Vertebrate gastrulation Flashcards
LO
- Describe the mechanisms of gastrulation and contrast the morphogenetic cell movements underlying amphibian and human gastrulation
- Understand the importance of morphogenetic cell movements for the specification of the three germ layers during gastrulation
- Describe the mechanisms of symmetry breaking in the fertilized amphibian egg, and understand the importance of that for gastrulation
What is gastrulation?
- “It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” Lewis Wolpert (1986)
- Literally: The formation of a digestive tract/gut. (Endoderm)
What are the stages from zygote to gastrula?
In the wider sense what does gastrulation produce?
- The process which produces three germ layers in embryo development.
- Germ layers in higher metazoans (triploblasts): Ectoderm, Mesoderm and Endoderm.
Tell me about the primary germ layers and early organs?
What does each germ layer generate?
- The ectoderm generates the outer layer of the embryo
- The endoderm becomes the innermost layer of the embryo
- The mesoderm becomes sandwiched between the ectoderm and endoderm
Gastrulation involves five major morphogenetic movements of cells from the external surface to the interior of the embryo. What are these movements?
Tell me a simple description of what each movement is?
- Invagination: infolding of cell sheet into embryo
- Involution: inturning of cell sheet over the basal surface of an outer layer
- Ingression: migration of individual cells into the embryo
- Delamination: splitting of one cell sheet into two more or less parallel sheets
- Epiboly: the expansion of one cell sheet over other cells
Tell me about Invagination
- Invagination starts with an epithelial sheet (Basal surface / Apical surface)
- The sheet forms an in pocketing towards the basal side
- The lumen of in pocketing is faced by the apical surface of the epithelial sheet
- Movement analogous to poking a poorly inflated football
- (Out pocketing of a cell sheet towards the apical side = Evagination)
Tell me the three types of invagination
Three types of invagination:
- Apical constriction
- Apical tractoring
- Swelling of proteoglycan
Tell me about Involution
- Involution starts with the epithelium expanding and turning over on itself
- Bulk movement of tissue by rolling inward
- Analogous to a conveyor belt, caterpillar tread
- Tissue from where the rolling started can move in deep underneath the original tissue and form new tissue sheets
Tell me about delamination
- Delamination starts by splitting of one cellular sheet into two more or less parallel sheets
- Resembles ingression
- Formation of a new (additional) epithelial sheet of cells
Tell me about ingression
- Ingression starts with an epithelium
- Individual cells undergo epithelial-to-mesenchymal transition (EMT):
- They lose adhesion, alter their shape, and become migrating mesenchyme cells
Primary mesenchymal cells (PMCs) loose cadherin complex components, such as E-cadherin, β-, and α-catenin, at their surface:
What do they lose/gain?
- Lose affinity for neighboring epithelial cells
- Lose affinity for the hyaline layer on the exterior of the embryo
- Gain affinity for the basal lamina
Tell me about epiboly
- Epiboly starts with movement of epithelial sheets, spreading out of an overlying sheet of cells over an underlying mass of stationary tissue.
- Enclose deeper layers
- Occurs by cell dividing, by cells changing their shape, or by several layers of cells intercalating into fewer layers
What does the frog egg have before fertilisation?
The egg has polarity before fertilization, with a dense yolk material in the vegetal pole and very little yolk in the animal pole
What does polarity of the frog egg develop upon?
Polarity also develops upon fertilization, determined by the point of sperm entry
At the point of sperm entry, what does the sperm centrioles organise?
At the point of sperm entry, the sperm centrioles organize the centrosomes and microtubules to set up the mitotic spindle in the animal pole
At the region opposite the point of entry of sperm, the cortical cytoplasm rotates relative to what?
At the region opposite to the point of entry of sperm, the cortical cytoplasm rotates relative to the internal cytoplasm. This is facilitated by the formation of parallel arrays of microtubules in the vegetal hemisphere
The region opposite to the point of sperm entry is where what begins?
The region opposite to the point of sperm entry, is where development begins with the formation of the gray crescent and the dorsal lip of the blastopore
What does the point of sperm entry determine?
The ventral-dorsal axis of the embryo
The cleavage division after fertilisation must go through what?
The gray cresent
Tell me about the initial events during frog gastrulation
- First visible sign of blastopore formation is a depression in a dorsal vegetal position to form dorsal blastopore lip, where gastrulation begins
- Marginal zone (MZ): region of equator where the vegetal and animal hemispheres meet
- Depression extends to form a circular blastopore with dorsal lip and ventral lip
- Note bottle cells around the blastopore
- Note deep cells in touch with ectoderm
- Invagination of tissue all around the blastopore, but mostly at the dorsal lip
Tell me the body axes establishment during frog gastrulation
The body axes (dorsal/ventral, anterior/posterior and left/right) are established at gastrulation
Tell me the cell movements during frog gastrulation in the diagram
tell me about early gastrulation shown in A&B
(A & B) Early gastrulation
1) The bottle cells of the margin invaginate and move inward to form the dorsal lip of the blastopore, and
2) the mesodermal precursors involute under the roof of the blastocoel and migrate apically. Involution of the mesodermal precursors is driven by vegetal rotation in the endoderm
Tell me the cell movements during frog gastrulation in the diagram
tell me about Mid-gastrulation shown in C&D
(C & D) Mid-gastrulation
1) The archenteron forms and displaces the blastocoel, and
2) cells migrate from the lateral and ventral lips of the blastopore into the embryo. 3) The cells of the animal hemisphere migrate down toward the vegetal regionT
Tell me the cell movements during frog gastrulation in the diagram
tell me about Late gastrulation shown in E&F
(E & F) Late gastrulation
1) The blastocoel is obliterated,
2) the embryo becomes surrounded by ectoderm through epiboly,
3) the endoderm has been internalized, and
4) the mesodermal cells have been positioned between the ectoderm and endoderm
What is the dorsal midline mesoderm responsible for?
Dorsal midline mesoderm = notochord. Will become responsible for inducing formation of neural plate and neural tube at gastrulation
What are each site responsible for?
(A) The site of the dorsal blastopore lip is evident by the pigmented cells at its rim, coming from the animal cap
(B, C) This region of involution later spreads to form the lateral lips.
(D) The blastopore eventually encircles a small yolk plug, with cells involuting along each side
(E) The outer cells converge to form ectoderm, and the yolky cells (comprising the endoderm) are internalized. The involuting cells between them become mesoderm
Gastrulation in the human embryo
Tell me about the formation of the three germ layers in the process of gastrulation in the human embryo
- Invagination of epiblast cells, invading and displacing the hypoblast cells to become endoderm
- Ingression of epiblast cells to form a third layer of mesoderm cells. Driven by epithelial-to-mesenchymal transition (EMT) through loss of the cell-cell adhesion molecule E-cadherin
- Epiblast cells remaining on the surface become ectoderm
Tell me about the stages of the TGF-beta mediated specification of the mesoderm
- The vegetal region of the oocyte accumulates maternal mRNA for the transcription factor VegT and (in the future dorsal region) mRNA for the Nodal (ligand which plays a role in specifying the mesoderm) paracrine factor Vg1 (released into the overlying mesoderm)
- At the late blastula stage, the Vg1 mRNA is translated and Vg1 induces the future dorsal mesoderm to transcribe the genes for several Wnt antagonists
- The VegT message is also translated, and VegT activates nuclear genes encoding Nodal proteins
- These TGF-β family members activate the expression of the transcription factor Eomesodermin (Eomes) in the presumptive mesoderm
- Eomes, with the help of activated Smad2 from the Nodal proteins, activates nuclear genes encoding VegT
- Mesodermal phenotype maintained by VegT being expressed in mesoderm in blastocyte
- Organising within the dorsal mesoderm (everything on following side shows how dorsalisation is coming about)
Tell me 4 abilities of the amphibian organiser?
- Ability to self-differentiate dorsal mesoderm
- Ability to dorsalize surrounding mesoderm into paraxial mesoderm
- Ability to dorsalize the ectoderm, inducing the formation of the neural tube
- Ability to initiate movements of gastrulation
What was the results of the experiment for determining Crucial changes in cell potency occur during gastrulation and what conclusions can be drawn from this
Results
(A) When the tissues are transferred between early gastrulae, the presumptive neural tissue develops into epidermis, and only one neural plate is seen
(B) When the same experiment is performed using late-gastrula tissues, the presumptive neural cells form neural tissue, thereby causing two neural plates to form on the host. (autonomous specification, they are in a new environment but can still develop into their own fate)
Conclusions
- Cells of the early gastrula exhibit conditional (induction-dependent) specification
- Cells of the late gastrula exhibit autonomous (mosaic, independent) specification. They are determined
What causes cells to become committed to epidermal and neural fate?
nb. Presumptive: prospective, fate mapping, define fate map of an embryo. Trace back any cell to early stage of development. Know what cell gives what function as its been shown my fate mapping
Explain this diagram
(A) Dorsal lip tissue from an early T. taeniatus gastrula is transplanted into a T. cristatus gastrula in the region that normally becomes ventral epidermis
(B) The donor tissue invaginates and forms a second archenteron, and then a second embryonic axis. Both donor and host tissues are seen in the new neural tube, notochord and somites
(C) Eventually, a second embryo forms, joined to the host
How does the organiser form?
- Mesodermal induction by vegetal endoderm
- A group of dorsal endodermal cells in the vegetal pole, expressing Vg1, VegT and β-catenin will form the Nieuwkoop centre
- The Nieuwkoop centre specifies the dorsal mesoderm and induce the organizer
With mechanisms of organizer induction, β-catenin-mediated dorsal signals, what happened after fertilisation?
After fertilization, these vegetal vesicles are translocated dorsally along subcortical microtubule tracks. Cortical rotation adds a “slow” form of diffusion of Wnt11 mRNA
After ferilisation with β-catenin-mediated dorsal signals , what happens to Wnt-11, Dsh and GBP?
(C) Wnt11, Dsh, and GBP are then released from the microtubules and are distributed in the future dorsal region of the 1-cell embryo
- *(D)** Dsh and GBP bind to and block the action of GSK3, thereby preventing the degradation of β-catenin on the dorsal side of the embryo. Wnt11 amplifies Dsh-mediated β-catenin stabilization
- *(E)** The nuclei of the blastomeres in the dorsal region of the embryo receive β-catenin, whereas the nuclei of those in the ventral region do not.
What is Noggin, Chordin and Cerberus?
Noggin, Chordin and Cerberus are paracrine factors secreted by the organizer to specify the neural plate
Tell me the vegetal nodal-related paracrine signals?
The amphibian organiser derivatives
