<P> Tight junctions are composed of a branching network of sealing strands, each strand acting independently from the others . Therefore, the efficiency of the junction in preventing ion passage increases exponentially with the 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 . Although more proteins are present, the major types are the claudins and the occludins . These associate with different peripheral membrane proteins such as ZO - 1 located on the intracellular side of plasma membrane, which anchor the strands to the actin component of the cytoskeleton . Thus, tight junctions join together the cytoskeletons of adjacent cells . </P> <P> They perform vital functions: </P> <Ul> <Li> They hold cells together . </Li> <Li> Barrier function, which can be further subdivided into protective barriers and functional barriers serving purposes such as material transport and maintenance of osmotic balance: <Ul> <Li> Tight Junctions help to maintain the polarity of cells by preventing the lateral diffusion of integral membrane proteins between the apical and lateral / basal surfaces, 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 . This aims to preserve the transcellular transport . </Li> <Li> Tight Junctions prevent the passage of molecules and ions through the space between plasma membranes of adjacent cells, so materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue . Investigation using freeze - fracture methods in electron microscopy is ideal for revealing the lateral extent of tight junctions in cell membranes and has been useful in showing how tight junctions are formed . The constrained intracellular pathway exacted by the tight junction barrier system allows precise control over which substances can pass through a particular tissue . (Tight junctions play this role in maintaining the blood--brain barrier .) At the present time, it is still unclear whether the control is active or passive and how these pathways are formed . In one study for paracellular transport across the tight junction in kidney proximal tubule, a dual pathway model is proposed: large slit breaks formed by infrequent discontinuities in the TJ complex and numerous small circular pores . </Li> </Ul> </Li> </Ul> <Li> They hold cells together . </Li>

Tight junctions are unique to which of the following