Tight junctions are regions of the plasma membrane surface between two cells where there is very close association resulting in a virtual impermeability to fluid.
It is usually found in the apicolateral part of contiguous cells where it forms a 'collar' around the entire circumference. With electron microscopy, the membrane at this point is seen as a trilaminar structure; the intercellular space is virtually obliterated and integral membrane lipids span the gap in regular arrays.
The presence of tight junctions in a sheet of cells e.g. within the bladder urothelium or the colonic epithelium, results in fluid being excluded from passage between apical and basal aspects of cells. The number of tight junctions are directly proportional to the 'leakiness' of a membrane. Therefore, tight junctions permit cells to regulate closely the type and number of ions and molecules traversing the tissue by a cytoplasmic route.
The tight junction is one part of the cellular junctional complex.
Last reviewed 01/2018