Paracellular transport is the transport of molecules around or between cells (Fig. 6.1). Tight junctions or similar interconnections exist between cells. At the level of tight junctions, cell membranes are brought into extremely close apposition, but are not fused, so as to occlude the extracellular space (Fig. 6.2). Because proteolytic activity is thought to be deficient in the paracellular space (in contrast to the presence of cytosolic enzymes in the transcellular space), the investigation of paracellular transport for hydrophilic compounds in general and for peptides and proteins in particular has recently become of great interest (Fasano, 1998). However, most of the commonly used absorption enhancers improve both the transcellular and paracellular pathways, albeit in different ratios (Junginger and Verhoef, 1998).
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