Introduction to epithelium

• Describe the major functions of epithelia

  • Barriers and linings: separates tissues
  • Transport: allows selective secretion and absorption across tissues
• Describe the different epithelial barriers

  Sheets of epithelial cells are linked by junctional complexes, these include:

  • Tight junctions
  • Adherens junctions
  • Desmosomes
  • Gap junctions: allow small molecules and ions to move between epithelial cells
• Describe paracellular transport

  This is when molecules move between cells via junctional complexes. This cannot occur in tight epithelia with tight junctions, such as the stomach and colon, which prevents microbes from entering the circulatory system. However, leaky epithelia allows a high rate of paracellular transport, such as the small intestine, which reabsorption and secretion.
• Describe claudins

  Claudins determine junctional permeability:

  • Claudin 1: tight epithelia
  • Claudin 2: allows transport of monovalent cations
  • Claudin 10a: allows movement of anions
  • Claudin 10b: allows transport of cations
  • Claudin 16: allows movement Mg2+ and Ca2+

  Claudin 10a, 10b and 16 are all found in the kidneys.
• Describe the main functions of junctional complexes

  1. Barrier: maintains mechanical integrity of epithelium
  2. Permeability: paracellular pathway
  3. Fence: maintain polarity by separating apical and basolateral membranes
• Describe transcellular transport

  This is when a substrate is transported through a cell and therefore must cross the plasma membrane. Lipid soluble molecules can cross via simple diffusion, however water-soluble molecules must cross via mediated transport.
• Describe the 3 transport proteins found in majority of cells

  1. Na-K ATPase: maintains negative membrane excitability
  2. Ion channels: K+ ion channel
  3. Na-2Cl-K co-transporter