Tight junction

Tight junctions (Zonula occludens)

Definition: Specialized contacts that occur at the very apical end of the junctional complex between adjacent epithelial cells. The adjoining membranes make contact at intermittent points, where integral proteins of the two adjacent membranes meet.

Tight junctions are specialized sites of contact that block solutes from diffusing between the cells in an epithelium.

• Some times called occlunding junctions.
• Organizing junction: holds cell together such that materials pass through but not between the cells
• Belts of proteins that close extracellular space between cells
• Prevent passage of water and water soluble substances
• Account for electrical resistance across epithelia
• Isolate parts of plasma membrane (apical and basolateral)
• Completely encircle polarized cells

• Look like honey comb
• Example-Junction between epithelial cells in the gut.
  
  
  Molecular Structure of Tight junctions
  
  • Claudins (membrane proteins) zip two membrane together
  • stabilized by spectrin
  • Connected to spectrin by adapter proteins ZO1 and ZO2.
  Regulation of tight junction
  • The tightness varies according to the barrier needs
  • Leaky epithelia where there is need for some traffic
  • Hormones-Vassopressin
  • Cytokines
  • Lack of ATP causes leak
  • Extravasating leukocytes open tight junctions