What are occluding junctions?Also what are its other names

Occluding junctions, also known as tight junctions or zonulae occludentes are the closely associated areas of two cells whose membranes join together forming a impermeable barrier to fluid. It is a type of junctional complex present only ion vertebrates. Corresponding junctions which occur in invertebrates are septate junctions.

Occluding junctions are composed of a branching network of sealing strands, each strand acting independently from others. Thus, efficiency of junction in presenting ion passage increases exponentially with number of strands. Each strand is formed from a row of transmembrane proteins embedded in both plasma membranes, with extracellular domains joining one another directly. Though more proteins are present, major types are claudins and occludins. These associate with different peripheral membrane proteins like ZO-1 located on intracellular side of plasma membrane, which anchor strands to actin component of cytoskeleton. So, tight junctions join together the cytoskeletons of adjacent cells.

They perform vital functions

They hold cells together

Barrier function, which can be further subdivided into protective barriers and functional barriers serving purposes like material transport and maintenance of osmotic balance:

Tight (Occluding) Junctions help to maintain polarity of cells by preventing lateral diffusion of integral membrane proteins between apical and lateral/basal surface, allowing specialized functions of each surface to be preserved (for example: receptor-mediated endocytosis at apical surface and exocytosis at basolateral surface). This aims to preserve transcellular transport.

Tight Junctions prevent passage of molecules and ions through space between plasma membranes of adjacent cells, so materials must actually enter cells in order to pass through the tissue. Investigation using freeze-fracture methods in electron microscopy is ideal for revealing lateral extent of occluding junctions in cell membranes and has been useful in showing how tight junctions are formed. Constrained intracellular pathway exacted by occluding junction barrier system allows precise control over which substances can pass through a particular tissue. (Occluding junctions play this role in maintaining blood-brain barrier). At present time, it is unclear whether control is active or passive and how these pathways are formed.

In human physiology, there are two main types of epithelia using distinct types of barrier mechanism. Dermal structures like skin form a barrier from many layers of keratinized squamous cells. Internal epithelia more often rely on occluding junctions for their barrier function. This kind of barrier is formed by only one or two layers of cells. It was long unclear whether occluding cell junctions also play any role in barrier junction of skin and similar external epithelia but this is indeed the case.

Epithelia are classed as ‘tight’ or ‘leaky’, depending on ability of occluding junctions to prevent water and solute movement.

Occluding epithelia have tight junctions which prevent most movement between cells. Examples of occluding epithelia include distal convoluted tubule, collecting duct of nephron in kidney and bile ducts ramifying through liver tissue.

Leaky epithelia do not have these occluding junctions or have less complex occluding junctions, for instance, occluding junction in kidney proximal tubule, a very leaky epithelium, has only two to three junctional strands and these strands exhibit infrequent large slit breaks.