Zoology

• Amphibian development starts with fertilization
• Fertilization


  Can occur anywhere in the animal hemisphere of the amphibian egg
• Amphibian egg


  • Contains the animal and the vegetal pole, the yolk is mostly in the vegetal pole due to gravity
  • Fertilization only occurs in the ANIMAL HEMISPHERE of animal pole
• Point of sperm entry


  Establishes the dorsal-ventral axis, with the point of sperm entry being the ventral side and the opposite being the dorsal side
• Cortical rotation

  1. Sperm enters the egg

  2. Gray crescent forms as cortical cytoplasm rotates 30 degrees with respect to internal cytoplasm

  3. Centrioles assemble microtubules into parallel array, dividing cytoplasm into cortical and vegetal

  4. Cortical cytoplasm in animal hemisphere is pigmented with melanin to protect from UV rays
• Cleavage


  1. First cleavage is meridional, bisecting the gray crescent

  2. Second cleavage is also meridional, perpendicular to first

  3. Third cleavage is equatorial, pushed in at animal pole due to yolk in vegetal pole

  4. Cells are called blastomeres, micromeres and macromeres

  5. Cleavage pattern is holoblastic unequal

  6. 16-64 cell stage is called morula

  7. E-cadherin proteins mediate blastomere adhesion, can be disrupted by antisense oligonucleotides

  8. 128 cell stage is called blastula, with blastocoel forming
• Blastocoel


  Allows change in shape for cell migration during gastrulation, and prevents premature interaction of cells in roof and base
• Mid-blastula transformation


  Prepares embryo for gastrulation by activating the entire genome, triggered by events like promoter demethylation and transcription factor binding (e.g. VegT)
• Gastrulation


  1. Leads to formation of 3 germ layers: endoderm, mesoderm, ectoderm

  2. Involves vegetal rotation and invagination of bottle cells to form blastopore lip

  3. Prechordal plate cells involute first, carrying pharyngeal endoderm anteriorly and expressing goosecoid, hex, and cerberus genes

  4. Chordamesoderm expresses Xbra/Brachyury gene for notochord formation

  5. Involution extends to ventral side, forming lateral and bilateral lips

  6. Epiboly supplies cells through division and intercalation, aided by fibronectin assembly
• Germ layer specification


  • VegT mRNA allows transcription of SOX17 gene for endoderm specification
  • Nodal paracrine factors signal neighboring cells to express Eomesodermin for mesoderm specification
  • Low VegT concentration leads to ectoderm specification
• Xenopus laevis (African clawed frog) has few genes active during early cleavage, with mid-blastula transition occurring around 12th cleavage furrow