Xenopus
laevis
The African
Clawed Foot Toad
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36 chromosomes, 3.1x109bp
ancestrally tetraploid
4 hr
The
Xenopus
Life
Cycle
6 hr
1-2 years
19 hr
110 hr
32 hr
10 hr
Oogenesis
Ovary filled with eggs
triggered by human chorionic gonadotropin
1-2 mm
Animal pole
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Female Xenopus
Vegetal pole
The
METRO
(MEssage
TRansport
Organizer)
organizes
mRNA
localizations for
Dorsal-Ventral
Specification
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VegT
Oct60
In vivo
In vitro
Sperm entry point
Fertilization
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Movie
D
A
Nieuwkoop
Center/
Spemann
Organizer
Gray
Crescent
P
V
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The first cleavage occurs along
the plane of cortical rotation
The First
Cleavages
V
Split the
Embryo on the
Three Axes of
the Fate Map
Cleavage
Continues to
Form the
Blastula
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View from the Animal Pole
Twin Frogs
Only Along the A-P Axis
Shift in the cell cycle
Archenteron
Gastrulation:
Involution
and Epiboly
Ectoderm
Mesoderm
Endoderm
Movie
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Gastrulation
layers the
embryo
Ectoderm
Mesoderm
Endoderm
skin
brain
spinal cord
all other neurons
sense receptors
notochord
muscles
blood
bone
sex organs
lining of gut
lining of lungs
lining of bladder
liver
pancreas
Inductive signals
Factor
Type
Source
Veg-1
TGF-Beta
Vegetal Cytoplasm
Activin
TGF-Beta
Bone Morphogenetic
Protein (BMP-4)
TGF-Beta
Entire Embryo
XWint-8
Wnt
Middle Embryo
Fibroblast Growth
Factor (FGF)
FGF
Middle Embryo
Noggin
Neural Inducer
Spemann Organizer
Binds BMP-4
Chordin
Neural Inducer
Spemann Organizer
Binds BMP-4
Spemann Organizer
Binds XWnt-8
Frizbee
Effect
Mesoderm Induction
And Embryonic
Patterning
Induction
Signaling molecules
The Roles of the
Nieuwkoop Center and
the Spemann Organizer
Veg1 (TGF-B)
signaling
TGF-Beta/SMAD Signaling
The Wnt Pathway
Wnt ligands (Wg)
Frizzled G-protein
coupled receptors (DFz2)
P
Axin
scaffold
adenomatous
polyposis coli
APC
GSK3
GSK3
Disheveled (dsh)
(signaling intermediate)
Glycogen Synthase
Kinase 3 (GSK3)
Beta-catenin (Arm)
(transcription factors)
A Key Signaling Pathway in Regulatory
Development and Cancer
New gene activity
-Axin
(activates)
Dorsalization
Wnt
-Beta catenin
(inactivates)
Ventralization
TGF-Beta
-Veg 1, VegT
Failure of Gastrulation
(Ectodermal ball)
WT
Sending the
Wrong Signals
FGF (Fibroblast
Growth Factor)
Is also Involved
in Mesodermal
Induction
FGF
Overexpression
Normal
FGF
Depletion
A
Downstream
Master
Switch
MyoD, a
myogenic
regulator
Gastrulation
Neural tube closure
Tail bud formation
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Movie
Tadpole
Morphogenesis:
Behold, the frog!
A Few Questions for Thought
•Integrate the contributions of mosaic and regulatory
mechanisms in Xenopus.
•What is a fate map and how is it applied to metazoan
development? Compare and contrast fate maps for our
different model organisms.
•Describe the molecular bases of inductive signaling via the
TGF-Beta and Wnt pathways.
•What is the gray crescent, how is it formed and what does
it mark? Using your understanding of METRO, suggest an
hypothesis for how the gray crescent determines the fate of
cells that ultimately inherit this cytoplasm.
•Describe the process of gastrulation in Xenopus and the
contributions of each of the three key tissue layers to the
development of the tadpole body.