Chapter 12. Frog Development

Page 1: Early Vertebrate Development

• Early vertebrate development stages

  • Frogs, tunicates, fish, and tetrapods (amphibians, reptiles, birds, mammals)

  • Notable features include notochord, vertebrae, and jointed limbs.

Page 2: Outline of Topics

• Main Topics Discussed

  • African clawed frog development

  • Cell and tissue behavior

  • Molecular causes behind development.

Page 3: Early Vertebrate Development

• Development Process

  • Begins with a fertilized egg.

  • Essential tasks:

    • Proper arrangement of germ layers

    • Determination of ventral and dorsal sides

    • Establishment of anterior-posterior axis (head and tail orientation)

  • Focus of content: Chickens and mammals, not frogs.

Page 4: Frog Cortical Rotation

• Cortical Rotation Mechanism

  • Sperm entry point sets up the dorsal-ventral axis.

    • Ventral side formed where sperm entered.

    • Sperm centriole promotes microtubule polymerization

  • Shift of cortical cytoplasm (30º) towards sperm entry point.

Page 5: Rotation of Cortical Cytoplasm

• Symmetry Changes Post-Fertilization

  • The embryo transitions to bilateral symmetry (as opposed to radial).

  • Key Features:

    • Diffuse black pigment, point of sperm entry, inner cytoplasm, shear zone, gray crescent.

Page 6: Gray Crescent

• Formation of Key Regions

  • The region devoid of darker cortical cytoplasm is the gray crescent.

  • Divided regions include:

    • Pigmented animal region

    • Gray crescent

    • Vegetal region.

Page 7: Microtubule Formation

• Microtubule Presence

  • Parallel microtubule arrays established by 70% completion of the first cell cycle.

    • Key images identified:

      • (A) 0.50

      • (B) 0.70.

Page 8: Blastula Stage Development

• Egg Development Stages

  • Various developmental stages labeled (A) to (H).

• Structure: Blastocoel forms in the embryo.

Page 9: Characteristics up to Blastula Stage

• Cell Size Variance

  • Smaller cells at the animal pole due to dense yolk at the vegetal pole.

  • Midblastula transition occurs at the 12th cell cycle.

  • The blastula stage exhibits hollow characteristics in the animal half and solid in the vegetal half.

Page 10: Early Gastrulation in Frogs

• Blastopore Formation

• Questions Explored:

  • Could blastopore form as it does in sea urchins?

  • Forms where animal and vegetal halves meet (lower gray crescent edge).

  • Blastopore shape resembles a horizontal slit, indicating the future dorsal side.

Page 11: Bottle Cells at the Blastopore

• Cellular Changes

  • Bottle cells shape change to form an indentation at the blastopore.

  • Dynamic shifts in cell structure: decreased external surface area, wider internal surface.

Page 12: Cell Migration through Blastopore

• Movement Patterns

  • Animal side cells migrate through the blastopore along the upper blastocoel surface.

  • Dorsal blastopore lip characterized by early mesodermal cell migration.

Page 13: Reiterated Cell Migration Through Blastopore

• Consistent Process

  • Re-affirmation of the migration process observed in the previous page.

Page 14: Gastrulation Comparison

• Sea Urchin vs. Frog Gastrulation

  • Comparison of dorsal mesoderm, blastocoel displacement.

  • Features named: Archenteron, mesoderm, and endoderm.

Page 15: Ectoderm Expansion

• Epiboly Process

  • Ectoderm cells spread from the animal hemisphere to cover more surface area (epiboly).

Page 16: Extended Blastopore Edges

• Cell Transition into Embryo

  • Blastopore edges extend and cells migrate into the embryo from all sides, leading to the internalization of the entire vegetal hemisphere.

Page 17: Continued Migration Process

• Internalization of Cells

  • Reiteration of the migration pattern where all cells from the vegetal hemisphere migrate inside.

Page 18: Gastrulation Video Resources

• Visual Learning

  • Gastrulation Resource 1

  • Gastrulation Resource 2

• Cellular Components: Mesenchyme, ectoderm, notochord, blastopore lips.

Page 19: Epiboly Causing Thinning of Ectoderm

• Changes in Ectoderm Layer

  • Ectoderm layer thins through cell rearrangement and division.

Page 20: Gastrulation Over Time

• Early vs. Late Stages

  • Comparison of mesoderm migration along a fibronectin layer in early vs. late gastrulation.

Page 21: Anticipated Effects of Fibronectin Blockage

• Impact of Blockage

  • If fibronectin binding is blocked, mesoderm precursor cells remain on the embryo surface.

Page 22: The Gray Crescent Importance

• Key Role in Gastrulation

  • Gray crescent presence is essential for the proper formation of the blastopore and subsequent development stages.

Page 23: Organizer Functionality of the Dorsal Blastopore Lip

• Inductive Influence

  • Dorsal blastopore lip can induce a secondary archenteron and generate additional dorsal and anterior structures based on the organizer framework.

Page 24: Induction of Secondary Structures

• Formation Mechanism

  • Description of induced structures versus primary structures during the gastrulation process.

Page 25: Continued Induction Examples

• Organizational Induction

  • Continuation of the discussion surrounding induction from the dorsal blastopore lip.

Page 26: Induction from Vegetal Side

• Induction Mechanics

  • Cells on the vegetal and dorsal sides of the blastula interact to induce the blastopore dorsal lip.

Page 27: Conversion of Presumptive Ectoderm to Mesoderm

• Microscopic Interaction

  • Factors released from vegetal cells trigger animal cap (presumptive ectoderm) conversion to mesodermal cells.

Page 28: Experiment on Organizers and Body Axis Formation

• Transplantation Under Normal Conditions

  • Points of sperm entry lead to the formation of new organizers and body axes in experimental setups.

Page 29: Data on Inductions in Different Developmental Contexts

• Induction Percentage Data

  • Quantification of induced structures based on the tier mechanism across various locations in the embryo.

• Differentiation of dorsal, intermediate, and ventral inductions.

Page 30: β-catenin Localization in Early Stages

• Localization Observations

  • Orange β-catenin presence noted predominantly on the dorsal side at the 2-cell and blastula stages.

Page 31: Nuclear β-catenin Analysis

• Comparison of Dorsal and Ventral Localization

  • Nuclear β-catenin identified exclusively on the dorsal side, absent on the ventral side throughout observation.

Page 32: Wnt Signaling Pathway Dynamics

• Wnt Signaling Process

  • Description of the absence of Wnt leading to different cellular dynamics concerning β-catenin stabilization, transcriptional activation.

Page 33: Disheveled Protein Dynamics

• Protein Movement Mechanism

  • Disheveled protein transport observed in association with microtubules, stabilizing β-catenin during cortical rotation.

Page 34: Dorsal Enrichment of Proteins**

• Key Observations

  • Dorsal concentrations of Disheveled, GBP, and β-catenin demonstrated, substantiating the signaling requirements in the dorsal region.

Page 35: Outcomes of GSK3 Inhibition

• Impact on Organizer Development

  • Injection of mRNA for dominant inactive GSK3 yields two organizers and formation of two body axes in experimental embryos.

Page 36: Gene Activation by Nuclear β-catenin

• Genomic Activation Mechanism

  • Nuclear β-catenin enhances the transcription of genes for organizer proteins, specifying mechanisms for chordin and noggin production.

Page 37: End Gastrulation Features in the Frog Embryo

• Embryonic Structuring

  • Overview of mesenchymal, ectodermal, and dorsal blastopore lip regions and their roles at the end of gastrulation.

Page 38: Neural Tube Formation

• Neural Development Overview

  • Structural formation and transverse sections indicating neural tube and associated layers during development.

Page 39: Dorsal Mesoderm Induction of Ectoderm

• Transplantation Insights

  • Ectodermal response to inductive signals and the formation of neural plate structures from ectoderm versus epidermis.

Page 40: TGF-β Signaling Pathway Dynamics

• Transcription Regulation Process

  • Interaction mechanisms of TGF-β ligands with receptors resulting in Smad activation and gene transcription.

Page 41: Effects of UV Exposure on Development

• Impact Study of UV Treatment

  • Examination of normal fertilization versus UV-irradiated embryonic development outcomes and organizer activity.

Page 42: Noggin mRNA Rescue in Embryonic Development

• Rescue Mechanism

  • Introduction of Noggin mRNA induces the formation of dorsal tissue and mitigates negative effects of UV treatment.

Page 43: Noggin mRNA Localization

• Embryonic Localization Studies

  • Observations of Noggin mRNA presence in organizer tissues.

Page 44: Chordin mRNA Localization

• Organizational Expressions

  • Illustration of Chordin mRNA within organizing tissue areas.

Page 45: BMP Signaling in Ectoderm

• Signal Interaction

  • BMP signaling induces epidermal development while organizer functions counteract BMP influences for neural tissue formation.

Page 46: Neural Fold Development under BMP Proteins Blockage

• Neural Development Conditions

  • Comparative analysis of normal embryonic development vis-a-vis BMP inhibition effects on neural structures.