Epithelial Transport

Question 1

Describe the basic characteristics of an epithelial cell.

Answer 1

• Epithelial cells line hollow organs and regulate the absorption and secretion of substances across these surfaces
• Characterised by tight junctions between cells that restrict passive diffusion of substances across the epithelial cell layer
• The apical membrane faces the lumen (hollow/fluid-filled chamber) and the basolateral membrane is normally close to a network of blood vessels

Question 2

List 5 examples of epithelial tissues.

Answer 2

1. Lung - growth, maintenance of pericillary layer
2. Kidney - maintenance of salt + water balance
3. Gut - absorption of nutrients, salt and water
4. Exocrine glands - secretion of fluid
5. Choroid plexus - regulation of CSF composition

Question 3

Describe the terms “primary active transport”, “passive transport”, “transcellular transport” and “paracellular transport”.

Answer 3

• Primary active transport - utilising energy in order to move ions across a membrane - e.g. Na-K-ATPase
• Passive transport - movement of ions by diffusion - i.e. passively due to concentration/electrical gradient
• Transcellular transport - movement of ions from the luminal side of the epithelium to the interstitium - i.e. across the epithelial cell
• Paracellular transport - movement of ions between adjacent epithelial cells

Question 4

Outline a method for measuring transepithelial transport in a whole organ and in isolated epithelium.

Answer 4

Whole organ:

• In vivo/in vitro water/solute transport:
  
  • Clearance methods
  • Gravimetric measurement
  • Marker infusion
• Net epithelial transport, overall function
• Little information on mechanics

Isolated epithelium:

• Ussing chambers:
  
  • Short circuit current
  • Ion flux ratios
  • Membrane permeabilisation
  • Microelectrodes
• Localisation of channels, mechanics of ion transport, representative of in vivo function?

Question 5

Explain how to identify transport processes which occur at either side of the epithelium i.e. mucosal vs luminal uptake.

Answer 5

*

Question 6

Describe the basic principles behind measurement of transepithelial transport.

Answer 6

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Question 7

Describe the difference between type I and II alveolar epithelial cells.

Answer 7

• Type I - large, squamous
• Type II - smaller, more cuboidal