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Year 2 Semester 2 Vertebrate Development > Gastrulation (Elias) > Flashcards

Gastrulation (Elias) Flashcards

1
Q

What are the 5 main morphogenetic movements caused by gastrulation?

A

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

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2
Q

What is invagination?

A

Invagination starts with an epithelial sheet (Basal surface / Apical surface)

The sheet forms an inpocketing towards the basal side

The lumen of inpocketing is faced by the apical surface of the epithelial sheet

Three types of invagination:
- Apical constriction
- Apical tractoring
- Swelling of proteoglycan

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3
Q

What is involution?

A

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

This is how the mesoderm is formed during embryogenesis

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4
Q

What is delamination?

A

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

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5
Q

What is ingression?

A

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:
- Lose affinity for neighbouring epithelial cells
- Lose affinity for the hyaline layer on the exterior of the embryo
- Gain affinity for the basal lamina

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6
Q

What is epiboly?

A

Epiboly start 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.

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7
Q

Symmetry breaking in the frog egg at fertilisation

A

The egg has polarity before fertilization, with a dense yolk material in the vegetal pole and very little yolk in the animal pole

Polarity also develops upon fertilization, determined by the point of sperm entry

At the point of sperm entry, the sperm centrioles organizes the centrosomes and microtubules to set up the mitotic spindle in the animal pole

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 development begins with the formation of the gray crescent and the dorsal lip of the blastopore

The point of sperm entry determines the ventral-dorsal axis of the embryo

The cleavage division after fertilization must go through the gray crescent

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8
Q

What are the intiail events during frog gastrulation?

A

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

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9
Q

What are the cell movements during early frog gastrulation?

A

A & B)

Early gastrulation

1) The bottle cells of the margin “invaginate” and move inward to form the dorsal lip of the blastopore

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

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10
Q

What are the cell movements during mid frog gastrulation?

A

(C & D)

Mid-gastrulation.

1) The archenteron forms and displaces the blastocoel

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 region

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11
Q

What are the cell movements during late frog gastrulation?

A

(E & F)

Late gastrulation

1) The blastocoel is obliterated

2) the embryo becomes surrounded by ectoderm through epiboly

3) the endoderm has been internalised, and 4) the mesodermal cells have been positioned between the ectoderm and endoderm

Dorsal midline mesoderm = notochord
- Will become responsible for inducing formation of neural plate and neural tube at gastrulation

The body axes (dorsal/ventral, anterior/posterior and left/right) are established at gastrulation

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12
Q

Epiboly of the ectoderm in the frog embryo

A

(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

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13
Q

What are the initial stages of gastrulation in the human embryo?

A

Formation of Bilaminar Germ Disc:
- The inner cell mass (ICM) cells segregate to form the epiblast and hypoblast (primitive endoderm), resulting in the bilaminar germ disc

Role of Epiblast and Hypoblast:
- The epiblast will give rise to the three primary germ layers of the embryo proper.
- The hypoblast contributes to the development of the extraembryonic mesoderm and the yolk sac, which forms the extraembryonic endoderm.

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14
Q

What is the importance of the hypoblast during gastrulation in the human embryo?

A

The hypoblast is essential for embryo patterning during gastrulation

Removal of the hypoblast experimentally results in the formation of multiple primitive streaks, underscoring its role in directing proper developmental cues

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15
Q

How do the germ layers develop during gastrulation in the human embryo?

A

Gastrulation begins with the formation of the primitive streak on the dorsal surface of the epiblast

Layer Formation:
- Endoderm: Epiblast cells invaginate, displacing hypoblast cells to form the endoderm.

  • Mesoderm: 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
  • Ectoderm: Epiblast cells that remain on the surface after these migrations become the ectoderm.
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16
Q

GASTRULATION VIDEO

A

https://www.youtube.com/watch?v=3AOoikTEfeo&t=2s&ab_channel=MedicalAnimations

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17
Q

How do maternal mRNA and signaling pathways regulate early embryonic development in the vegetal region of the oocyte?

A

The vegetal region of the oocyte accumulates maternal mRNA for the transcription factor VegT and (in the future dorsal region) mRNA for the Nodal paracrine factor Vg1

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

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18
Q

When do frog body axes determined?

A

Specification of body axes are triggered at fertilization but realized during gastrulation

Tissues organize to change fates

The three axes of the fully developed embryo

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19
Q

What is the amphibian organiser?

A

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

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20
Q

What did mangolds experiments find?

A

Cells of the early gastrula exhibit conditional (induction-dependent) specification

Cells of the late gastrula exhibit autonomous (mosaic, independent) specification. They are determined

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21
Q

What happpens when a dorsal lip is transplanted from one gastrula into another?

A

The dorsal lip cells and grey crescent form an organizing center that initiates gastrulation and patterns the embryo

When 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 –>

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

Eventually, a second embryo forms, joined to the host.

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22
Q

How does the organiser form?

A

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

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23
Q

How does β-catenin-mediated dorsal signaling occur before and after fertilization in the embryonic development process?

A

(A)
- Before fertilization, Disheveled (Dsh) and GSK3-binding protein (GBP) associate with the microtubule-associated motor kinesin at the vegetal pole. Wnt11 mRNA is also in vesicles at the vegetal portion of the egg

(B)
After fertilization, these vegetal vesicles are translocated dorsally along subcortical microtubule tracks. Cortical rotation adds a “slow” form of diffusion of Wnt11 mRNA

(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.

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24
Q

β-catenin role in cleavage

A

During cleavage, β-catenin enters the nuclei and binds with Tcf3 to form a transcription factor that activates genes encoding proteins such as Siamois and Twin

Siamois and Twin interact with the Smad2 transcription factor activated by vegetal TGF-β family members (Nodal-related proteins, Vg1, etc.).

Together, these three transcription factors activate the “organizer” genes such as chordin, noggin, and goosecoid.

Goosecoid is a transcription factor that specifies the dorsal mesoderm, which becomes the organizer

Noggin, Chordin and Cerberus are paracrine factors secreted by the organizer to specify the neural plate

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25
Q

What is the role of the organizer in dorsalization and neural plate induction?

A

The organizer, a region of dorsal mesoderm, expresses specific genes and releases ligands that act on the ectoderm, dorsalizing it and inducing the formation of the future nervous system.

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26
Q

What happens to the remaining ectoderm after part of it is dorsalized?

A

The remaining ectoderm differentiates into the epidermis and other surface ectoderm derivatives.

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27
Q

What are neural crest cells and when do they emerge?

A

Neural crest cells emerge at the interface between the surface ectoderm and the neural tube just at the end of neural tube closure. They start to migrate and contribute to key parts of the embryo.

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28
Q

What are some of the key ligands released by the organizer?

A

Key ligands released by the organizer include noggin, chordin, and follistatin. These ligands inhibit BMP signaling to promote neural specification.

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29
Q

What is the role of BMP signaling in ectoderm differentiation?

A

BMP signaling promotes surface ectoderm differentiation. Its inhibition is necessary for neural specification.

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30
Q

What is the function of the TGF-beta signaling pathway in neural plate induction?

A

BMP, a component of the TGF-beta signaling pathway, binds to tyrosine kinase receptors, triggering an intracellular cascade that activates SMAD proteins, which then act as transcription factors. The organizer inhibits this pathway to promote neural tube induction.

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31
Q

How do noggin, chordin, and follistatin influence BMP signaling?

A

These ligands bind to BMPs and prevent their action, leading to neural specification by inhibiting BMP signaling.

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32
Q

What happens if BMP signaling is overactivated?

A

Overactivation of BMP signaling promotes epidermal differentiation, leading to an excess of epidermis and inhibition of neural tube formation.

33
Q

What happens if BMP signaling is completely inhibited?

A

Complete inhibition of BMP signaling results in an embryo made entirely of neural ectoderm, leading to the loss of the dorsal-ventral axis and body axis formation.

34
Q

How does Sox2 contribute to neural plate induction?

A

Sox2 is a pluripotency gene found in the ICM and is crucial for neural plate induction. It can be visualized using in situ hybridization, highlighting the elongation of the neural plate.

35
Q

What is the significance of the medial and dorsolateral hinge points in neural tube formation?

A

The medial hinge point forms first due to apical constriction controlled by the notochord, elevating neural folds. The dorsolateral hinge points form later to help converge and close the neural folds, ensuring proper neural tube formation.

36
Q

What role does the notochord play in neural tube formation?

A

The notochord releases sonic hedgehog, creating a gradient that influences the formation of the medial and dorsolateral hinge points through apical constriction and inhibition of noggin.

37
Q

How does sonic hedgehog influence noggin and BMP signaling?

A

Sonic hedgehog inhibits noggin. Away from sonic hedgehog, noggin can inhibit BMP, leading to the formation of dorsolateral hinge points necessary for neural tube closure.

38
Q

What is the importance of cadherins in neural tube formation?

A

Cadherins mediate cell-cell adhesion. N-cadherin in the neural plate and E-cadherin in the prospective epidermis facilitate the closure of the neural tube by allowing cells to establish communication and physical contact.

39
Q

What is anencephaly and how is it related to neural tube defects?

A

Anencephaly is a neurodegenerative disease caused by the failure of the neural tube to close, exposing it to toxic amniotic fluid, leading to massive apoptosis and neurodegeneration.

40
Q

What are the three primary vesicles formed in the chick embryo and their derivatives?

A

The three primary vesicles are the forebrain, midbrain, and hindbrain. They later mature into five vesicles that give rise to all derivatives of the central and peripheral nervous systems.

41
Q

What happens during the neural tube formation in the chick embryo?

A

Neural tube formation occurs along an anterior-posterior axis, starting in the anteriormost part. It involves dynamic processes like folding, elevation of neural folds, convergence, and closure, regulated by spatial and temporal mechanisms.

42
Q

How does the apical constriction contribute to neural tube formation?

A

Apical constriction, due to actomyosin cytoskeleton activity, generates driving forces for neural plate folding, forming the medial hinge point, which then elevates the neural folds and promotes their convergence and closure.

43
Q

What is the impact of manipulating BMP signaling in neural development experiments?

A

Manipulating BMP signaling by inhibiting its components using morpholinos leads to enhanced neural tube formation. Overactivation or complete inhibition of BMP signaling results in either excessive epidermis or neural ectoderm, disrupting normal development.

44
Q

How do experimental models like mouse and tunicate help in understanding neural tube formation?

A

Models like mouse and tunicate provide insights into conserved mechanisms of neural tube formation, allowing high-resolution live imaging and manipulation to study processes like zippering of neural folds and cadherin-mediated cell adhesion.

45
Q

What is the significance of noggin expression in the organizer and its role in neural development?

A

Noggin expression in the organizer is crucial for inhibiting BMP signaling, promoting neural specification. Its absence or disruption leads to defects in neural tube closure and body axis formation.

46
Q

How does the balance between BMP and noggin regulate neural and epidermal specification?

A

Proper balance between BMP and noggin is essential for normal development. Noggin inhibits BMP to promote neural specification, while BMP promotes epidermal specification. Imbalance leads to either excessive neural tissue or epidermal tissue, disrupting development.

47
Q

What are the roles of sonic hedgehog and noggin in regulating apical constriction during neural tube formation?

A

Sonic hedgehog, released by the notochord, creates a gradient inhibiting noggin in specific regions, allowing controlled apical constriction and formation of medial and dorsolateral hinge points necessary for neural tube closure.

48
Q

How does the dynamic process of neural tube closure occur, and what are its key regulatory mechanisms?

A

Neural tube closure involves coordinated cell movements like folding, elevation, and convergence, driven by apical constriction. Key regulatory mechanisms include gradients of morphogens like BMP and sonic hedgehog, and cell adhesion molecules like cadherins.

49
Q

What is the importance of in situ hybridization in studying genes like Sox2 and noggin during neural development?

A

In situ hybridization allows visualization of gene expression patterns, providing insights into the spatial and temporal dynamics of genes like Sox2 and noggin, which are crucial for neural plate induction and organizer function.

50
Q

How do cadherins contribute to the separation of the neural tube from the surface ectoderm?

A

E-cadherin in the prospective epidermis and N-cadherin in the neural plate facilitate cell-cell communication and adhesion, driving the closure of the neural tube and its separation from the surface ectoderm.

51
Q

What experimental evidence supports the role of BMP and noggin in neural tube formation?

A

Experiments show that manipulating BMP signaling, either by inhibiting or overactivating it, affects neural tube formation. Inhibiting BMP promotes neural tissue, while overactivating it promotes epidermal tissue, highlighting the importance of balanced signaling.

52
Q

What are neural crest cells and why are they important?

A

Neural crest cells are a multipotent population of cells that arise from the neural tube, undergo epithelial-to-mesenchymal transition (EMT), and migrate to form diverse tissues such as cranial nerves, cartilage, and parts of the adrenal medulla.

53
Q

What are the two main learning outcomes for studying neural crest cells?

A

Understanding where neural crest cells come from and how they find their final destination, which determines their fate, specification, and differentiation path.

54
Q

What is the epithelial-to-mesenchymal transition (EMT)?

A

EMT involves the loss of cell adhesions and epithelial features, allowing cells to become migratory. This process is repurposed from mechanisms seen in mesoderm formation and cancer metastasis.

55
Q

What are some key processes neural crest cells undergo during their development?

A

Neural crest cells undergo EMT, delaminate from the neural tube, and migrate to various locations where they differentiate into multiple cell types

56
Q

How do neural crest cells challenge the developmental dogma?

A

Neural crest cells reactivate multipotency after neural specification, allowing them to give rise to various derivatives, reversing their differentiation program.

57
Q

What do electron micrographs reveal about neural crest cells?

A

Electron micrographs show tightly packed epithelial cells and more elongated, mesenchymal neural crest cells ready to delaminate and migrate.

58
Q

What are cranial neural crest cells and their contributions?

A

Cranial neural crest cells migrate through the pharyngeal arches to form the face’s cartilage, bone, and cranial nerves, possessing high developmental potential.

59
Q

Where are vagal neural crest cells located and what do they contribute to?

A

Vagal neural crest cells are between somite numbers 1 and 7, and posterior somites up to number 28, contributing to the parasympathetic nervous system, mainly in the gut.

60
Q

What is the role of cardiac neural crest cells?

A

Cardiac neural crest cells form the septum between the pulmonary artery and aorta, controlling blood flow in the heart and preventing congenital heart defects

61
Q

What are trunk neural crest cells and their functions?

A

Trunk neural crest cells span the trunk of the embryo, contributing to the adrenal medulla and other tissues, releasing cortisol and adrenaline.

62
Q

What are the two main migratory pathways for neural crest cells?

A

The dorsolateral pathway (between the dermis and epidermis) and the ventral pathway (between the somites), which determine the derivatives’ nature and identity.

63
Q

How do neural crest cells acquire migratory behavior?

A

Through the regulatory gene network involving BMP signaling, transcription factors like Snail2 and ZEB2, and loss of N-cadherin, facilitating EMT and migration.

64
Q

What role do Rho GTPases play in neural crest cell migration?

A

Rho GTPases like Rac1 and RhoA enable the formation of lamellipodia and stress fibers, allowing neural crest cells to migrate effectively.

65
Q

What is contact inhibition in neural crest cell migration?

A

Contact inhibition involves neural crest cells touching each other and moving away, directing the migration and preventing clustering.

66
Q

How is live imaging used to study neural crest cells?

A

By tagging actin proteins with fluorescent markers like Lifeact-mCherry, allowing visualization of cytoskeletal dynamics and cell adhesions in real-time.

67
Q

What is the “kiss-and-run” process in neural crest cell migration?

A

Neural crest cells briefly touch each other and then move away, remodeling their cytoskeleton to continue migrating.

68
Q

What is collective migration in neural crest cells?

A

A process where neural crest cells migrate together, guided by leader cells, maintaining low levels of N-cadherin for adhesion, and following signals like complement factor C3a.

69
Q

How do neural crest cells navigate various environments during migration?

A

They respond to attractant and repellent signals, guiding their migration towards their final destination and away from undesirable areas.

70
Q

What is the “chase-and-run” process in cranial neural crest cells?

A

Neural crest cells are attracted by SDF1/CXCL12, touch target cells, lose focal adhesions, then re-establish them to chase target cells, directing migration.

71
Q

How do neural crest cells demonstrate multipotency?

A

By differentiating into various cell types, including neurons, cartilage, bone, and adrenal medulla cells, reactivating multipotency after initial specification.

72
Q

Why is the final destination of neural crest cells crucial?

A

The final destination determines their fate, including specification and differentiation into various tissues based on signals encountered during migration.

73
Q

How is the migration of neural crest cells similar to haematopoiesis?

A

Neural crest cells use similar mechanisms involving SDF1/CXCL12 and CXCR4 for guided migration to their target tissues, akin to haematopoietic stem cells finding the bone marrow

74
Q

What role do transcription factors Snail2 and ZEB2 play in neural crest cells?

A

These transcription factors downregulate cadherins, facilitating the loss of epithelial features and promoting the migratory phenotype of neural crest cells.

75
Q

How does contact inhibition influence neural crest cell directionality?

A

Neural crest cells contact each other and move away, remodeling their actin cytoskeleton and forming new adhesions to ensure directed migration.

76
Q

How do neural crest cells respond to SDF1/CXCL12 during migration?

A

Neural crest cells expressing CXCR4 are attracted to SDF1/CXCL12, guiding them to their final destination through dynamic cytoskeletal reorganization.

77
Q

What mechanisms allow neural crest cells to maintain collective migration?

A

Low levels of N-cadherin for adhesion, leader cells guiding the migration, and release of signals like complement factor C3a to potentiate collective behavior.

78
Q

What is the significance of cadherin regulation in neural crest cell migration?

A

Proper regulation of cadherin levels ensures cells lose epithelial characteristics, acquire migratory abilities, and maintain coordination during migration.

Year 2 Semester 2 Vertebrate Development (5 decks)

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