Download Developmental Biology 8/e

Cortical Rotation
Cleavage of a frog egg
Frog egg cleavage
Fate maps of the blastula of the frog Xenopus laevis
Cell movements during frog gastrulation
Cell movements during frog gastrulation
Surface view of an early dorsal blastopore lip of Xenopus
Epiboly of the ectoderm
Importance of localized cytoplasmic determinants
Spemann’s first classical
experiment
There are determinants in the cytoplasm of the
fertilized egg
At the vegetal half – maternal m-RNAs
Vg-1 – TGF-β signal – mesoderm induction
Xwnt-11 – dorso-ventral axis specification
VegT – transcription factor-mesoderm
induction
Summary of the results of experiments by Nieuwkoop and by Nakamura and Takasaki, showing
mesodermal induction by vegetal endoderm – vegetal cells are a signaling center
Organization of a secondary axis by dorsal blastopore lip tissue
Chronologically this is first evidence of induction by a
signaling center
Organization of a secondary axis by dorsal blastopore lip tissue
Now we have two levels of induction:
1. The vegetal cells
2. The dorsal lip of the blastopore
Dorsal lip of the blastopore patterns a new
D/V axis and a new A/P axis. How does
the dorsal lip of the blastopore become a
signaling center?
Model resulting from the experiments by Nieuwkoop and by Nakamura and Takasaki
Grafting cells containing the Nieuwkoop center to ventral
side results in formation of a second axis.
Model of the mechanism by which the Disheveled protein stabilizes -catenin in the dorsal portion
of the amphibian egg – PATTERNING THE AXES
Identifying the dorsal signal
Model of the mechanism by which the Disheveled protein stabilizes -catenin in the dorsal portion
of the amphibian egg (Part 2)
Signaling center established in
the dorsal vegetal side
Nieuwkoop center
Summary of events hypothesized to bring about the induction of the Organizer in the dorsal
mesoderm
Β-catenin, VegT, and Vg1
interact to activate Nodalrelated genes (TGF-β)
Model for mesoderm induction and organizer formation by the interaction of -catenin and TGFproteins
Defined by
β-catenin &
goosecoid
Steps to the establishment of a dorsal side in a
Xenopus embryo:
1. cortical rotation
2a. induction of mesoderm by vegetal cells maternal
factors
2b. establishment of Nieuwkoop center - dorsalization
3. establishment of the Organizer
4. gastrulation – dorsal lip of blastopore
Cell movements during frog gastrulation
The movement of the involuting mesoderm establishes the anterior-posterior axis.
The first cells to migrate over the dorsal lip are responsible for anterior structures.
Dorsal mesoderm “dorsalizes” ectoderm above.
Model for the action of the organizer
BMP visualized by using antibodies against
Smad1 (highlights region where BMP is active.
Arrows show dorsal side ( by dorsal lip of the
blastopore)
Rescue of dorsal structures by Noggin protein – embryos ventralized by UV light
UV irradiation = no cortical rotation
noggin mRNA injection = rescue of
dorsal structures in a dosage dependent
manner
Localization of noggin mRNA in the organizer tissue, shown by in situ hybridization
Dorsal marginal
Zone at gastrulation
Precursors of
notochord,
prechordal plate,
pharyngeal
endoderm during
convergent
extension
Dorsal
blastopore
lip after
involution
Beneath
ectoderm
Localization of chordin mRNA
Prior to gastrulation
Future dorsal lip
Gastrulation in
dorsal lip
In organizer tissues
Control of neural specification by the levels of BMPs
Expression of
Sox2 a neural
marker
Control embryo
Neural marker
BMP4 signaling
not present
BMP inhibitors
knocked out
All ventralizing
signals absent
Regional specificity of induction can be demonstrated by implanting different regions (color) of the
archenteron roof into early Triturus gastrulae
Regionally specific inducing action of the dorsal blastopore lip
Paracrine factor antagonists from the organizer are able to block specific paracrine factors to
distinguish head from tail
Cerberus mRNA injected into a single D4 blastomere of a 32-cell Xenopus embryo induces head
structures as well as a duplicated heart and liver
The Wnt signaling pathway and posteriorization of the neural tube (Part 2)
Model for axes
formation in amphibians
Model of organizer function and axis specification in the Xenopus gastrula