Study Notes on Cell-Cell Adhesion and Tissue Composition

Introduction to Cell-Cell Adhesion and Communication

  • Individual cells must recognize, adhere, and communicate with each other to form tissues, organs, and systems.
  • Focus on cell-cell contacts and adhesion in epithelial tissues.
  • Epithelial tissue consists of sheets of cells held together by various cell junctions.

Types of Cell Junctions

Tight Junctions

  • Composed of proteins such as occludins and claudins.
  • Tight junctions seal cells together, controlling passage of molecules.
  • Visual representation through immunohistochemistry:
    • Occludins stained red.
    • Claudins stained green.
    • Overlapping areas (yellow) indicate tight junction formation.
  • DAPI stain used to visualize cell nuclei (blue).

Overview of Animal Tissue Types

  • Four main tissue types in animals:
    • Epithelial tissue
    • Connective tissue
    • Muscle tissue
    • Nervous tissue
  • Discussion limited to epithelial and connective tissues.

Epithelial Tissue

  • Forms sheets lining organs, acting as protective barriers.
  • Polarized structure with distinct regions:
    • Apical region: faces lumen of hollow organs.
    • Basolateral region: adjacent to the basal lamina and connects to underlying tissues.
  • The basal lamina is a specialized extracellular matrix produced by epithelial cells.
  • Connects to underlying connective tissue.

Connective Tissue

  • Generally more loosely organized than epithelial tissue.
  • Contains fewer cells with more extracellular matrix.
  • Cell adhesion in connective tissue primarily involves interaction with the extracellular matrix rather than adjacent cells.

Cell-Cell Adhesion Receptors

  • Cell-cell adhesion proteins fall into a few main classes:
    • Immunoglobulin Superfamily (IGSF)
    • Cadherins
    • Selectins
    • Integrins
  • Homophilic interactions: adhesion proteins on adjacent cells bind identical molecules.
  • Heterophilic interactions: adhesion proteins bind different molecules on adjacent cells.
  • Linker proteins connect transmembrane adhesion receptors to the cytoskeleton, facilitating structural integrity and signaling.

Cell Adhesion Molecules (CAMs)

  • CAMS: Cell adhesion molecules belonging to the immunoglobulin superfamily.
  • First identified CAM: NCAM (Neural Cell Adhesion Molecule).
  • Discovered as important for cell adhesion between neuronal cells; its disruption led to the understanding of its role in adhesion due to antibody interaction.
  • Characterized by organized structural loops which facilitate homophilic interactions.
  • Adhesion systems are dynamic; proteins undergo recycling and regulated expression based on cellular needs.

Cadherins

  • Cadherins are glycoproteins crucial for adhesion and recognition in cells:
    • Require calcium for stability and binding.
    • Form dimers within cells and participate in homophilic interactions.
  • Structure:
    • Transmembrane proteins with distinct extracellular, transmembrane, and cytosolic regions.
    • Extracellular region has multiple repeated domains, which bind calcium.
  • Specifically, E-cadherin found in epithelial tissue and N-cadherin found in neuronal tissue.
  • Importance in embryonic development; blocking cadherin interactions can impair tissue formation.
  • In cancer cells, reduced cadherin expression allows for metastasis by preventing cell adhesion.

Cadherin Superfamily

  • Divided into classical cadherins (like E, N, and P cadherins) and non-classical cadherins (like desmocollin and desmoglein).
  • Classical cadherins typically found in adherence junctions.
  • Non-classical cadherins are numerous in specific tissues, playing various roles.

Role of Carbohydrates in Cell Adhesion

  • Carbohydrates attached to CAMs and cadherins affect their adhesive properties.
  • Lectins: carbohydrate-binding proteins secreted by cells, promoting adhesion with specific sugars on other cells.
  • Selectins: located on the surface of cells, playing a key role in cell-cell interactions during processes like leukocyte rolling.

Leukocyte Rolling

  • Selectins facilitate the binding of leukocytes (white blood cells) to endothelial cells in inflamed tissues.
  • Cytokines trigger endothelial cells to express selectins, allowing leukocytes to roll along vessel walls.
  • Low-affinity binding allows leukocytes to adhere temporarily, aiding their migration into tissues during inflammation or injury.

Human Blood Types and Cell Surface Carbohydrates

  • Blood type determined by specific glycolipids on erythrocytes:
    • Type O: expresses neither N-acetyl galactosamine nor galactose.
    • Type B: expresses galactose.
    • Type A: expresses N-acetyl galactosamine.
    • Type AB: expresses both.
  • Antibody reactions can occur if blood types are mismatched, highlighting the significance of cell surface carbohydrates in immune responses.