EM 079 Brush Border

Brush Border

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 an epithelial cell in the simple columnar epithelium that lines the small intestine is shown. This method reveals the two distinct membrane faces of the plasma membrane.

• Protoplasmic Face (P-Face; tan)
  • Inner layer of the cell membrane facing the cytoplasm
  • Rich in integral membrane proteins, visible as bumps or particles
• Extracellular Face (E-Face; brown)
  • Outer layer of the cell membrane facing the extracellular space
  • Fewer integral membrane proteins, visible as indentations or pits, resulting a smoother appearance

Tight junctions (zona occludens) form a 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

The apical surface \is covered by densely packed microvilli (blue), collectively forming the brush border. They dramatically increase the surface area to absorb nutrients from the intestinal lumen.

Subcellular structures:

• Membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum; grayscale)
• Cytoplasm (green)