Lecture 9: Gastrulation Flashcards
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
First stage in forming the body plan
establishes the basic organization
gastrulation
The process places the blastomeres in their proper positions to form tissues and organs of the adult animal
gastrulation
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
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
What is the process in which two layers merge to form a single layer?
Intercalation
Increases surface area
Surface extends in all directions
What is the process in which cells move to form a single line?
Convergent extension
Increases length
Surface generally extends in one direction
These cell movements are dependent on changes in
cell adhesion (e.g. paraxial and axial protocadherins) and cytoskeletal remodelling
involves non-canonical Wnt pathway
Gastrulation in Sea Urchins
What is the primitive gut called?
Archenteron
What is EMT?
Epithelial-to-mesenchymal transition (EMT)
Where can fertilization occur?
Fertilization can occur anywhere in the animal hemisphere (pole) of amphibians
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
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
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.
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)
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)
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
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)
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?
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
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
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.
Gastrulation in Amphibians
The mesoderm migrates over the blastocoel roof until
The mesoderm migrates over the blastocoel roof until
the first cells to involute reach the opposite side of the embryo
Gastrulation in Amphibians
As the involution is happening, ______ migrates to cover the space left by involuted mesoderm.
the ectoderm migrates (epiboly)
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
