What are also known as tight junctions?

• A. Zonula occludens
• B. Zonula adherens
• C. Macula adherens
• D. Gap junctions

Answer: (A)

Tight junctions form a seal between neighboring epithelial cells at the apical surface, creating a belt-like barrier that blocks paracellular diffusion of substances. This sealing function is why they’re called zonula occludens—“zonula” for a belt and “occludens” for closing off the space between cells. They also help maintain cell polarity by restricting the movement of membrane proteins between the apical and basolateral surfaces. The other junctions do different jobs: zonula adherens (adherens junctions) connect to actin filaments to provide mechanical cohesion; macula adherens (desmosomes) anchor intermediate filaments for strong adhesion; gap junctions form channels that permit direct cytoplasmic exchange of ions and small molecules. So the term for tight junctions is zonula occludens.