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
21_24_Adherens_junct.jpg
Alberts Fig

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

6. Types of Movement in Gastrulation
Groups of cells
Individual cells
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
Fig. 5.4

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

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

9. Sea urchin gastrulation
Our “simple” model

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

11. by changes in cell adhesion
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 5.15
Figure 5.17

16. Apical constriction and changes in the extracellular matrix create a
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. Cell intercalation (convergent extension) converts
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
Ectoderm
These secondary mesenchyme cells will become muscle (mesoderm)
Figure 5.21
Endoderm (gut)

21. Pluteus larva
Pluteus larva
Figure 5.14

22. Gastrulation: frogs

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

24. 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

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

26. Frog gastrulation: added complexity but similar mechanisms
1. Blastopore Formation
sperm
entry
Figures\Chapter10\DevBio7e10071.jpg
(That looks
familiar!)
Fig. 7.6

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

28. Frog gastrulation: added complexity but similar mechanisms
2. Involution of Marginal zone cells
Mechanism #2
INVOLUTION
around dorsal lip
Figures\Chapter10\DevBio7e10071.jpg
Marginal Zone Cells
inside MZ
Fig. 7.6
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
MIGRATION
Movement of individual cells over other cells or matrix
Figure 5.4
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

30. 2. Involution of marginal zone cells
movement of inside MZ cells dependent on ectoderm cells of blastocoel roof secreting fibronectin
Figures\Chapter10\DevBio7e10072.jpg
inside MZ
Figure 10.7
outside MZ

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

32. convergence and extension in three dimensions
3. Formation of the Archenteron = Convergent Extension of the Dorsal Mesoderm
convergence and extension in three dimensions
Figures\Chapter10\DevBio7e10072.jpg
Figure 7.6

33. 4. Epiboly of the Ectoderm
Figures\Chapter10\DevBio7e10072.jpg
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
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

35. 4. Epiboly of the Ectoderm
Figures\Chapter10\DevBio7e10092.jpg
Figure 7.9

36. 5. mesenchyme migration Just like sea urchin Figure 7.6
Figures\Chapter10\DevBio7e10073.jpg
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
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

38. Gastrulation: Mission Accomplished
Ectoderm
Mesoderm
Endoderm

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

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