1. Blastopore FormationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 361. Blastopore Formation(That looks familiar!)Gastrulation begins 180° from site of sperm entryFig. 7.6spermentryHow do cells choose fates before gastrulation?The dorsal-ventral axis in the frogA frog’s mother sets up the initial axis of polarityAnimal poleVegetal poleAn outline of frog gastrulationFuture Dorsal side (back)Future Ventral Side (belly)Sperm always enters at the animal hemisphereSperm entry is the critical asymmetric cue in setting up the dorsal-ventral axisof the frog XenopusSee Figure 7.130Sperm entry triggers a rotation of the outer cortex relative to the inner cytoplasm, fixing the site of gastrulation initiationYolky cytoplasmAnimal cytoplasmAnimal cortexSperm entrypoint cortex rotationGray crescent = junction of vegetal cortex with animal cytoplasmgastrulation initiates here.Vegetal cortexRotation occurs during the first division,and requires the sperm centrioleExperimental alteration of cortical rotation changes the blastopore location Sperm entry point3. Gravity rearranges cytoplasm so that the junction of vegetal pole cortex with animal pole cytoplasm is next to sperm entry point instead of opposite it.Sperm entry pointgray crescentSperm entry point1. Rotate egg so that sperm entry point is now on top.2. Prevent normal cortical rotation4. Gastrulation now occurs next to sperm entry pointMother cell Cell type ACell division places daughter cells in different environments, which can lead to different cell fate choicesCell type BCell divisionInformation from neighbors:Induction: information from neighbors influences cell fateinducerresponderinducerresponderCompetence: ability to respond to a certaininductive signalCell not competent to respondresponderinducerSuccesive inductions: can generate many cell types from just a few interactionsCell not competent to respondTypes of signalsInducerResponderRelease into the Fallopian tubeSignals can act globally throughout the bodySignals can also actin a graded fashionInduction:An initial difference can beamplified into many cell typesHow is cell fate determined-- the dorsal-ventral axis in newts See p. 255Spemann & Mangold: the organizer can influence neighboring cells toform a secondary body axis:Figure 7.17Induction of theorganizer can influence neighboring cells to form a secondary body axis:If one transplants a second inducerof the organizer the embryo forms two body axesFigure 7.19Induction of mesoderm in the frog embryoAnimal poleVegetal poleInitial asymmetry of Xenopus egg leads to some cell fate differencesMesoderm arises at endoderm/ectoderm junctionExperiment: Neither vegetal or animal pole alone can make mesoderm. Figure 7.18Vegetal pole cells can induce animal pole cells to make mesodermConclusion: a signal in the vegetal part is needed to induce mesoderm in the animal cap. Fig. 7.18Induction of dorsal mesoderm by dorsalmost vegetal cells#1#3#2The 3-step model of mesoderm inductionFigure 7.18#2#1#3The 3-step model of mesoderm inductionModel for mesoderm induction and organizer formationFigure 7.23Neurulation separates the ectoderm intoneural and epidermal cell fatesAlberts Figure 20.26Neurulation is driven by cell shape and adhesion changes similar to those that drive gastrulationNeural tube formationDorsal-Most CellsFigure 9.4Differential adhesion helps the neural tube and skin to separate from each otherHatta and Takeichi (1986) NatureE-cadherinN-cadherinectodermSee Fig 9.6Experiment: Differential adhesion helps the neural tube and surface ectoderm separate from each otherectodermFig 9.6Inject N-cadherin mRNA so that surface ectoderm expresses N-cadherinNeural crest cellsmigrate from the CNS to form the peripheral nervous systemDefects in neural tube closure are among the most common birth defectsNumbers indicate regions of neural tube closure.Figure 9.5It is estimated that 50% of human neural tube defects would be prevented if all women of childbearing age take supplemental folic acid(vitamin B12)400 g/dayExpression of a folate receptor in mice neural tubes prior to fusionFigure
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