Slide 1Slide 2Slide 321_24_Adherens_junct.jpg21_21_cell_cell_junction.jpgSlide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 251. Blastopore FormationSlide 27Slide 28Slide 29Slide 30Slide 313. Formation of the Archenteron = Convergent Extension of the Dorsal Mesoderm4. Epiboly of the EctodermSlide 34Slide 355. mesenchyme migrationSlide 37Slide 38Slide 39Slide 40Slide 41Cleavage and GastrulationSlide 43Gastrulation The goal is to form three GERM LAYERS (starting from a hollow ball of cells)Ectoderm: Outside skin, nervesMesoderm: Blood, Muscle, some organsEndoderm: Inside skin- -gut lining, inside layers of skinGastrulation involves changes in cell shape and changes in cell adhesionCytoskeletal events drive cell shape changesContraction of the adhesion belt drives apical constriction(see Alberts Fig 20-26)21_24_Adherens_junct.jpgAlberts Fig. 20-2521_21_cell_cell_junction.jpgE-cadherinAlberts Fig. 20-22Types of Movement in GastrulationLocal inward bucklingof an epithelium Inward movement of a cell layer around a point or edge Movement of individual cells or small groups from an epithelium into a cavity Spread of an outside cell layer (as a unit) to envelop a yolk mass or deeper layerSplitting layers of cells (sometimes used to describe coordinated ingression) Migration Movement of individual cells over other cells or matrixFig. 5.4Groups of cells Individual cellsMore complex changes in cell shape can drive elongation or shortening of a flat sheet of cells15 cells4 cellsCell intercalationNarrowed and lengthened sheet of cells 30 cells2 cells“Convergent Extension”Sea urchin gastrulationOur “simple” modelblastocoelFig. 5.14Sea urchin gastrulationOur “simple” modelStep 1: Primary mesenchyme cells ingressMesenchyme cells-cells that are unconnected to one another and operate as independent unitsSee also Figure 5.16Outside (apical)InsidePrimary mesenchyme ingression is drivenby changes in cell adhesionFigure 5.16Changes in cell adhesion drive the first step of gastrulationbasal lamina and extracellular matrixInvaginating primarymesenchyme cellsbeginning tomigrate on theextracellular matrixlining the blastocoelPrimary mesenchyme cells migrate along the extracellular matrixusing filopodia to detect chemical cuesPrimary mesenchyme cells eventually fuse and form the spicules (skeletal rods)Figure 5.17Figure 5.15Step 2:Apical constriction and changes in the extracellular matrix create a dome-shaped invagination =archenteron (primitive gut)blastopore = openingFigure 5.19Apical constriction drives invaginationInvagination of the Vegetal Plate involves changes in the extracellular matrix(CSPG)Step 3:Cell intercalation (convergent extension) convertsthe dome (archenteron) into an elongated tubeFigure 5.20Step 4: Secondary mesenchyme cells at the leading edge of the gut tube use filopodia to look for cues at the animal pole and pull themselves to that siteThese secondary mesenchyme cells will become muscle (mesoderm)EctodermEndoderm (gut)Figure 5.21Figure 5.14Pluteus larvaPluteus larvaGastrulation: frogsanimalvegetalEarly cleavage in XenopusHere is where gastrulation startsSea urchinFig. 7.2Two functions of the blastocoel: 1. Prevents cells from interacting too soon2. allows space for cell migrations during gastrulationanimalvegetalEarly cleavage in XenopusA Fate Map of the Xenopus BlastulaMost Exterior Cellsform ectoderm or endoderm Most Interior Cellsform mesodermFig. 7.5Sea urchinMesoderm1. Blastopore Formation(That looks familiar!)Frog gastrulation: added complexity but similar mechanismsFig. 7.6spermentryApical constriction of bottle cells drivesblastopore invaginationMechanism #1Figure 7.7ArchenteronINVOLUTIONaround dorsal lipMechanism #2Marginal Zone CellsFrog gastrulation: added complexity but similar mechanismsFig. 7.62. Involution of Marginal zone cellsinside MZoutside MZTypes of Movement in GastrulationLocal inward bucklingof an epithelium Inward movement of a cell layer around a point or edge Movement of individual cells or small groups from an epithelium into a cavity Spread of an outside cell layer (as a unit) to envelop a yolk mass or deeper layerSplitting layers of cells (sometimes used to describe coordinated ingression) MIGRATION Movement of individual cells over other cells or matrixFigure 5.4Figure 10.72. Involution of marginal zone cellsinside MZoutside MZ movement of inside MZ cells dependent on ectoderm cells of blastocoel roof secreting fibronectinFibronectin is essential for mesodermal cell involution during gastrulationControlembryoEmbryo injectedwith fibronectin competitorYolk PlugFigure 7.123. Formation of the Archenteron = Convergent Extension of the Dorsal Mesodermconvergence and extension in three dimensionsFigure 7.64. Epiboly of the EctodermFigure 7.6Types of Movement in GastrulationLocal inward bucklingof an epithelium Inward movement of a cell layer around a point or edge Movement of individual cells or small groups from an epithelium into a cavity Spread of an outside cell layer (as a unit) to envelop a yolk mass or deeper layerSplitting layers of cells (sometimes used to describe coordinated ingression) MIGRATION Movement of individual cells over other cells or matrixFigure 5.44. Epiboly of the EctodermFigure 7.95. mesenchyme migration Just like sea urchinFigure 7.6Types of Movement in GastrulationLocal inward bucklingof an epithelium Inward movement of a cell layer around a point or edge Movement of individual cells or small groups from an epithelium into a cavity Spread of an outside cell layer (as a unit) to envelop a yolk mass or deeper layerSplitting layers of cells (sometimes used to describe coordinated ingression) MIGRATION Movement of individual cells over other cells or matrixFigure 5.4Gastrulation: Mission AccomplishedEndodermMesodermEctodermEctoderm (outer layer) will produce skin & the central nervous system (brain, spinal cord) through later invagination of the neural tube. In vertebrates, migrating neural crest cells form the peripheral nervous system & many other structures, including some bone, cartilage, and connective tissue in the head.EctodermMESODERM (middle layer) will produce muscles, connective tissue, blood and blood vessels. In vertebrates also the notochord (progenitor of vertebrae), bones & cartilage, circulatory and urogenital systems (kidneys, gonads).MesodermENDODERM (inner layer) will produce the
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