EM 069 Junctional Complex

Gap Junctions

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 epithelial cells in the simple columnar epithelium that lines the small intestine is visible. Their plasma membranes have been split to reveal the Protoplasmic Face (P-face; tan) and Extracellular Face (E-face; brown) of adjacent cells.

Small gap junctions (#1,#2; red) are visible as clusters in the P-face.

• Gap junctions are made of protein subunits called connexins, which assemble into connexons
• Each connexon from one cell aligns with a connexon from the adjacent cell, forming a continuous aqueous pore between cytoplasms.
• Each cluster contains many connexons, packed in a highly ordered hexagonal array.

Gap junctions allow the direct passage of ions, metabolites, and signaling molecules (e.g., Ca²⁺, cAMP) between cells. This enables the coordination of processes across the epithelium. Such as synchronized contraction, secretion, and cellular homeostasis.

Also visible are tight junctions as a continuous network of anastomosing ridges in the P-face (yellow) and grooves in the E-face (orange).