Zonula occludens are located in which epidermal layer?

• A. Stratum corneum
• B. Stratum granulosum
• C. Stratum spinosum
• D. Stratum basal

Answer: (B)

Zonula occludens are tight junctions that seal the space between neighboring cells to control paracellular diffusion and water loss. In the epidermis, these seals are formed mainly in the stratum granulosum, the layer where keratinocytes are differentiating and releasing lipid-rich lamellar bodies to build the skin’s waterproof barrier. This placement aligns with the need to create a robust barrier just before cells become the dead, keratin-filled cells of the stratum corneum. The other layers differ in the junction types they primarily use: the basal layer relies on hemidesmosomes to attach to the basement membrane, the spinous layer uses desmosomes for strong cell–cell adhesion, and the corneum consists of dead cells with a lipid matrix rather than active junctions.