Lecture 9: Gastrulation Flashcards by Rachel Chin

What is gastrulation?

A process in which the coordinated movement of blastomeres of an embryo (relative to each other) results in the formation of the three germ layers:

Ectoderm
Mesoderm
Endoderm

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First stage in forming the body plan

establishes the basic organization

gastrulation

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The process places the blastomeres in their proper positions to form tissues and organs of the adult animal

gastrulation

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Gastrulation

Directional movement of the cells results in:

Changes in cellular neighbourhood (e.g. new cells, new neighbours)

Internalization of endoderm and mesoderm

Externalization of the ectoderm

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What are the five basic types of cell movement?

INVAGINATION
Inward bending of a sheet of epithelial cells

INVOLUTION
Inward movement of an expanding outer layer –> spreads over the internal surface

INGRESSION
Individual cells become mesenchymal and migrate from surface to the interior – independent migration

DELAMINATION
Splitting of one cellular sheet into two –> resembles ingression but results in formation of a new sheet of epithelial cells

EPIBOLY
Spreading of the sheets of cells by a combination of cell division, shape changes, intercalation

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What is the process in which two layers merge to form a single layer?

Intercalation

Increases surface area

Surface extends in all directions

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What is the process in which cells move to form a single line?

Convergent extension

Increases length

Surface generally extends in one direction

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These cell movements are dependent on changes in

cell adhesion (e.g. paraxial and axial protocadherins) and cytoskeletal remodelling

involves non-canonical Wnt pathway

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Gastrulation in Sea Urchins

What is the primitive gut called?

Archenteron

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What is EMT?

Epithelial-to-mesenchymal transition (EMT)

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Where can fertilization occur?

Fertilization can occur anywhere in the animal hemisphere (pole) of amphibians

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The point of sperm entry develops into the _____ side of the embryo.

The point of sperm entry develops into the VENTRAL side of the embryo

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Where does gastrulation start ?

site 180o from the point of sperm entry

this is where gastrulation starts and will eventually develop into dorsal side

Cortical cytoplasm (thin layer of gel-like cytoplasm under the cell membrane) rotates ~30 degrees w.r.t. inner cytoplasm

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Gastrulation in Amphibians

Generally:

Animal hemisphere/pole = ?
Vegetal hemisphere = ?
Cells opposite to sperm entry =?

Animal hemisphere = ectoderm

Vegetal hemisphere = endoderm

Cells opposite to point of sperm entry = neural ectoderm, notochord etc.

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Gastrulation in Amphibians

Start of gastrulation is marked by:

Start of gastrulation is marked by

the elongation and involution of cells on future dorsal side of the embryo (region opposite to the site of sperm entry)

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Gastrulation in Amphibians

Where does the start of gastrulation begin?

Hint: which zone?

begins in MARGINAL ZONE (‘belt’ region around the equator where AP and VP meet)

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Gastrulation in Amphibians

Cells invaginate to form the slit-like blastopore –> change in cell shape to form elongated _________ —> initiate the formation of archenteron

Bottle cells

Bottle cells involute to form the dorsal lip of the blastopore, and the mesodermal precursor cells involute under the roof of the blastocoel

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Gastrulation in Amphibians

What is the mid-gastrulation stage?

The involution of endoderm produces the archenteron (later forms lumen of the digestive tube) and displaces the blastocoel, and cells migrate from the lateral and ventral lips of the blastopore into the embryo (Mid-gastrulation stage)

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Gastrulation in Amphibians

Experiment:

Remove dorsal marginal zone cells (give rise to dorsal blastopore lip) from salamander embryo
Graft lip cells to inner prospective endoderm
Formed bottle cells and sank below the surface

–> Showed that these lip cells have an intrinsic property that initiates gastrulation - What could it be?

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Gastrulation in Amphibians

The cells of the animal hemisphere migrate down toward the vegetal region (what is this process called?) blastopore moves towards the region near the vegetal pole

The layers of mesoderm and endoderm start to involute around the blastopore into the blastula interior (blastocoel is eliminated)

vegetal rotation

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Gastrulation in Amphibians

The first mesodermal cells to involute give rise to the _______________

How do these cells continue to migrate?

The first mesodermal cells to involute give rise to the mesoderm of the head

(prechordal mesoderm)

They continue to migrate as single cells

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Gastrulation in Amphibians

The prechordal mesoderm is followed by what?

These are followed by the precursors of the notochord (axial mesoderm, chordamesoderm)

which remain in an epithelial sheet.

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Gastrulation in Amphibians

The mesoderm migrates over the blastocoel roof until

the first cells to involute reach the opposite side of the embryo

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Gastrulation in Amphibians

As the involution is happening, ______ migrates to cover the space left by involuted mesoderm.

the ectoderm migrates (epiboly)

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Gastrulation in Amphibians Both the first mesoderm and endoderm cells that involuted reach (same/opposite) sides of the embryo.

opposite

Gastrulation in Amphibians The ______ of the archenteron increases substantially.

size archenteron = primitive gut

Gastrulation in Amphibians By the end of gastrulation:

Ectoderm surrounds the embryo Endoderm is internalized Mesodermal cells get positioned between the ecto and endoderm

Gastrulation in Amphibians What is the IMZ? What does it include

involuting marginal zone In amphibians, it is composed of chordamesoderm Head mesoderm

Gastrulation in Amphibians What are the two transcription factors in the IMZ? Chordamesoderm Head mesoderm

Chordamesoderm controlled by Xbra - Xbra (Brachyury) is the TF critical for notochord formation Head mesoderm controlled by Goosecoid -Goosecoid is a TF that controls head formation genes (represses genes that normally repress head development like Wnt8)

Gastrulation in Amphibians What does Goosecoid control?

Goosecoid controls the head mesoderm in the IMZ (involuting marginal zone) It is a TF that controls head formation genes It represses genes that repress head development like Wnt8

Gastrulation in Amphibians What does Xbra contol?

It controls the chordamesoderm in the IMZ It is critical for notochord formation

Gastrulation in Amphibians The region of involution spreads to form

the lateral lips

The blastopore eventually encircles a small _________ as the cell involute along each side. _______ is eventually internalized.

yolk plug

Gastrulation in Amphibians As the mesoderm and endoderm involute, they undergo what process?

convergent extension | cells move in single direction, increase length

Gastrulation in Amphibians How can convergent extension of the mesoderm and endoderm be visualized?

using in-situ hybridization using Xbra Xbra = TF that controls genes important in notochord formation

Gastrulation in Amphibians The movement of the cells is guided by different ______ _____.

guidance cues

Gastrulation in Amphibians Guidance cues The presumptive ectoderm of the blastocoel roof secretes _________

extracellular matrix fibronectin assembled into fibrils

Gastrulation in Amphibians Guidance cues Involuting mesodermal precursors migrate towards the animal pole by traveling on what?

fibronectin network | that results from the secretion of extracellular matrix from the presumptive ectoderm of the blastocoel

Gastrulation in Amphibians Guidance cues How would you stop gastrulation?

Inhibiting fibronectin interaction

Mammalian Gastrulation Days 5-9, what happens?

Implantation of the blastocyst

Mammalian Gastrulation Days 5-9 Implantation of the blastocyst Trophoblast develops through several stages to become the

chorion

Mammalian Gastrulation Day 9 The trophoblast develops into

cytotrophoblast - inner layer of the trophoblast between the synsytiotrophoblast and the external wall of the blastocyst

Mammalian Gastrulation Day 9 What is the cytotrophoblast?

The trophoblast develops into the cytotrophoblast cytotrophoblast - inner layer of the trophoblast between the synsytiotrophoblast and the external wall of the blastocyst

Mammalian Gastrulation Day 9 Trophoblast develops into cytotrophoblast And then?

Trophoblastic stem cells differentiate and proliferate to form diff cell types for multiple function

Mammalian Gastrulation Day 9 and 10-12 What is the syncytiotrophoblast?

the epithethial covering of the embryonic placental villi invades the uterine wall to establish nutrient circulation between mother and embryo

Mammalian Gastrulation Day 9 and 10-12 What does the synsytiotrophoblast establish?

By invading the uterine wall, it establishes a nutrient circulation between the embryo and the mother

Mammalian Gastrulation Day 12-15 What happens here?

Gastrulation and formation of the PRIMITIVE STREAK

Mammalian Gastrulation Day 12-15 What is the primitive streak?

first morphological sign of gastrulation in amniotes establishes symmetry

Mammalian Gastrulation What is the first morphological sign of gastrulation in amniotes?

primitive streak

Mammalian Gastrulation What is the primitive streak in mammals homologous to in amphibians?

blastopore the site of cell migration within the embryo

Mammalian Gastrulation The first segregation of the ICM forms two layers:

Primitive endoderm (hypoblast) Epiblast

Mammalian Gastrulation The first segregation of the ICM forms two layers: the primitive endoderm and the epiblast Define them.

Primitive endoderm (aka hypoblast) is the lower layer in contact with the blastocoel Epiblast is the outermost layer of the embryo that eventually becomes the ectoderm and mesoderm

Mammalian Gastrulation Both the primitive endoderm and epiblast form what?

bilaminar germ disc

Mammalian Gastrulation Primitive endoderm cells expand to lien the blastocoel cavity

Give rise to the yolk sac

Mammalian Gastrulation Fate mapping experiments have shown the movement of epiblast cells which eventually give rise to the

mesoderm and endoderm cells

Mammalian Gastrulation What is the functional equivalent in mammals of the the dorsal lip in amphibians?

node

Mammalian Gastrulation Mammalain mesoderm and endoderm cells:

Originate in the epiblast Undergo epithelial-mesenchymal transition Lose E-cadherin Migrate through primitive streak as individual mesenchyme cells

Mammalian Gastrulation Cell migration and specification are coordinated partly by

fibroblast growth factors (FGF) cells and both produce and respond to FGF

Twins in humans Two kinds:

monozygotic twins dizygotic twins

What are monozygotic twins?

identical twins arising from a single egg (1:400 births)

What are dizygotic twins?

Fraternal twins that arise from two fertilization events - maybe produced by the separation of early blastomeres or of ICM into two regions in the same blastocyst

In twins, what is important?

Timing of the split!

If splitting occurs before the formation of the trophoblast, what happens?

Each twin has its own chorion and amnion.

If splitting occurs after trophoblast formation but before amnion formation, what happens?

Twins having individual amniotic sacs but share one chorion

If splitting after amnion formation, what happens?

Twins in one amniotic sac and a single chorion