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 animal-

vegetal 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

• 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

Importance of Gray Crescent Region

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