Developmental Biology Lecture 4

Cellular Communication During Development

  • Importance of Cell Communication

    • Cells must communicate effectively during development as organs are formed from different layers of cells.

    • Communication is enabled by signals produced by one group of cells and received by another group.

  • Induction

    • Definition: A process through which one group of cells alters the behavior of an adjacent group of cells.

    • Two Main Components of Induction:

    1. Inducer: The tissue that produces signals.

    2. Responder: The tissue that has its behavior altered by the inducing signals.

Competence in Tissues

  • Responder Limitations

    • Not all types of tissues can act as responders to inductive signals.

    • An inductive signal is effective only on competent cells.

  • Competence

    • Definition: The ability of a cell to respond to a specific inductive signal.

    • Example: In the case of Xenopus lens induction, certain genes determine competence.

Factors Contributing to Competence

  • Gene Expression's Role

    • Cells gain competence through a specific combination of genes that they express.

    • Competence Factor:

    • Definition: Specific genes required for a tissue to respond to induction signals.

    • Example: Pax6, a competence factor involved in rat lens induction.

    • Key Question: Is Pax6 required in the induced tissue for successful lens development?

Experimental Findings Regarding Pax6

  • Experimental Observations

    • Only the Pax6 mutant ectoderm results in the loss of the lens.

    • Experimental groups:

    • Wild-type vs. Pax6- / Pax6- comparisons show that Pax6 is critical for lens induction.

    • Lens induction with the presence of the optic vesicle influences the competence of ectodermal tissues.

Cascades of Induction

  • Mechanism of Induction

    • A cascade of induction processes makes ectoderm competent for lens formation, with Pax6 being the final factor needed prior to induction by the optic vesicle.

    • Stages of Induction:

    1. Pharyngeal endoderm and heart-forming mesoderm initiate interactions.

    2. The anterior neural plate becomes involved in the next stage.

    • Key Inducing Molecules:

    • BMP4 and FGF8 are identified as important inducing factors.

Inductive Interactions

  • Sequential Induction Events

    • Upon induction, tissues can also become inducers themselves:

    1. Lens is induced by the optic vesicle.

    2. The lens induces the optic vesicle to form the optic cup, resulting in two layers: pigmented retina and neural retina.

    3. The lens additionally induces the overlying ectodermal cornea.

  • Differentiation Events

    • The process of differentiation involves secretion of collagen, which guides neural crest cells to complete differentiation.

    • A third signal is necessary to dehydrate the cornea, making it transparent.

Types of Tissue Interactions

  • Two Types of Interactions:

    1. Instructive Interaction:

    • A specific signal is essential for a cell to initiate new gene expression.

    • Characteristics:

      • Tissue B develops in the presence of tissue A.

      • If tissue A is absent, tissue B does not get induced.

      • Replacing tissue A with another does not induce tissue B.

    1. Permissive Interaction:

    • The surrounding environment does not provide additional information but allows cells to continue their existing expression.

    • Example: Use of a solid substance with fibronectin and laminin to permit cell growth.

Epithelial-Mesenchymal Interactions

  • Critical Role of Interactions

    • Organs typically consist of mesenchymal layers interacting with epithelial layers.

    • Epithelial cells can originate from any cell layer and are arranged in specific formations (e.g. sheets or tubes).

    • Mesenchymal layers can be derived from the mesoderm or neural crest.

Modes of Induction Based on Studies

  • Induction in Kidney and Tooth Development

    • Induction mechanisms can vary:

    • Some induction relies on soluble factors that diffuse through membranes.

    • Others require direct cell-to-cell contact.

    • Paracrine Interactions:

    • Defined as soluble factors acting over short distances, crucial for signaling.

    • Paracrine Factors:

      • Organisms utilize a small set of paracrine factors to promote organ development.

      • Paracrine factors are categorized into four major families.

Mechanism of Action of Paracrine Factors

  • Interaction with Receptors

    • Paracrine factors exert their influence by binding to specific receptors on responding cells.

    • Process of Signal Transduction:

    1. Paracrine factors bind to receptors fostering a conformational change on the extracellular side of the receptor.

    2. This alteration induces a change in the cytoplasmic domain.

    3. Activates enzymatic activity, predominantly kinases beginning the signaling cascade.

Receptor Tyrosine Kinase (RTK) Pathway

  • Components of RTK Pathway:

    • Cell surface receptors that participate with ligands like FGF, EGF, and platelet-derived growth factors.

    • Specific RTK pairs with a limited set of ligands.

    • Activation Sequence:

    1. Ligand binding induces dimerization of RTK.

    2. Results in autophosphorylation of the cytoplasmic domain.