Some
claudins form
tight junction-associated pores that allow paracellular ion transport. The tight junctions have a net negative charge, and are believed to preferentially transport positively charged molecules. Tight junctions in the intestinal epithelium are also known to be size-selective, such that large molecules (with molecular radii greater than about 4.5
Å) are excluded. Larger molecules may also pass the intestinal epithelium via the paracellular pathway, although at a much slower rate and the mechanism of this transport via a "leak" pathway is unknown but may include transient breaks in the epithelial barrier. Paracellular transport can be enhanced through the displacement of zona occludens proteins from the junctional complex by the use of permeation enhancers. Such enhancers include medium chain fatty acids (e.g. capric acid), chitosans, zona occludens toxin, etc. == References ==