Early Development and Axis Formation in Amphibians

Fertilization

• Sperm entry

  • Animal hemisphere

  • Dorsal-ventral axis

    • Cells closest to sperm entry becomes ventral side

    • Cortical rotation leads to formation of gray crescent

Mid-blastula transition

• Turn on different genes in different cells

• Blastomeres can become motile

• Cell cycle gets gap phases

• Triggers

  • Demethylated promoters

  • Synthesis of new transcription factors from maternal mRNA

Ventral vs. Dorsal Structures

• Ventral

  • Blood

  • Kidneys

• Dorsal

  • Spinal cord

Gastrulation Initiation

• Dorsal side of the embryo 

• Below the equator (in the marginal zone)

• Bottle cells

  • Invaginate and involute to form blastopore opening

Formation of Dorsal lip of blastopore

• Involution of marginal zone cells

• Order of involution:

  • Pharyngeal endoderm (and bottle cells)

  • Prechordal plate (precursor of head mesoderm)

  • Chordamesoderm (will form notochord)

  • Prospective heart, kidney, ventral mesoderm (ventral and lateral blastopore lips)

Mid-Gastrulation

• Archenteron forms

  • Forms alimentary canal

  • Displaces blastocoel

• Endoderm internalized

• Mesoderm positioned in between layers

Epiboly of the Ectoderm

• Not invading marginal zone cells expand (NIMZ)

• Ectoderm covers embryo

• Yolk plug

Yolk plug 

endoderm remaining at the blastopore lip

From Gastrulation to Neurulation

• Notochord separates from mesoderms on either side

• Notochord cells elongate

Organizer Function

• Spemann-Mangold organizer

• Self-differentiate dorsal mesoderm

• Dorsalize the surrounding mesoderm into paraxial mesoderm

• Dorsalize the ectoderm, inducing formation of the neural tube

• Initiate movements of gastrulation

Determination of the Axes

• Early fate mapping

  • Animal/vegetal poles

    • Transcription factor VegT and TGF-β family paracrine factor Vg1

    • mRNAs positioned in egg prior to fertilization

• Nieuwkoop center

• Signal from vegetable pole to organizer

• Beta catenin (Wnt)

Dorsal signal: β-catenin

• Synthesized throughout embryo  from maternal mRNA

  • Accumulates in dorsal region of egg during cytoplasmic movements after fertilization and during early cleavage

  • Kinesin moves β-catenin to dorsal side after fertilization

  • Specific removal of β-catenin from ventral region

Neural Ectoderm

• Converting ectoderm to neural tissue

  • Ectoderm is defaulted to be neural

  • Signal induces epidermis to form 

    • Bone morphogenetic proteins (BMPs)

    • Ectoderm that blocks BMP binding 🡪 neural tissue

      • Inhibitors: Noggin, Chordin