrectsize:retainsconsiderablecapacitytoregulateuntillateingastrulation.Innercellmassremainpluripotentupto4.5daysafterfertilization-theycangiverisetomanycelltypes.CellsinICMcanbeculturedtoproduceembryonicstemcells(EScells)小鼠發育相關突變的產生Studyingtheroleofaparticulargeneindevelopment,tostudytheeffectsofamutationinthatgeneMice:transgenictechniques,overexpressorknock-outagivengeneIntroducingembryonicstemcells(EScells)carryingthemutationintotheblastocyst小鼠的中胚層誘導和模式建成發生在原條形成時期

6days:theAVEinduceanteriorcharacterintheunderlyingepiblastandearlymarkersofthefutureprimitivestreak(PS)arerestrictedtotheproximalepiblast

6.5days:

BMPactivatestheexpressionofmesodermmarkersandtheposteriormovementofcellsexpressingPSmarkersresultsinPSformation小鼠的中胚層誘導和模式建