Early Development and Axis Formation in Amphibians Development and Axis Formation in Amphibians...
Transcript of Early Development and Axis Formation in Amphibians Development and Axis Formation in Amphibians...
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Early Development and Axis
Formation in Amphibians
Biology 4361
July 9, 2009
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The Amphibian Model
1) How are the body axes established?
- How do relatively homogeneous cells (i.e. zygotes)
establish polarity (complexity from simplicity)?
2) How are the germ layers determined?
- How do zygotes send their cellular progeny
(i.e. blastomeres) down pathways toward different fates?
3) How is development organized in a regulative system?
Common vertebrate (e.g. Rana, Xenopus)
Manipulable, observable (i.e. large eggs and embryos)
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Amphibian Development - Overview
Cleavage
Fertilization, cortical rotation
Gastrulation
Axis and germ layer determination
The “Organizer”
Mesoderm
Dorsal/organizer
Inductions:
Ectoderm
Axes Summary(Photo by Harland lab/UC Berkeley)
Left – Right Asymmetry
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Cortical Rotation
Fertilization – animal hemisphere
Microtubular network originates
at fertilization point
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Cleavage Patterns
Meroblastic (incomplete cleavage)
Holoblastic (complete cleavage)
cephalopod molluscs
fish, reptiles, birds
most insects
echinoderms,
amphioxis
annelids, molluscs,
flatworms
tunicates
mammals, nematode
amphibians
Species
Spiral
Bilateral
Radial
Rotational
Displaced
radial
Bilateral
Discoidal
Superficial
CleavageYolk
classification
Isolecithal
Telolecithal
Centrolethical
Mesolecithal
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Unequal Radial Holoblastic Cleavage
Cell cycles regulated by mitosis-promoting factor (MPF)
- no G phases
… until mid-blastula transition
- promoters demethylated (i.e. de-repressed)
- transcription factors formed in vegetal cytoplasm
- embryonic control of development
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Mechanics of GastrulationFormation of the dorsal lip
- vegetal rotation
- invagination of bottle cells
- involution of marginal zone cells
Involuting Marginal Zone
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Mechanics of Gastrulation - 2
motive force
(D)
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Xenopus Gastrulation
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Cell Movements during Gastrulation
Movements
- invagination
- involution
- epiboly (animal cap)
- intercalation and
convergent extension
- migration
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Xenopus Gastrulation
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Xenopus Gastrulation - Blastopore
NIMZ
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Amphibian Development - Overview
Cleavage
Fertilization, cortical rotation
Gastrulation
Axis and germ layer determination
The “Organizer”
Mesoderm
Dorsal/organizer
Inductions:
Ectoderm
Axes Summary(Photo by Harland lab/UC Berkeley)
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2-cell
stage
Determination of Amphibian Axes
If one blastomere received no
gray crescent material, resulted in
- “belly piece” – blood,
mesenchyme, gut cells
- no dorsal structures(e.g. notochord, somites)
The gray crescent area is critical
for proper development
Gray crescent = future
dorsal lip of the blastopore
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Dorsal Lip Transplant
Doral lip = “Organizer” - organizes secondary D-V axis- induced ventral cells to change fates
(Spemann,
Mangold
experiment)
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Amphibian Development - Overview
Cleavage
Fertilization, cortical rotation
Gastrulation
Axis and germ layer determination
The “Organizer”
Mesoderm
Dorsal/organizer
Inductions:
Ectoderm
Axes Summary(Photo by Harland lab/UC Berkeley)
Left – Right Asymmetry
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The Organizer
- head mesoderm (prechordal plate)
- chordamesoderm (notochord)
Transplantation experiments established the organizing propertiesof the dorsal blastopore lip, which …
2) Establishes the dorsal-ventral axis
3) Specifies multiple tissues, including…
1) Self-differentiates (all other tissues conditionally specified)
- dorsal mesoderm, which includes…
4) Dorsalizes surrounding mesoderm into paraxial mesoderm
5) Induces the neural tube
6) Initiates the movements of gastrulation
How is the dorsal lip specified?
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Dorsal Signal: β-Catenin
β-catenin is initially distributed throughout the embryo,
- accumulates only in prospective dorsal cells.
- concentrated in the Nieuwkoop center and organizer
Organizer
Nieuwkoop
center
β-catenin
Dorsalization of β-catenin:
a) protect β-catenin in dorsal area,
b) degrade β-catenin everywhere else.
Mechanism - cortical rotation …
(in sea urchins, specifies micromeres, endomesoderm)
nuclear transcription factor (in Wnt pathway)
β-catenin
(in Xenopus, specifies dorsal structures; e.g. organizer)
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Disheveled/β-Catenin/Cortical Rotation
β-catenin is initially distributed
throughout the oocyte
β-catenin induces cells to dorsal fates
GSK3 marks β-catenin for degradation
Glycogen synthase kinase 3 (GSK3)
is also distributed throughout oocyte
Dishevelled (Dsh) blocks GSK3 activity
Dsh localizes in the cytoplasmic
cortex at the vegetal pole
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Disheveled/β-Catenin/Cortical Rotation
β-catenin, GSK3 distributed throughout
GBP – GSK3
binding protein
Dsh – Dishevelled
Kinesin - motor
protein
- anywhere Dsh exists, β-catenin survives
Dishevelled - blocks GSK3-mediated
β-catenin degradation
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Disheveled/GSK3/ β-Catenin
but Disheveled and GBP block GSK3;
- resulting in β-catenin present only in
the marginal area opposite the point
of sperm entry (i.e. future dorsal lip)
GSK3 mediates β-catenin destruction ……
At the blastula stage, β-catenin is located
exclusively in the future dorsal region
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Organizer Inductionβ-catenin acts with Tcf3 (transcription
factor); stimulates expression of
dorsalizing genes:
Siamois – TF; activates
Xlim, goosecoid(dorsal determinants)
Goosecoid protein –TF responsible for
organizer properties
Goosecoid also plays a part in
specifying dorsal mesoderm;
however, additional vegetal
factors are needed: e.g.
Vegetal TGF-β signals
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Mesoderm/Organizer Induction
(mid-blastula)
VegT, Vg1, Xnr(s) – Xenopus nodal related genes; TGF-β family
(late blastula) (early gastrulation)
, dorsal and
lateral mesoderm
- somites, notochord
- mesenchyme, blood
muscle, kidney
& intermediate
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Determination of Ectoderm
Later gastrula cells
are determined
- exhibit autonomous
development
Early gastrulation stage
cells are uncommitted
- exhibit regulative
development
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Establishment of Axes - Summary
D-V axis – set up at fertilization
A-P axis – established by gastrulation
movements across the dorsal lip of
the blastopore
L-R axis – Nodal expression on
Left, not Right
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Left – Right Asymmetry
Left–Right axis established
by Xenopus nodal-related(Xnr1)
Xnr1 expression is limited to
the left side in a process
involving cortical rotation
and Vg1
gut coiling – counter-clockwiseheart loops to the left
Block Xnr1 expression =
random gut coiling,
heart looping
Nodal expression:
- common to all vertebrates
- expressed on the left side!
wild-type Xnr1-/-