amphibian axis - part 1

AXIS FORMATION in AMPHIBIANS

Orientation of an Embryo

• Orientation Terms:

  • Dorsal

  • Ventral

  • Anterior

  • Posterior

  • Right

  • Left

  • Caudal

  • Rostral

Introduction to Axis Formation

• Critical first step in embryogenesis that defines the layout of the embryo.

• Determines the spatial arrangement and identity of embryonic structures.

Establishing Axis in Amphibians

Early Development

• Eggs are spherical; both structural and positional properties are crucial for development.

• Oocytes are arrested in metaphase II of meiosis, prior to fertilization.

Axis Characteristics

• First Axis (Animal-Vegetal Pole):

  • Not fertilized oocytes show nuclear migration during second meiotic division.

  • Pigmentation Asymmetry:

    • Animal pole: pigmented

    • Vegetal pole: un-pigmented

Composing the First Axis

• RNA Localization:

  • Asymmetrical localization of mRNAs such as Vg1, VegT, and Wnt11.

  • Associated with the animal-vegetal (AP-VP) axis formation.

Functional Importance of RNA Gradients

• Cytoplasmic determinants influence developmental patterns.

• Major significance in early embryonic layer formation is maternally determined.

Induction Processes Through Vegetal Cells

• Essential Factors:

  • VegT acts as a transcription factor crucial for mesoderm formation.

  • Induces target genes associated with mesoderm function including nodal family.

• Oocytes depleted of VegT showing significant developmental changes at late tailbud stage (Stage 34).

Second Axis Formation: Dorso-Ventral Axis

Grey Crescent Theory

• The formation of the grey crescent post-fertilization is vital for dorso-ventral asymmetry.

• Explains initiation of gastrulation and potential shifts in cellular identity (potency).

Dynamics Post-Fertilization

• Cortical Rotation: Indicated by sperm entry, establishes the dorso-ventral axis.

• Microtubule arrangement is crucial in this process impacting inner cytoplasmic density and orientation.

Grey Crescent Functions

• Creates the Dorsal Blastopore Lip (Spemann's organizer) at the grey crescent, crucial for initiating gastrulation.

The Role of Microtubules

• Microtubules assist in establishing the orientation of embryonic development.

• Asymmetrical localization of proteins such as GBP and Dsh indicates the dorsal side of the embryo.

  • These proteins are involved in the regulation of the Wnt signaling pathway, affecting cellular development.

Dorsal Enrichment and Gene Activation

• Beta-catenin Stabilization:

  • Through cortical rotation and inhibition of GSK3, beta-catenin accumulates on the dorsal side, activating organizer genes like chordin and noggin.

• The transcription of important proteins occurs, influencing mesoderm specification and further developmental processes.

Key Points Discussed:

• Spemann's Experiments:

  • Significance of the grey crescent and its derivatives.

  • Nieuwkoop center's role in axis determination.

  • Inquiry into whether all vegetal cells are equivalent, and methods to determine equivalence.

• Beta-Catenin:

  • Localization, regulation, and the genes it activates.

• Determination of right/left and dorso-ventral axes occurs early in embryonic development.