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The two pathway model. Idealized data show the permeability of noncharged polyethylene glycol molecules of different sizes across an epithelium. The pathway for molecules below 4 Å is formed by claudin pores that are size and charge selective. The pathway for molecule above 4 Å in diameter shows no charge selectivity and appears to be defined by the status of cell-to-cell adhesion and the cytoskeleton.
