Gastrulation Flashcards
(104 cards)
1
Q
“It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life”
A
Lewis Wolpert (1986)
2
Q
Major morphogenetic moements during gastrulation result in the rearrangement of the embryo from blastula to astage characterized by the presence of the _____ _____ _____.
A
three germ layers
3
Q
- Profound well-ordered rearrangements of the cell in the embryo
- Acquisition by the cells ofthe capacity for undergoing morphogenetic movements which often result in the reorganization of the entire embryo.
A
Gastrulation
4
Q
major morphogenetic movements
A
Invagination
Involution
Epiboly
Ingression
Delamination (Poly-ingression)
5
Q
All organisms use at least ___ of the mechanisms to gastrulate
A
one
6
Q
Infolding of cell sheet into embryo
Ex. Sea urchin endoderm
A
Invagination
7
Q
Inturning of cell sheet over the basal surface of an outer layer
Ex. amphibian mesoderm
A
Involution
8
Q
Migration of individual cells into the embryo
Ex. Sea urchin msoderm, Drosophila neuroblasts
A
Ingression
9
Q
Splitting or migration of one sheet into two sheets
Ex. Mammalian and bird hypoblast formation
A
Delamination
10
Q
The expansion of one cell shet over other cells
Ex. Ectoderm formation amphibians, sea urchin and tunicates
A
Epiboly
11
Q
The outward folding of a cell sheet
A
Evagination
12
Q
Cells move in an amoeba-like fashion, often seen in some invertebrates
A
Amoeboid motion
13
Q
Two main strategies to handle gastrulation
A
- Spherical Context (Low yolk eggs - e.g., Amphioxus)
- Yolk-Rich Context (Large Yolk Eggs - e.g., Birds, Reptiles)
14
Q
Gastrulation occurs in a spherical or nearly symmetrical embryo.
Mechanism:
Invagination of the blastula forms a double-layered cup.
The inner cavity formed is the archenteron (primitive gut).
The opening to the archenteron is called the blastopore.
Outcome:
The blastula reorganizes into the three germ layers:
Ectoderm (outer layer)
Endoderm (inner layer)
Mesoderm (middle layer, formed by cell
migration/involution)
A
Spherical Context (Low Yolk Eggs - e.g., amphioxus)
15
Q
Gastrulation is adapted due to the large amount of yolk.
Mechanism:
Formation of a primitive streak instead of a blastopore.
Cells migrate inward and spread to form the germ
layers.
Outcome:
Despite the yolk, ectoderm, mesoderm, and endoderm
still form through epiboly, ingression, and migration.
A
Yolk-Rich Context (Large Yolk Eggs - e.g., Birds, Reptiles)
16
Q
Three main stages of Gastrulation
A
Initiation
Germ layer formation
Body Axis Establishment
17
Q
Formation of a blastopore (amphioxus, amphibians) or
primitive streak (birds, mammals).
Early cell movements begin.
A
Initiation
18
Q
Cells move and differentiate into:
Ectoderm
Mesoderm
Endoderm
A
Germ Layer Formation
19
Q
Formation of:
Anterior–posterior axis
Dorsal–ventral axis
Left–right symmetry
Sets the foundation for organ development and body
plan
A
Body Axis Establishment
20
Q
Morphogenetic movements begin in the late blastula stage
A
Gastrulation in Sea Urchin Embryos
21
Q
Gastrulation in sea urchin embryos: The focus of gastrulation n the movement and arrangemet of the _____ ______ ______.
A
Primary mesenchymal cells (PMCs)
22
Q
are a specific group of cells that arise from the vegetal pole of the blastula
A
Primary mesenchymal cells (PMCs)
23
Q
Two main processes in the gastrulation in sea urchin embryos
A
- The ingression of Primary Mesenchyme cells
- The invaginatn of the Archeon or the primitive gut
a. First invagination of Archenteron
b. Second and Third stage of Archenteron invagination
24
Q
The PMCs in sea urchin emerge from the
epithelium of the blastulla wall and move into
the blastocoel (a process called Ingression)
A
The ingresion of Primary Mesenchyme cells
25
First Invagination of Archenteron
Second and Third stage of Archenteron Invagination
The invagination of the Archenteron or the primitive gut
26
- When primary mesenchyme cells leaves
the vegetal region. The cells in the vegetal
plate remain bound to one another to the
hylaine layer of the egg, they move to fill
the gaps caused by the ingression.
- Then, the vegetal plate moves inward and
invaginates into the blastocoel . It will then
suddenly cease. The invaginated region
(called archenteron, or the primitive gut)
First invagination of Archenteron
27
- After a brief pause, second phase
happens. The archenteron extends
dramatically. Forming a long thin tube, the
cells of archenteron also migrates and
flatten themeselves.
- The filopodia are extended from these cells
through the blastocoel fluid, therefore
having contact in the inner surface, once
the filopodia attach to the wall, the junction
between the blastoderm cells and then
shorten ,pulling the archenteron.
Second and Third stage of archenteron
28
Afte contact with the all of the blastocoel, they begin to migrate and differentiate into four destined types: What are those?
Pigment cells
Blastocoelar cells
Two coelomic pouches which protrude from the tip of archenteron
The circumesophageal musculature
29
After the archenteron reaches the opposite wall of the blastula, the resulting bilaminar layer soon ruptures to form the _____ ______.
oral opening
30
becomes the anus
blastopore
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Gastrulation: crucial process after cleavage and blastulation
•Establishes the three germ layers: ectoderm, mesoderm, endoderm
•In Xenopus, influenced by yolk distribution and cortical rotation
Gastrulation in amphibian embryo (Xenopus)
32
Blastula organized into animal and
vegetal poles
•Fate map shows future positions of germ
layers
•Dorsal side determined by sperm entry
→ cortical rotation
Xenopus Blastula Fate map
33
Cortical rotation → Nieuwkoop center forms in dorsal vegetal region
•Induces overlying cells to become Spemann Organizer
•Organizer coordinates body axis and mesoderm development
Nieuwkoop center and mesoderm induction
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•Involution: cells roll inward at blastopore
•Epiboly: ectoderm spreads to enclose embryo
•Convergent extension: notochord and mesoderm elongate
•Migration: mesodermal and endodermal cells move into the interior
Morphogenetic movements in Xenopus
35
•Archenteron = future gut cavity
•Displaces the blastocoel
•Cells move toward the anterior end, defining body axes
Formation of the Archenteron
36
Begins at dorsal lip of blastopore
•Bottle cells form and invaginate
•First cells to involute: future
pharyngeal endoderm
Initiation of Gastrulation
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•Regulates dorsal development
•Expresses genes like goosecoid, noggin, chordin
•Initiates neural induction and notochord formation
Spemann Organizer Function
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•Blastopore becomes circular with yolk plug in center
•Germ layers positioned:
Ectoderm: outer covering
Mesoderm: muscles, skeleton, notochord
Endoderm: gut lining
Final stages of gastrulation in Xenopus
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a crucial phase in early embryonic development
where the single-layered blastula is reorganized into a trilaminar
structure, establishing the three primary germ layers: ectoderm,
mesoderm, and endoderm. These layers will subsequently give rise to all
the tissues and organs of the developing chick.
Gastrulation in birds
40
By the time a hen lays an egg, the blastoderm, a disc of cells on top of the
yolk, contains approximately 50,000 cells. Most of these cells form the
epiblast, an upper layer. Beneath the epiblast are some scattered cells
that will contribute to the primary hypoblast.
Blastoderm Formation
41
Shortly after the egg is laid, a thickening of the epiblast called Koller's
sickle appears at the posterior edge of the area pellucida (the central,
translucent region of the blastoderm). Adjacent to Koller's sickle and
bordering the area opaca (the opaque peripheral region) is the posterior
marginal zone (PMZ). The PMZ plays a vital role in initiating gastrulation.
Formation of Koller’s sickle and the posterior marginal zone (PMZ)
42
A sheet of cells at the posterior boundary between the area pellucida and
the PMZ migrates anteriorly beneath the epiblast. Simultaneously, cells in
more anterior regions of the epiblast delaminate (separate) and form
clusters called hypoblast islands. These islands migrate and coalesce to
form the primary hypoblast, a layer beneath the epiblast.
The migration of cells and formation of the primary hypoblast
43
The primitive streak is the hallmark of gastrulation in birds, reptiles, and
mammals. It appears as a thickening in the epiblast at the posterior
region, just anterior to Koller's sickle. This thickening results from the
ingression (inward migration) of epiblast cells. Cells from the lateral
regions of the posterior epiblast migrate towards the center, contributing
to the formation of the streak.
Formation of the primitive streak
44
As more cells ingress, the primitive streak elongates anteriorly,
extending towards the future head region of the embryo. The secondary
hypoblast cells also migrate anteriorly from the posterior margin of the
blastoderm, and this migration appears to be coordinated with the
elongation of the primitive streak. The primitive streak eventually extends
about 60-75% of the length of the area pellucida.
Elongation of the primitive streak
45
A groove, known as the primitive groove, forms along the midline of the
primitive streak. At the anterior end of the primitive streak is a
specialized region called Hensen's node (or the primitive knot). Hensen's
node is a critical signaling center that directs the organization of the
embryo.
Formation ofthe primitive groove and Hensen’s node
46
Epiblast cells migrate through the primitive streak and into the space between the epiblast and the hypoblast (the blastocoel, although it's a relatively small space in birds). The cells that ingress through the anterior part of the primitive streak and Hensen's node contribute to the formation of the endoderm (which displaces the hypoblast) and the
head mesoderm. Cells that ingress through the more posterior portions of the primitive streak form the majority of the mesodermal tissues. The
remaining epiblast cells that do not migrate inward become the ectoderm.
Cell migration and Germ layer formation
47
As gastrulation progresses and the germ layers are established, the primitive streak begins to regress posteriorly. Hensen's node also moves posteriorly, laying down the notochord (a rod-like structure that
provides support and signaling cues) as it moves.
Regression of the primitive streak
48
- Transition from bilaminar germ disc to trilaminar germ disc
- Cellular rearrangement, which involves migration, invagination, and differentiation of the epiblast.
- Egg type: Microlecithal (little yolk)
- Cleavage type: Holoblastic
- Grastulation initiation: Primitive streak
- Germ layer origin: Epiblast cell ingress through primitive plate
Gastrulation in Humans
49
Origin of the cranial mesoderm; development of the anterior forebrain and other head structures.
Prechordal plate
50
Area where ectoderm and endoderm meet without mesoderm; marks future mouth
Prechordal membrane
51
Ectoderm-endoderm junction at the tail end; marks future anus.
Cloacal membrane
52
Midline groove on epiblast; site of cell migration to form germ layers.
Primitive streak (groove)
53
Swelling at head end of streak; organizer of notochord
Primitive node (pit)
54
Midline rod from mesoderm; signals nervous system development and forms spine base.
Notochord
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Disintegration of primitive streak and primitive node leaves a cavity
primitive groove and primitive pit
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Gastrulation in humans:
hickening of epiblast results to ______ ___and _____ ____.
Primitive streak and primitive node
56
Cells near the primitive groove produce _______ ______ _______.
Fibroblast Growth Factor-8 (FGF-8)
57
FGF-8 inducce the production of ______.
SNAI1
58
SNAI1 inhibits the _________.
E-cadherine (holds the cell together), which allows cell to migrate
59
Cell migration is called ______ ______.
epithelial migration
60
Lateral epiblast cells migrate towards primitive groove, essentially replacing
the hypoblast. The replaced hypoblast is now called ________.
endoderm
61
The epiblast becomes the ________.
ectoderm
62
As more ectodermals move between
the epiblast and hypoblast layers,
they begin to spread laterally and
cranially, which forms the _______.
mesoderm
63
The formation of the ectoderm,
endoderm, and mesoderm is
called _______.
Trilaminar disc
64
The end of grastulation is when the
three germ layers are estalished, _____ is formed, and _____ _____ regresses.
notochord, primitive groove
65
Contiued production of FGF-8 allows ectodermal cells to continue _______.
migrating
66
- High levels of Nodal specify endoderm and dorsal/anterior mesoderm fates (high levels of Nodal signalling promote endoderm development, whereas lower doses specify mesoderm identity)
- Required for primitive streak formation; the antagonists (Lefty, Cerberus) released by tissues like the hypoblast help ensure the proper localization of the primitive streak by restricting Nodal activity (to avoid multiple streaks). It also triggers the migration and internalization of endodermal cells.
- Furthermore, Nodal cooperates with the Planar Cell Polarity (PCP) pathway to
regulate cell movements like convergence and extension.
Nodal signaling
67
- BMP signalling often patterns the mesoderm (promoting ventral fates) and ectoderm (promoting epidermal fates vs. neural fates dorsally). It frequently acts against the signals from the organizer region (like the primitive node), which
secretes BMP inhibitors (e.g., Chordin, Noggin).
- BMP signalling gradients are important; for example, lower BMP levels are typically found near the primitive streak, while higher levels are present in surrounding areas. High levels can inhibit streak formation in chick embryo.
Bone Morphogenetic Protein (BMP) signaling
68
- This pathway is essential for coordinating the morphogenetic movements of gastrulation, particularly convergent extension. Convergent extension involves the narrowing (convergence) and lengthening (extension) of tissues, driven by
mediolateral cell intercalation.
- The Wnt/PCP pathway polarizes the cells within the plane of the tissue, organizing cell behaviors like directed migration and intercalation. Key components of the pathway include Frizzled (Fz), Dishevelled (Dsh), Vangl, Prickle, and downstream
effectors like Rho GTPases. Defects in PCP signalling lead to failures in convergent extension and subsequent developmental problems like neural tube defects. The pathway is regulated by Nodal.
Non-canonical WNT/planar cell polarity (PCP) Pathway
69
- Induces the organizer region (like the Spemann organizer or primitive node) early in development. It often interacts with TGF-β signals (like Vg1/Nodal) to initiate this.
- Wnt/β-catenin signalling often promotes posteriorly and antagonizes proteins
involved in the formation of the head
1. Activation of the Wnt/β-catenin pathway (+Wnt) leads to headless tadpoles;
2. its inhibition (-Wnt) results in enlargement of anterior and concomitant reduction of trunk structures (Kiecker & Niehrs, 2013).
- Wnt inhibitors secreted by the head organizer help establish an anterior-posterior gradient of Wnt activity (Head = low, posterior = high). Wnt signalling interacts with BMP and Nodal pathways in a complex cascade to pattern the germ layers.
WNT signaling
Canonical WNT/B-catenin pathway
70
- FGF signalling contributes in mesoderm induction and patterning, cell migration, and coordinating cell movement during gastrulation, specifically, for the expression of genes involved in cell migration and lineage specification. FGF signalling can act
with or against other pathways.
Example:
Wnt signalling: It cooperates with Wnt signalling to induce posterior structure
BMP signalling: Germ layer specification
- FGF signalling is involved in primitive streak formation and subsequent germ layer development through regulation of gene expression.
Fibrblast Growth Factor (FGF) Signaling
71
Interactions of signaling pathways
Nodal & Wnt
BMP & Organizer antagonists (Chordin/Noggin)
Wnt/FGF vs. Anterior Antagonists
Nodal gradient
Wnt/PCP
FGF
72
Cooperate to initiate the primitive streak (posterior). Inhibited anteriorly.
Nodal & Wnt
73
Define the Dorsal-Ventral (D-V) axis (BMP high on the ventral side, inhibited dorsally)
BMP & Organizer Antagonists
74
Define the Anterior-Posterior (A-P) axis
(Wnt/FGF high posteriorly, inhibited anteriorly).
Wnt/FFGF ns. Anterior antagonists
75
Specify different mesendoderm fates along D-V and A-P axes
Nodal gradient
76
Coordinates cell migration, often downstream or parallel to fate specification signals.
Wnt/PCP
77
Broad role in mesoderm induction, cell migration, interacts with Wnt/BMP for patterning
FGF
78
Genes involved in gastrulation
Nodal
BMP4
FGF8
WNT3
Goosecoid
Brachyury (T)
79
Role in Gastrulation: Mesoderm & endoderm induction
Nodal
80
Role in Gastrulation: axis and mesoderm patterning
BMP4
81
Role in Gastrulation: Cell migration
FGF8
82
Role in Gastrulation: Primitive streak initiation
WNT3
83
Role in Gastrulation: Head organizer
Goosecoid
84
Role in Gastrulation: Notochord and tail development
Brachyury (T)
85
Mutaion Consequence: Nodal
Embryonic lethality
86
Mutation consequence: BMP4
Tissue differentiation defects
87
Mutation consequence: FGF8
Shortened or malformed axis
88
Mutation consequence: WNT3
No gastrulation
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Mutation consequence: Goosecoid
Head and neural defects
90
Mutation consequence: Brachyury (T)
No tail/mesoderm
91
Key events in gastrulation
- Cell movement and rearrangement
- Formation of the primitive streak (in mammals and birds)
- Establishment of Body axes (Anterioir-posterior, Dorsal-Ventral)
92
Cells change position relative to each other to form the three layers (ectoderm, mesoderm, endoderm).
Cell movement and rearrangement
93
The primitive streak is a visible line or groove that forms on the surface of the epiblast (top layer of the embryo).
Formation of the Primitive streak (in mammals and Birds)
94
Defines where the head and tail will be. Defines where the back and belly will form.
Establishment of Body Axes (anterior-posterior, Dorsal-ventral)
95
The primitive streak forms as a narrow groove on the surface of the epiblast (the upper layer of the
early embryo).
Primitive streak appearance
96
Cells of the epiblast move toward the primitive streak, sink inward (invaginate), and migrate through it.
Function as a site for Cell migration
97
Cells of the epiblast move toward the primitive streak, sink inward (invag inate), and migrate through it.
rganizer of Body plan
98
Formation of the Primitive streak
Primitive streak appearance
Function as a site for cell migration
Organizer of Body plan
99
major cell movements in gastrulation
Involution
Invagination
Epiboly
Delamination
100
A sheet of cells rolls inward and spreads over the inner surface of the embryo.
Involution
101
The infolding of a sheet of cells into the embryo.
Invagination
102
The movement of epithelial sheets (thin layers of cells) to spread and cover the entire embryo.
Epiboly
103
The splitting of one cellular sheet into two parallel sheets.
Delamination
