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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 organs
Endoderm: Inside skin- -gut lining, inside layers of skin
Gastrulation involves changes in cell shape
and changes in cell adhesion
Cytoskeletal events drive cell shape changes
Contraction of the adhesion belt drives apical constriction(see Alberts Fig 20-26)
21_24_Adherens_junct.jpg
Alberts Fig. 20-25
21_21_cell_cell_junction.jpg
E-cadherin
Alberts Fig. 20-22
Types of Movement in Gastrulation
Local 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 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 cells Individual cells
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”
Sea urchin gastrulationOur “simple” model
blastocoel
Fig. 5.14
Sea urchin gastrulationOur “simple” model
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
Primary mesenchyme ingression is drivenby changes in cell adhesionFigure 5.16
Changes in cell adhesion drive the first step of gastrulation
basal lamina and extracellular matrix
Invaginating primarymesenchyme cellsbeginning tomigrate on theextracellular matrixlining the blastocoel
Primary mesenchyme cells migrate along the extracellular matrixusing filopodia to detect chemical cues
Primary mesenchyme cells eventually fuse and form the spicules (skeletal rods)
Figure 5.17
Figure 5.15
Step 2:Apical constriction and changes in the extracellular matrix create a dome-shaped invagination =archenteron (primitive gut)
blastopore = openingFigure 5.19
Apical constriction drives invagination
Invagination of the Vegetal Plate involves changes in the extracellular matrix
(CSPG)
Step 3:Cell intercalation (convergent extension) converts
the dome (archenteron) into an elongated tube
Figure 5.20
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
Figure 5.14Pluteus larva
Pluteus larva
Gastrulation: frogs
animal
vegetal
Early cleavage in Xenopus
Here is where gastrulation starts
Sea urchin Fig. 7.2
Two functions of the blastocoel: 1. Prevents cells from interacting too soon2. allows space for cell migrations during gastrulation
animal
vegetal
Early cleavage in Xenopus
A Fate Map of the Xenopus Blastula
Most Exterior Cellsform ectoderm or endoderm
Most Interior Cellsform mesoderm
Fig. 7.5Sea urchin
Mesoderm
1. Blastopore Formation
(That looks familiar!)
Frog gastrulation: added complexity but similar mechanisms
Fig. 7.6
spermentry
Apical constriction of bottle cells drivesblastopore invagination
Mechanism #1
Figure 7.7Archenteron
INVOLUTIONaround 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
Types of Movement in Gastrulation
Local 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 layer
Splitting layers of cells (sometimes used to describe
coordinated ingression)
MIGRATION
Movement of individual cells over other cells or matrix
Figure 5.4
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
Fibronectin is essential for mesodermal cell involution
during gastrulation
Controlembryo
Embryo injectedwith fibronectin competitorYolk Plug
Figure 7.12
3. Formation of the Archenteron = Convergent Extension of the Dorsal
Mesoderm
convergence and extension
in three dimensions
Figure 7.6
4. Epiboly of the Ectoderm
Figure 7.6
Types of Movement in Gastrulation
Local 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 layer
Splitting layers of cells (sometimes used to describe
coordinated ingression)
MIGRATION
Movement of individual cells over other cells or matrix
Figure 5.4
4. Epiboly of the Ectoderm
Figure 7.9
5. mesenchyme migration
Just like sea urchin
Figure 7.6
Types of Movement in Gastrulation
Local 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 layer
Splitting layers of cells (sometimes used to describe
coordinated ingression)
MIGRATION
Movement of individual cells over other cells or matrix
Figure 5.4
Gastrulation: Mission Accomplished
Endoderm
Mesoderm
Ectoderm
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
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
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
Fig. 8.15
Fig. 8.20
Cleavage and Gastrulation
Gastrulation
Hatch from Zona Pellucida
In mammals, gastrulation initiates AFTER formation of the placental connection to mom
Fig. 8.23