Diagram of Tight junction.
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 only present in vertebrates. The corresponding junctions that occur in invertebrates are septate junctions.
Structure
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 located on the intracellular side of plasma membrane which anchor the strands to the actin cytoskeleton. Thus, tight junctions join together the cytoskeletons of adjacent cells.
Complexity of Tight Junction Structure
Functions
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. This aims to preserve the transcellular transport.
- 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.) Currently 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 1.
Classification
Epithelia are classed as 'tight' or 'leaky' depending on the ability of the tight junctions to prevent water and solute movement:
- Leaky epithelia do not have these tight junctions or have less complex tight junctions. For instance, the tight juction in the kidney proximal tubule, a very leaky epithelium, has only two - three junctional strands and these strands exhibit infrequent large slit breaks.
References
- ^ Guo, Weinbaum and Weinstein. A dual-pathway ultrastructural model for the tight junction of rat proximal tubule epithelium
See also
TEM of negatively stained proximal convoluted tubule of Rat kidney tissue at a magnification of ~55,000x and 80KV with Tight junction. Note that the three dark lines of density correspond to the density of the protein complex, and the light lines in between correspond to the paracellular space.
External links
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