Xenopus laevis

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36 chromosomes, 3.1x109bpancestrally tetraploid

The African Clawed Foot Toad

4 hr

6 hr

10 hr

19 hr

32 hr

110 hr

1-2 yearsThe

Xenopus Life

Cycle

Oogenesis

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Ovary filled with eggs

Animal pole

Vegetal poleFemale Xenopus

1-2 mm

triggered by human chorionic gonadotropin

The METRO

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Oct60

VegT

(MEssage TRansport Organizer)organizes

mRNA localizations for Dorsal-Ventral Specification

FertilizationQuickTime™ and a

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In vivo

In vitro

Sperm entry point

Ca++ indicator

Movie

Gray Crescent

The first cleavage occurs along the plane of cortical rotation

Nieuwkoop Center/

Spemann Organizer

A P

D

V

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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

Gastrulation:Involution

and Epiboly

EctodermMesodermEndoderm

Archenteron

Gastrulation layers the embryo

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Ectoderm Mesoderm Endoderm skin notochord lining of gut brain muscles lining of lungs spinal cord blood lining of bladder all other neurons bone liver sense receptors sex organs pancreas

Movie

Inductive signals

Factor Type Source Effect

Veg-1 TGF-Beta Vegetal Cytoplasm

Mesoderm Induction

And Embryonic Patterning

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

Frizbee Spemann Organizer Binds XWnt-8

Induction Signaling molecules

Veg1 (TGF-B) signaling

The Roles of the Nieuwkoop Center and the Spemann Organizer

TGF-Beta/SMAD Signaling

Wnt ligands (Wg)

Frizzled G-protein coupled receptors (DFz2)

Disheveled (dsh) (signaling intermediate)

Glycogen Synthase Kinase 3 (GSK3)

Beta-catenin (Arm)(transcription factors)

New gene activity

The Wnt Pathway

GSK3

PAxin

APC

GSK3

A Key Signaling Pathway in Regulatory Development and Cancer

scaffold

adenomatous polyposis coli

Sending the Wrong Signals

-Veg 1, VegT WT

Failure of Gastrulation(Ectodermal ball)

Dorsalization

Ventralization

-Axin(activates)

-Beta catenin(inactivates)

Wnt

TGF-Beta

FGF (Fibroblast Growth Factor)

FGFOverexpression

Normal

FGF Depletion

Is also Involved in Mesodermal

Induction

MyoD, a myogenic regulator

A Downstream

Master Switch

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GastrulationNeural tube closureTail bud formation

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.