EM 298 Junctional Complex

Junctional Complex

Transmission electron microscopy (TEM) of the apical region of two adjacent epithelial cells in the simple columnar epithelium that lines the small intestine. The cell on the left is shown in tan, and the cell on the right in brown.

Near the apical surface, a closely associated group of intercellular junctions forms the junctional complex. Together, they regulate permeability, maintain tissue integrity, and allow communication between neighboring cells.

It is comprised of three main types of intercellular junctions arranged in a specific sequence:

• Tight Junctions (zonula occludens; yellow) - seal the space between adjacent cells, preventing the passage of molecules and pathogens
  • Located just below the microvilli
  • Adjacent plasma membranes are brought into close contact
  • Help maintain cell polarity by restricting the movement of membrane proteins between the apical and basolateral surfaces
• Adherens Junctions (zonula adherens; red) - provide mechanical attachment between adjacent cells
  • Found just below tight junctions
  • Ribbon-like structures of cadherins that span the intercellular space and connect to actin filaments in the cytoskeleton
  • Help transmit contractile forces and maintain cohesion of the epithelial sheet
• Desmosomes (Macula Adherens) (#1, #2, and#3; purple) - spot-like adhesions with dense plaques on each cell
  • Found further down the lateral surfaces
  • Anchored to intermediate filaments (keratin) that extend into the cytoplasm
  • Provide resistance to mechanical stress

Under the light microscope, they appear as a dense, dark line at the apical lateral borders of epithelial cells, forming the terminal bar.

The apical surface of these cells is covered with microvilli, which contain core bundles of actin filaments (orange). This brush border dramatically increases the surface area for absorption of nutrients from the intestinal lumen.