= Tight junction
A <tight junction> is a belt-like <cell junction> near the apical boundary of many <epithelial cells>. Interacting strands of <membrane proteins> regulate passage between neighbouring <cells>, while helping keep <apical-basal polarity> in membrane components in different domains. A <tight junction> can be selectively permeable to <ions> and small solutes; tight does not mean universally impermeable. Its transport function must be measured rather than inferred solely from an apparently continuous contact. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 19, Tight Junctions Form a Selective Permeability Barrier Across Epithelial Cell Sheets]{ref}
To test strand assembly, investigators expressed <claudin>-1 or <claudin>-2 in <fibroblasts> lacking <tight junctions>. Freeze-fracture <electron microscopy> revealed networks of strands at the new contacts, and coexpressed <occludin> was recruited into the <claudin>-based structures. This gain-of-function experiment identified major structural components, rather than merely finding <proteins> near an existing junction. <A single gene product, claudin-1 or -2, reconstitutes tight junction strands and recruits occludin in fibroblasts by Mikio Furuse and others>[1998, abstract and freeze-fracture replica experiments]{ref}
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