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Tight junctions are intercellular protein complexes that form continuous belt-like seals at cell–cell contacts in epithelial tissues, preventing paracellular movement of water, ions, and solutes between cells. In the epidermis, tight junctions are assembled in the stratum granulosum, where they constitute the paracellular barrier – the second of the two independent barrier systems operating in skin, alongside the stratum corneum lipid bilayer. ^[1]

The epidermal TJ complex consists of:

• Claudins – the primary sealing proteins; claudin-1 provides paracellular water retention; claudin-4 governs ion selectivity; leaky claudins such as claudin-2 increase paracellular permeability when aberrantly expressed
• Occludin – a four-pass transmembrane protein that contributes to TJ strand integrity and barrier regulation; its loss increases permeability without fully disrupting TJ strand architecture
• ZO-1 and ZO-2 – intracellular scaffold proteins of the zonula occludens family that anchor claudins and occludin to the actin cytoskeleton, maintaining TJ complex geometry and stability
• JAM proteins (junctional adhesion molecules) – contribute to TJ assembly and cell–cell adhesion initiation during TJ formation

The TJ barrier governs the paracellular route of trans-epidermal flux; the SC lipid bilayer governs the transcellular route. Neither substitutes for the other – claudin-1 knockout demonstrates that an intact SC cannot compensate for absent TJ function. ^[1] In atopic dermatitis and type 2 inflammatory states, TJ integrity is disrupted through cytokine-driven suppression of claudin-1 and occludin, increasing paracellular permeability and allergen penetration independently of SC lipid status.

→ See Claudin-1 for the detailed mechanism of TJ barrier dysfunction and its clinical implications.