Which of the following statements is true about zonula occludens?

• A. They form gap junctions between cells.
• B. They are always located at the basal surface.
• C. They function as the most apical tight junctions that seal the paracellular space.
• D. They are the primary components of desmosomes.

Answer: (C)

Zonula occludens are tight junctions that form a belt-like seal just below the apical surface of adjacent epithelial cells. Their main role is to seal the paracellular space, preventing unrestricted movement of ions and solutes between cells and helping establish and maintain the distinct apical and basolateral membrane domains. This apical position and sealing function are what define them as the most apical tight junctions.

They are not gap junctions, which are channels that allow direct cytoplasmic communication between neighboring cells. They are not located at the basal surface, and they are not the primary components of desmosomes, which are strong adhesion junctions that connect intermediate filaments between cells.