Cell Adhesion and the Extracellular Matrix

10.3 Cell Adhesion through Junctions

  • Types of junctions:
    • Gap junctions: allow direct communication between adjacent cells.
    • Tight junctions: prevent leakage of materials between cells.
    • Adherens junctions: provide cell-to-cell adhesions and connect to the actin cytoskeleton.
    • Desmosomes: connect cells through intermediate filaments, providing structural integrity.
    • Hemidesmosomes: anchor cells to the extracellular matrix (ECM).

Key Experiments in Cell Adhesion

  • Sponge Experiment: Cells sorted by species when mixed, indicating species-specific cell adhesion molecules.
  • Amphibian Embryo Experiment: Individual cells sorted into neural and epidermal tissues, suggesting tissue-specific cell adhesion molecules.

Cadherins

  • Function: Cadherin proteins are cell-to-cell adhesion molecules.
  • Specificity: Different species and tissue types produce varying cadherins, highlighting both species-specificity and tissue-specificity.
  • Mechanism: Cadherins are transmembrane proteins that link adjacent cells.
    • Extracellular Domain: Interacts with cadherins on neighboring cells.
    • Cytoplasmic Domain: Links to the cytoskeleton (microfilaments or intermediate filaments).

Integrins

  • Function: Cell-ECM adhesion molecules.
  • Composition: Dimeric transmembrane proteins that provide structural integrity to tissues under stress.
  • Mechanism:
    • Extracellular Domain: Binds to ECM proteins.
    • Cytoplasmic Domain: Links to cytoskeletal components (microfilaments or intermediate filaments).

Extracellular Matrix (ECM)

  • Importance: Provides structural support to tissues; contains various proteins including collagens, elastins, and glycoproteins.
  • Specific Structure: Varies between tissue types but consistently includes elastic fibrous proteins and proteoglycans that form a gel-like network.

Cellular Junction Types and Functions

  • Anchoring Junctions:
    • Example: Adherens junctions contribute to cell-cell adhesion via cadherins and microfilaments.
    • Example: Desmosomes provide structural integrity through cadherins and intermediate filaments.
    • Example: Hemidesmosomes anchor to ECM using integrins.
  • Barrier Junctions:
    • Example: Tight junctions create impermeable layers to prevent substance movement between cells, crucial in epithelial tissues.
  • Communicating Junctions:
    • Example: Gap junctions allow passage of ions and small molecules (e.g., glucose, Ca²⁺) for rapid communication between cells.

Cell Adhesion and Cancer

  • Cancer Basics: Increased cell division leading to tumor formation.
    • Benign Tumors: Localized and non-invasive.
    • Malignant Tumors: Invasive and spread to other tissues (metastasis) through modified integrins, implicating changes in cell adhesion properties.
  • Metastasis Process: Involves detachment and migration of cancer cells, which may cross basal lamina.

Summary of Important Junctions and Proteins

  • Cell-Cell Adhesion: Cadherins (e.g. adherens junctions, desmosomes).
  • Cell-ECM Adhesion: Integrins (e.g. hemidesmosomes, focal adhesions).
  • Barrier Functions: Tight junctions prevent movement of substances.
  • Communication: Gap junctions facilitate intercellular communication.