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Tight junction

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Image:Cellular tight junction.svg

Image:Tight junction blowup.jpg

Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtual impermeable barrier to fluid. It is a type of junctional complex. They perform three vital functions:

They hold cells together
They block the movement of integral membrane proteins between the apical and basolateral surfaces of the cell, allowing the specialized functions of each surface (for example receptor-mediated endocytosis at the apical surface and exocytosis at the basolateral surface) to be preserved.
They prevent the passage of molecules and ions through the space between cells. So materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides control over what substances are allowed through. (Tight junctions play this role in maintaining the blood-brain barrier.)

They are formed by claudin and occludin proteins, joining the cytoskeletons of the adjacent cells.

Epithelia are classed as 'tight' or 'leaky' depending on the ability of the tight junctions to prevent water and solute movement: tight epithelia have tight junctions that prevent most movement between cells, leaky epithelia do not.

An example of a tight epithelium is the distal convoluted tubule, part of the nephron in the kidney. An example of a leaky epithelium is the proximal convoluted tubule.