tight junction

Imagine a row of boats tied so closely along their top edges that no water can slip between them. Many sheets of cells in your body act like such a leak-proof barrier: the cells lining your gut, your bladder, and the tiny blood vessels of your brain all have to keep the fluid on one side from leaking through to the other. A tight junction is the band of tightly stitched contacts, running like a belt around the top of each cell, that seals neighboring cells together so almost nothing can sneak through the gaps between them.

Up close, a tight junction is made of rows of membrane proteins — chiefly claudins and occludins — that reach out from one cell and clasp the matching proteins on its neighbor, like the teeth of two zippers meshing. These protein rows form continuous strands that completely encircle the cell near its top, welding the two membranes so closely that there is essentially no space left between them. The seal does two jobs at once: it blocks fluid from leaking through the spaces between cells (the barrier job), and it acts as a fence that keeps membrane proteins from drifting from the top surface of the cell to the sides and bottom, which lets the top and bottom of the cell stay different (the fence job).

Tight junctions are why a sheet of cells can be a true wall, not just a loose pile. They let your intestine absorb nutrients across the cells, by the controlled routes, instead of having food leak straight through the cracks; they help form the blood–brain barrier that keeps many toxins out of brain tissue. They are not perfectly impermeable, though — different tissues build tighter or leakier junctions on purpose, and some claudins even form tiny selective pores, so a tight junction is better thought of as an adjustable seal than an absolute wall.