Early Vertebrate Development• Axis Determination• Amphibian Early Development: Frog• Fish Early Development: Zebra Fish

Heads or Tails? Axis specification • Common theme in early

development• Conserved mechanisms• Divergent mechanisms

Early Vertebrate Development: Xenopus

Xenopus: Cleavage• Sperm entry point:

Dorsal-Ventral (D-V) axis• Microtubule array• Cortical rotation.

Xenopus: Cleavage• Radially Symmetrical; Holoblastic• High amount of yolk in vegetal half.• Second cleavage before first finishes.• Third= equatorial• 16-64 cell stage: Morula• 128 cell stage: Blastula

Mid-Blastula Transition• DNA demethylation• H3K4 methylated (trimethylation) transcription activation• Ex. Vegetal cells endoderm. Produce paracrine factors that

induce mesoderm specification of cells above.

Xenopus: Gastrulation• Same goals as other organsims: get cells to appropriate

locations!• Movements start in Gray Crescent, cells invaginate to form

slit-like blastopore.

Xenopus Gastrulation: Involution

Vegetal Rotation: Movement of Pharyngeal endoderm above involuting mesoderm.

Involuting Marginal Zone (IMZ)

Xenopus: Gastrulation

Xenopus Gastrulation: Convergent extension

Axis specification in Xenopus: Induction• Animal-Vegetal axis

determines tissue type• A-V, D-V, and L-R axis

specification is triggered by fertilization

• Primary embryonic induction.

• Hans Spemann and Hilde Mangold- The Organizer

Axis Specification in Xenopus: Molecular Mechanism• Dorsal Signal: b-catenin, Nodal, and Vg1.• Bone Morphogenic Proteins (BMPs)– noggin– chordin– follistatin

Regional Neural Induction in Xenopus• Head inducer: Wnt

inhibitors– Cerberus– Frzb, Dickkopf– Insulin-like Growth

factors• Trunk patterning:

– Wnt and Retinoic Acid

Frogs are left handed!• Xenopus Nodal related

protein (Xnr-1) determines left-right axis

• Xnr-1 is expressed on Left side.

• Pitx2 is activated by Xnr-1 and also persists on left side. Injection of Pitx2 on right side alters gut coiling and heart placement.

Early Vertebrate Development: Danio rerio

Danio Gastrulation

Epiboly and beginning formation of notocord

Axis Formation: Danio• D-V axis: BMP/Wnt inhibitors regulated by b-catenin and

Nodal-like proteins.• Fish Nieuwkoop center = Organizer.

Danio A-V axis formation

Left-Right Axis formation in Danio

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement

Early Vertebrate Developement