Early Development of VertebratesBiology 4361July 13, 2009Vertebrate Development - OverviewModels: chick & mouseMouseRotational cleavage: formation of the blastocystImplantation &formation of embryonic and extraembryonic tissuesGastrulation and derivation of germ layersPatterning the axesDiscoidal cleavage: formation of a three-layered blastodiscGastrulation and primitive streak regressionChickAxis formationPrimitive streak formationCleavage PatternsMeroblastic (incomplete cleavage)Holoblastic (complete cleavage)cephalopodmolluscsfish, reptiles,birdsmost insectsechinoderms, amphioxisannelids, most molluscs, flatwormstunicatesmammals, nematodeamphibiansSpeciesSpiralBilateralRadialRotationalDisplaced radialBilateralDiscoidalSuperficialCleavageYolkIsolecithalCentrolethicalMesolecithalTelolecithalDisocoidal Meroblastic CleavageFertilization in oviductchickscope.beckman.uiuc.edu/…/day05/ovary.htmlBlastodisc cytoplasm ~ 2-3 mmEquatorial and vertical cleavages produce 5-6 cell layers- vertical layers eventually reducedto single layer - epiblastThree-Layered Blastodisc- 5 – 6 cell layer blastodisc sheds cells in the center- single layer remains = area pellucida (epiblast)- edges – area opaca- marginal zone – important for determining cell fate- some area pellucida cells detach; form poly-invagination islandsThree-Layered Blastodisc - 2- sheets of tissue from posterior marginal zone (Koller’s sickle) migrate anteriorly- poly-invagination islands combine; form the primary hypoblast- forms the secondary hypoblast- space between epiblast and hypoblast forms blastocoelThree-Layered Blastodisc - 3- epiblast cells at the midline thicken, accumulate, move anteriorly- primitive streak moves anteriorly by intercalation and convergent extension- depression forms within the primitive streak: the primitive groove- cells gastrulate through primitive groove- form primitive streakPrimitive Streak FormationPrimitiveStreak - 2Hensen’s node – anteriorthickening; leading edgeof the primitive streak- primitive pit- primitive groove- equivalent of the dorsal lip of the blastopore (amphibians) - cells pass through groove individually- epithelial to mesenchyme Primitive streak defines axes:- posterior to anterior extension- cells enter dorsal to ventral- separates left from rightPrimitiveStreak - 2somitesPrimitive Streak - 3Hensen’s node extends 60 – 75% length of area pellucida;then regresses- creates posteriordorsal axisPosterior forms anal regionNOTE – avian embryos exhibit distinct anterior-to-posteriorgradient of developmental maturityVertebrate Development - OverviewModels: chick & mouseMouseRotational cleavage: formation of the blastocystImplantation &formation of embryonic and extraembryonic tissuesGastrulation and derivation of germ layersPatterning the axesDiscoidal cleavage: formation of a three-layered blastodiscGastrulation and primitive streak regressionChickAxis formationPrimitive streak formationEndodermal and Mesodermal Cell MigrationPrechordal plate & notochordNotochord & somitesIntermediate mesodermLateral plate mesodermEndodermal and Mesodermal Cell MigrationFGF8 expressed in primitive streak- repels migrating cells awayFGF4 produced by chordamesoderm- attracts migrating mesoderm cellsDeep lateral migrating cells displace hypoblast- form endodermShallower cells form mesodermal mesenchymeChick Gastrulation24 h – full extensionof primitive streak25 h – two somite stageChick Gastrulation27 h – four somite stage28 h – 7 somite stageprimitive streak regressionPrimitive Streak RegressionChick - Primitive Streak InitiationThe role of Gravity: - ovum rotates ~ 20 h in reproductive tract- lighter yolk components shift to lie beneath one side of blastoderm- contents? – probably maternal determinants- portion becomes the Posterior Marginal Zone (PMZ)- primitive streak forms herePMZ acts as an equivalent to the amphibian Nieuwkoop center(expresses Vg1 and Nodal)PMZ initiates primitive streak; - also prevents other regions from initiating their own primitive streaksHensen’s node forms just anterior to the PMZNieuwkoopcenterHensen’s node: the equivalent to amphibian dorsal blastopore lip- gastrulation initiation site- cells become chordamesoderm- cells can organize a second embryonic axis when transplantedChick - Left-Right Axis FormationLeft side requires active:1) Nodal (paracrine factor)2) Pitx2 (transcription factor)Snail blocks Pitx2expressionNodal blocks snail(cSnR) expressionNodal activates Pitx2expressionActivin inhibits ShhexpressionActivin stimulates Fgf8expressionWhat limits Activinexpression to the right side??Vertebrate Development - OverviewModels: chick & mouseMouseRotational cleavage: formation of the blastocystImplantation &formation of embryonic and extraembryonic tissuesGastrulation and derivation of germ layersPatterning the axesDiscoidal cleavage: formation of a three-layered blastodiscGastrulation and primitive streak regressionChickAxis formationPrimitive streak formationEarly Development in MammalsMammalian development is difficult to study.- zygotes are very small; ~ 100 μm diameter- produced in relatively low numbers- development inside another body makes observationvery difficultAlthough mammalian eggs are isolecithal and containvery little yolk, their embryos act as if they are sitting on top a large imaginary ball of yolk- i.e. gastrulate like fish, reptiles, and birdsMammalian Fertilization- mammalian cleavage is among the slowest- 1stcleavage ~ 1 day after fertilization- subsequent cleavages 12 – 24 h apartCleavage PatternsMeroblastic (incomplete cleavage)Holoblastic (complete cleavage)cephalopodmolluscsfish, reptiles,birdsmost insectsechinoderms, amphioxisannelids, most molluscs, flatwormstunicatesmammals, nematodeamphibiansSpeciesSpiralBilateralRadialRotationalDisplaced radialBilateralDiscoidalSuperficialCleavageYolkIsolecithalTelolecithalCentrolethicalMesolecithalRotational CleavageMouse CleavageRotational cleavage- asynchronous- no MBT2-cell 4-cellCompaction – 8-cell stage- tight junctions between outside cellsseal off inside of sphere8-cell compacted 8-cellMouse CleavageBlastocyst - unique to mammalsMorula – 16-cell stage- small group of internal cells; inner cell mass (ICM)- ICM will form the embryo proper- larger group of external cells; trophoblast (trophectoderm)- trophoblast will form extraembryonic structures- secretes hormones causing uterus to retain fetus- cavitation – trophoblast secretes fluid into morula (via
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