EM 069 Junctional Complex

Intestinal Epithelium

A powerful method to visualize intracellular junctions is freeze-fracture electron microscopy, which splits membranes through the hydrophobic core of the lipid bilayer.

The apical region of two adjacent epithelial cells in the simple columnar epithelium that lines the small intestine is shown. Two distinct membrane faces are exposed.

• Epithelial Cell, left (tan) - shows the Protoplasmic Face (P-face)
  • Inner layer of the cell membrane facing the cytoplasm
  • Contains many intramembranous proteins that appear as bumps
• Epithelial Cell, right (brown) - shows the Extracellular Face (E-face)
  • Outer layer of the cell membrane facing the extracellular space
  • Fewer intramembranous proteins appear as indentations or pits

The apical surface of the epithelial cells is covered by microvilli that dramatically increase the surface area to absorb nutrients from the intestinal lumen.

The intracellular junctions are described on the next page.

Junctional Complex

A closely associated group of intercellular junctions found near the apical region of epithelial cells is known as the junctional complex. Together, they regulate permeability, maintain tissue integrity, and allow communication between neighboring cells.

• Tight Junctions (zonula occludens) - immediately beneath the microvilli
  • Continuous network of anastomosing ridges in the P-face (yellow) and grooves in the E-face (orange)
  • Seals the space between adjacent cells, preventing passage of water, ions, molecules, proteins, and pathogens
  • Helps maintain cell polarity by restricting the movement of membrane proteins between the apical and basolateral surfaces
• Adherens Junctions (Zonula Adherens) - found just below the tight junctions
  • Not visible along the folded plasma membranes between these two cells
  • See EM 298 Junctional Complex
• Desmosomes (Macula Adherens) - found further down the lateral surface
  • Not visible in the broken and folded cell membranes between the two cells)
  • See EM 298 Junctional Complex

Gap Junctions

Also visible are Gap junctions (#1,#2; red), which are specialized intercellular connections made of protein channels called connexons. These align between adjacent epithelial cells to create continuous aqueous pores.

• Allows the rapid and regulated exchange of ions, second messengers, and small metabolites
• Play a crucial role in coordinating processes across the small intestine epithelium, such as synchronized contraction, secretion, and cellular homeostasis