Developmental Bio Ch3

Chapter 3: Cell-Cell Communication in Development

Morphogenesis Questions

  • Key inquiries:

    • How does matter organize into the tissues of an embryo?

    • How do separate tissues originate from a population of cells?

    • What processes lead to organ formation from tissues?

    • Mechanisms behind the location-specific formation of organs and migration of cells to their destinations.

    • Coordination of organ and cell growth throughout development.

    • Regeneration capabilities of certain organs (e.g., skin and intestine).

    • Achieving polarity in organs.

Types of Embryonic Cells

  • Epithelial Cells:

    • Form sheets and tubes via attachment to each other.

  • Mesenchymal Cells:

    • Migrate individually and create extracellular matrices to separate cells.

Morphogenic Variations

  • Limited range of morphogenic processes in epithelial and mesenchymal cells:

    • Direction and number of cell divisions.

Cell Adhesion and Communication

  • Three Behaviors Requiring Communication:

    • Cell adhesion

    • Cell migration

    • Cell signaling

Cell Adhesion

  • Initial hypothesis suggested uniformity in cell membranes.

  • Early embryonic development shows differential characteristics based on cell type.

  • Research Example: Townes and Holtfreter (1955) demonstrated selective cell re-aggregation in amphibians.

    • Certain cells display affinity for others; e.g., mesoderm shows positive affinity for both ectoderm and endoderm.

Selective Affinity Summary

  • Cells sort themselves based on selective affinities that change during development.

  • Tissues from later embryonic stages demonstrate variance in adhesion properties.

Thermodynamic Model of Cell Interactions

  • Differential Adhesion Hypothesis: Cell sorting influenced by thermodynamic principles.

  • Steinberg's experiment (1964): Certain combinations of cell types sort differently (centrally vs. peripherally).

  • Surface Tension: Measured in dynes per centimeter; significantly impacts cell aggregation during sorting.

Cadherins and Cell Adhesion

  • Major cell adhesion molecules:

    • Cadherins: Calcium-dependent transmembrane proteins affecting cell interactions.

    • Diverse types: E-cadherin, P-cadherin, N-cadherin, R-cadherin; each with specific roles in tissue binding and development.

  • Importance of cadherin levels:

    • Differential expression correlates with sorting and tissue organization.

Cell Migration

  • Common between epithelial and mesenchymal cells.

  • Key Concept: Polarization - determining the front and back of a migrating cell.

  • Integrins: Connect extracellular matrix to the actin cytoskeleton, forming focal adhesion points, facilitating migration.

Cell Signaling in Development

  • Regulates:

    • Adhesion

    • Migration

    • Differentiation

    • Division

  • Induction Process: Involves interaction between inducers (signal-producing cells) and responders (cells responding to signals).

Inductive Interactions

  • Instructive Interaction: Necessary signal from inducing cells initiates new gene expression.

  • Permissive Interaction: Responding cells have inherent potentials that need permissive environment to express traits.

Paracrine Factors

  • Inductive events can sometimes occur across barriers, while others require direct contact.

  • Major Families of Paracrine Factors:

    • FGF, Hedgehog, Wnt, TGF-β.

Specific Pathways

  • Fibroblast Growth Factors (FGFs): Essential in multiple developmental processes, including skin and limb formation.

  • Hedgehog Pathway: Critical in tissue boundary formation; includes Sonic hedgehog involved in various developmental signals and conditions like Holoprosencephaly.