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Gastrulation  The goal is to form three GERM LAYERS (starting from a hollow ball of cells) Ectoderm: Outside skin, nerves Mesoderm: Blood, Muscle, some.

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Presentation on theme: "Gastrulation  The goal is to form three GERM LAYERS (starting from a hollow ball of cells) Ectoderm: Outside skin, nerves Mesoderm: Blood, Muscle, some."— Presentation transcript:

1 Gastrulation  The goal is to form three GERM LAYERS (starting from a hollow ball of cells) Ectoderm: Outside skin, nerves Mesoderm: Blood, Muscle, some organs Endoderm: Inside skin- -gut lining, inside layers of skin

2 Gastrulation involves changes in cell shape and changes in cell adhesion

3 Cytoskeletal events drive cell shape changes Contraction of the adhesion belt drives apical constriction (see Alberts Fig 20-26)

4 21_24_Adherens_junct.jpg Alberts Fig. 20-25

5 21_21_cell_cell_junction.jpg E-cadherin Alberts Fig. 20-22

6 Types of Movement in Gastrulation Local inward buckling of 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 layer Splitting layers of cells (sometimes used to describe coordinated ingression) Migration Movement of individual cells over other cells or matrix Fig. 5.4 Groups of cellsIndividual cells

7 More complex changes in cell shape can drive elongation or shortening of a flat sheet of cells 15 cells 4 cells Cell intercalation Narrowed and lengthened sheet of cells 30 cells 2 cells “Convergent Extension”

8 Sea urchin gastrulation Our “simple” model blastocoel Fig. 5.14

9 Sea urchin gastrulation Our “simple” model

10 Step 1: Primary mesenchyme cells ingress Mesenchyme cells- cells that are unconnected to one another and operate as independent units See also Figure 5.16 Outside (apical) Inside

11 Primary mesenchyme ingression is driven by changes in cell adhesion Figure 5.16

12 Changes in cell adhesion drive the first step of gastrulation basal lamina and extracellular matrix

13 Invaginating primary mesenchyme cells beginning to migrate on the extracellular matrix lining the blastocoel

14 Primary mesenchyme cells migrate along the extracellular matrix using filopodia to detect chemical cues

15 Primary mesenchyme cells eventually fuse and form the spicules (skeletal rods) Figure 5.17 Figure 5.15

16 Step 2: Apical constriction and changes in the extracellular matrix create a dome-shaped invagination = archenteron (primitive gut) blastopore = opening Figure 5.19

17 Apical constriction drives invagination

18 Invagination of the Vegetal Plate involves changes in the extracellular matrix (CSPG)

19 Step 3: Cell intercalation (convergent extension) converts the dome (archenteron) into an elongated tube Figure 5.20

20 Step 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 site These secondary mesenchyme cells will become muscle (mesoderm) Ectoderm Endoderm (gut) Figure 5.21

21 Figure 5.14 Pluteus larva

22 Gastrulation: frogs

23 animal vegetal Early cleavage in Xenopus Here is where gastrulation starts Sea urchin Fig. 7.2

24 Two functions of the blastocoel: 1. Prevents cells from interacting too soon 2. allows space for cell migrations during gastrulation animal vegetal Early cleavage in Xenopus

25 A Fate Map of the Xenopus Blastula Most Exterior Cells form ectoderm or endoderm Most Interior Cells form mesoderm Fig. 7.5Sea urchin Mesoderm

26 1. Blastopore Formation (That looks familiar!) Frog gastrulation: added complexity but similar mechanisms Fig. 7.6 sperm entry

27 Apical constriction of bottle cells drives blastopore invagination Mechanism #1 Figure 7.7 Archenteron

28 INVOLUTION around dorsal lip Mechanism #2 Marginal Zone Cells Frog gastrulation: added complexity but similar mechanisms Fig. 7.6 2. Involution of Marginal zone cells inside MZ outside MZ

29 Types of Movement in Gastrulation Local inward buckling of 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 layer Splitting layers of cells (sometimes used to describe coordinated ingression) MIGRATION Movement of individual cells over other cells or matrix Figure 5.4

30 Figure 10.7 2. Involution of marginal zone cells inside MZ outside MZ  movement of inside MZ cells dependent on ectoderm cells of blastocoel roof secreting fibronectin

31 Fibronectin is essential for mesodermal cell involution during gastrulation Control embryo Embryo injected with fibronectin competitor Yolk Plug Figure 7.12

32 3. Formation of the Archenteron = Convergent Extension of the Dorsal Mesoderm convergence and extension in three dimensions Figure 7.6

33 4. Epiboly of the Ectoderm Figure 7.6

34 Types of Movement in Gastrulation Local inward buckling of 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 layer Splitting layers of cells (sometimes used to describe coordinated ingression) MIGRATION Movement of individual cells over other cells or matrix Figure 5.4

35 4. Epiboly of the Ectoderm Figure 7.9

36 5. mesenchyme migration Just like sea urchin Figure 7.6

37 Types of Movement in Gastrulation Local inward buckling of 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 layer Splitting layers of cells (sometimes used to describe coordinated ingression) MIGRATION Movement of individual cells over other cells or matrix Figure 5.4

38 Gastrulation: Mission Accomplished Endoderm Mesoderm Ectoderm

39 Ectoderm (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. Ectoderm

40 MESODERM (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). Mesoderm

41 ENDODERM (inner layer) will produce the gut (entire digestive system) and other internal organs that arise as outpocketings of gut in vertebrates such as liver, lungs, pancreas, and salivary glands. Endoderm

42 Fig. 8.15 Fig. 8.20 Cleavage and Gastrulation Gastrulation Hatch from Zona Pellucida

43 In mammals, gastrulation initiates AFTER formation of the placental connection to mom Fig. 8.23


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