Tight Junctions
Epithelia are sheets of cells that seperate two compartments by functioning as a selectively permiable barrier between compartments. The small intestinal epithelium, for instance, seperates the lumen of the intestine from the vascular system and allows only certain things to travel from lumen to blood. Tight junctions glue individual cells together within an epithelium, and in order to cross this sheet, molecules and ions must pass either through the epithelial cells (transcellular route) or between the cells and thus through the junctions (paracellular route). The permiability barrier of epithelial sheets is defined by two characteristics of tight junctions:
- Tight junctions prevent most molecules from crossing the epithelium between cells. Water is able to diffuse through tight junctions relatively unimpeded, but most ions and even small macromolecules are completely blocked. Simple sugars and amino acids, for example, are unable to penetrate tight junctions.
- Tight junctions contribute to maintainance of membrane asymetry in epithelial cells, which is necessary for much of the transcellular transport. In polarized epithelial cells, the composition of the plasma membrane facing one compartment is distinctly different from that facing the other compartment. These two faces are usually called apical and basolateral membranes. Tight junctions encircle the cells and function as a 'fence' to prevent diffusion of proteins in the liquid membrane between apical and basolateral compartments and thus play a major role in maintaining membrane asymetry. This is important because it allows one-way or vectorial transport of molecules through the epithelial cells. As an example, glucose is transported from the lumen of the small intestine into blood, but never from blood into the lumen. This is because glucose transporters on the apical (lumen-facing) plasma membrane transport it into the cell, while other transporters on the basolateral (blood-facing) membrane transport it out of the cell. Clearly, if both types of transporters were present on both membrane faces, glucose would not be efficiently absorbed from lumen to blood.
in polarized epithelial cells
It should be noted that not all tight junctions are functionally identical. Diversity is seen especially in the ability of different tight junctions to restrict passage of inorganic ions like sodium, where leakiness can vary by several orders of magnitude.
Structurally, tight junctions are regions of very close cell-to-cell contact that encircle cells. The plastic sheet that holds cans together in a six-pack is a crudely analagous structure. With an electron microscope, tight junctions are seen as regions of plasma membrane that appear to touch or even be fused together at intervals, usually with no discernable intercellular space. Freeze fracture preparations reveal them as an encircling net of interdigitating membranous particles. The molecular nature of tight junctions is poorly characterized.
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Last updated on November 27, 1996
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