Download Axis formation in Amphibians

Early Development in
Amphibians
Cleavage in Amphibians
• Radial, holoblastic
• Affected by yolk
• First two through
AV axis
• Third “equatorial”
• Blastula of ca
100,000 cells
Xenopus Fate Map
When Do Zygotic Genes Turn
On?
• Almost all zygotic expression occurs after
12th cycle in mid-blastula
• = mid-blastula transition
• Senses changing chromatin cytoplasm ratio
When Do Axes Form?
• Anterior-posterior axis develops from the
animal-vegetal axis
– Originates during oogenesis
• Dorsal-ventral axis established after
fertilization
– Before first cleavage
Animal-Vegetal Axis in Oocyte
• Yolk protein (vitellin)
accumulates in vegetal
half
• Cortical cytoplasm of
animal half accumulates
dark pigment
What Else Localizes Along A-V?
• Transport system (MTs)
localizes some maternal
mRNAs to different poles
– ex. Vg1 and VegT mRNAs
localize to the cortex of the
vegetal hemisphere
– Vg1 = TGF-b
– VegT = transcription factor
What Controls A-V Axis?
• Asymmetric
distribution of factors
such as Vg1 and VegT
imposes animalvegetal fates
How Do We Know? VegT
• VegT antisense RNA
– Artificial depletion of VegT
• Results in altered fate-map:
How Do We Know? Vg1
•
•
•
•
Deplete Vg1
No endoderm
No dorsal mesoderm (why?)
Therefore, pre-localized cortical vegetal
determinants specify germ layers
Can Xenopus Regulate Along
Animal-Vegetal Axis?
• Fates along the A-V axis are specified through a series of
cell interactions
• Experiment: isolate blastomeres at 32-cell stage:
Mesoderm not autonomous
Can Xenopus Regulate Along
Animal-Vegetal Axis?
• Repeat same experiment on a later stage embryo:
– isolate blastomeres at 128-cell stage:
– now each produces proper structures in isolation
– so induction of mesoderm between 32 and 128 by ?
Regulative Specification Along
Animal-Vegetal Axis
• Which cells are competent to respond to
induction by vegetal pole?
• Nieuwkoop’s recombination experiments:
mesoderm forms
Regulative Specification Along
Animal-Vegetal Axis
• Recombination
experiments:
– Presumptive endoderm
induces pre-ectoderm to
form mesoderm
Further Regulative Specification
Along Animal-Vegetal Axis
– Regionally specific
• Ventral and lateral vegetal cells induce ventral and
intermediate mesoderm (blood, kidney)
• Dorsal vegetal cells induce dorsal mesoderm
(somites, notochord)
Model A-P Polarity
• Oocyte
early blastula
mid-blastula
Marginal zone (= region surrounding equator of blastula
where animal and vegetal hemispheres meet) is induced by vegetal region
early Bl
DV Axis
Dorsal-Ventral Axis
• Arises from cytoplasmic rearrangements following
fertilization
– Unfertilized egg is still radially symmetrical
– Fertilized egg is not
D
V
How Does Dorsal-Ventral Axis Arise?
• Cortical cytoplasm rotates 30o relative to internal
cytoplasm
– Reveals underlying gray pigment
– “Gray crescent”
– Bisected by first cleavage plane
Importance of Gray Crescent Region
• Hans Spemann
• Artificially displace first
cleavage, get
– Bauchstuck
– ecto, meso, and endoderm, but
no dorsal structures (nervous
system, notochord, somites)
• Or by blocking rotation
• So rotation related to DV
What Determines Rotation and the
Dorsal-Ventral Axis?
• Greatest displacement of
cortical cytoplasm occurs
opposite point of sperm
entry (SEP)
• Gastrulation will begin in
this region (blastopore lip)
• Identifies future dorsal
side
Twins Produced by Rotation
• Move SEP up just before first cleavage
• Gravity displacement forms new axis