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