6) Cell Junctions and Tissues

• Cell Junctions allows cells to

  1. Adhere to each other
  2. Withstand Mechanical Pressure
  3. Facilitate Cell signalling
  4. Allow Cell movement
• During tissue repair, cells detach and adhere to each other to migrate to the site of injury whey can recruit immune cells and proliferate
• Types of Junctions found in Cells:

  1. Tight Junctions
  2. Adherens Junctions
  3. Desmosomes
  4. Gap Junctions

  
• Summary of Tight Junctions:

  • Cell-Cell Attachment
  • Membrane Proteins: Occludin, Claudins
  • Adaptor Proteins: ZO
  • Link to: Actin Cytoskeleton
  • Functions: Seals gaps between cells, Maintains cell polarity and controls diffusion of macromolecules between cells
• Tight Junction Structure:

  • Cell-Cell Attachment
  • Made up of transmembrane proteins ‘Claudin’ + ‘Occludin’
  • Forms sealing strands between cells through the apical region of cells
  • Adaptor proteins (like ZO) and tight junctions act as a bridge between two cells actin cytoskeleton

  These can be found in “belts”, providing overall structural support and stability to junctions

  
• There are two main types of Cell attachments:

  1. Cell-Cell attachments (Lateral)
  2. Cell-ECM attachments (Basal)

  
• Adherens Junctions Summary:

  • Cell-Cell attachment
  • Membrane Proteins: Cadherins
  • Adaptor Proteins: p120-catenin, β-catenin, α-catenin
  • Links to: Actin filaments
  • Functions: Mediates cell movement, maintain tissue integrity
• Adheren junctions structure:

  • Composed of transmembrane protein ‘Cadherins’
  • Cadherins exhibit homophilic binding when Ca2+ are present
  • Linker proteins p120/α/β-catenin link cadherins to the actin cytoskeleton
  • When signalling pathways activate, the actin cytoskeleton contracts/relaxes, facilitating cell movement

  
• Desmosomes Summary:

  • Cell-cell attachment
  • Membrane proteins: Cadherins
  • Adaptor proteins: Plakogoblin, Plakophilin, Desmoplakin
  • Links to: Intermediate filaments (such as keratin)
  • Functions: Cell-Cell Adhesion, Tensile Strength

  
• Desmosome Structure:

  • Made up of non-classical cadherins ‘desmogleins’ + ‘desmocollins’ which exhibit heterophilic and homophilic binding.
  • extracellular domain of desmosome link cadherins to each other
  • intracellular domain of desmosomes links to the ‘intermediate filaments’ of cells using linker proteins
  • As a result, cells are indirectly connected to each other throughout the tissue, providing tensile strength

  
• Gap Junction Summary:

  • Cell-Cell attachment
  • Membrane Proteins: Connexins
  • Functions: Cell-Cell communication, facilitates passage of small, water soluble molecules, Adhesion

  
• Gap Junctions Structure:

  • Connexons (made up of Connexins) form Pores/Channels between cytoplasms
  • Allows passage of small, water-soluble molecules and ions, facilitating cell communication and additional cell adhesion
  • Gap junctions can also be opened/closed