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
Groups of cells
Local inward buckling
of an epithelium
Individual cells
Inward movement of a cell
layer around a point or edge
Movement of individual cells
or small groups from an
epithelium into a cavity
Migration
Movement of individual cells
over other cells or matrix
Splitting layers of cells
(sometimes used to describe
coordinated ingression)
Spread of an outside cell layer
(as a unit) to envelop a
yolk mass or deeper layer
Fig. 5.4
More complex changes in cell shape can drive
elongation or shortening of a flat sheet of cells
“Convergent Extension”
15 cells
4 cells
Cell intercalation
Narrowed and lengthened sheet of cells
30 cells
2 cells
Sea urchin gastrulation
Our “simple” model
Fig. 5.14
blastocoel
Sea urchin gastrulation
Our “simple” model
Step 1: Primary mesenchyme cells ingress
Inside
Outside (apical)
Mesenchyme cellscells that are unconnected to
one another and operate as
independent units
See also Figure 5.16
Primary mesenchyme
ingression is driven
by changes in cell
adhesion
Figure 5.16
Changes in cell adhesion
drive the first step
of gastrulation
basal lamina and
extracellular matrix
Invaginating primary
mesenchyme cells
beginning to
migrate on the
extracellular matrix
lining the blastocoel
Primary mesenchyme cells
migrate along the extracellular matrix
using filopodia to detect chemical cues
Primary mesenchyme cells eventually
fuse and form the spicules (skeletal rods)
Figure 5.15
Figure 5.17
Step 2:
Apical constriction and changes in
the extracellular matrix create a
dome-shaped invagination =
archenteron
(primitive gut)
blastopore = opening
Figure 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
Ectoderm
These secondary mesenchyme cells will become muscle (mesoderm)
Figure 5.21
Endoderm (gut)
Pluteus larva
Pluteus larva
Figure 5.14
Gastrulation: frogs
Early cleavage in Xenopus
animal
vegetal
Sea urchin
Fig. 7.2
Here is where
gastrulation starts
Early cleavage in Xenopus
animal
vegetal
Two functions of the blastocoel:
1. Prevents cells from interacting too soon
2. allows space for cell migrations during gastrulation
A Fate Map of the Xenopus Blastula
Most Exterior Cells
form ectoderm or endoderm
Sea urchin
Mesoderm
Most Interior Cells
form mesoderm
Fig. 7.5
Frog gastrulation: added complexity
but similar mechanisms
1. Blastopore Formation
sperm
entry
(That looks
familiar!)
Fig. 7.6
Mechanism #1
Apical constriction of bottle cells drives
blastopore invagination
Archenteron
Figure 7.7
Frog gastrulation: added complexity
but similar mechanisms
2. Involution of Marginal zone cells
Mechanism #2
INVOLUTION
around dorsal lip
Marginal Zone Cells
Fig. 7.6
inside MZ
outside MZ
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
MIGRATION
Movement of individual cells over
other cells or matrix
Splitting layers of cells
(sometimes used to describe
coordinated ingression)
Spread of an outside cell layer
(as a unit) to envelop a
yolk mass or deeper layer
Figure 5.4
2. Involution of marginal zone cells
 movement of inside MZ cells dependent on ectoderm cells
of blastocoel roof secreting fibronectin
Figure 10.7
inside MZ
outside MZ
Fibronectin is essential for mesodermal
cell involution during gastrulation
Yolk Plug
Control
embryo
Embryo injected
with fibronectin competitor
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 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
MIGRATION
Movement of individual cells over
other cells or matrix
Splitting layers of cells
(sometimes used to describe
coordinated ingression)
Spread of an outside cell layer
(as a unit) to envelop a
yolk mass or deeper layer
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 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
MIGRATION
Movement of individual cells over
other cells or matrix
Splitting layers of cells
(sometimes used to describe
coordinated ingression)
Spread of an outside cell layer
(as a unit) to envelop a
yolk mass or deeper layer
Figure 5.4
Gastrulation: Mission Accomplished
Ectoderm
Mesoderm
Endoderm
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
Cleavage and Gastrulation
Hatch from Zona Pellucida
Fig. 8.20
Gastrulation
Fig. 8.15
In mammals, gastrulation initiates AFTER
formation of the placental connection to mom
Fig. 8.23