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
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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
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Early Vertebrate Developement
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Early Vertebrate Developement
Early Vertebrate Developement
Early Vertebrate Developement