Figure 1.

A diagram of six epithelial cells packed into a portion of an epithelium. The apical and basolateral membranes are separated by tight junctions (shown as continuous red rings joining the cells). Selective epithelial permeability involves both the transcellular and paracellular routes. In the former, solutes are transported across the apical plasma membrane, diffuse through the cytoplasm (dashed arrow), and are then retransported out of the cell into the extracellular space beneath the tight junctions. In the paracellular route, solutes move through channels in the tight junction itself (solid arrow) and gain access to the paracellular spaces without entering the cytoplasm of the cells. The small box indicates the plane of section shown in diagram in Fig. 3.