Gap and tight junctions in liver: Structure, function, and pathology

This chapter is an update that integrates new findings on both tight junctions and gap junctions that have emerged. Tight junctions surround the bile canaliculi, where they seal the paracellular spaces between hepatocytes, regulating movement of solutes, ions, and water through the extracellular space, and act as a fence to block lateral diffusion of membrane-embedded molecules between basolateral and apical membrane, establishing and maintaining cellular polarity. Gap junctions are found in the basolateral membrane nearby the tight junctions. In contrast to the linear strands of intramembrane particles that form tight junctions, gap junction particles are clustered together to form discoid areas or gap junction plaques. The dependence of gap junction regulation on cytoskeletal components such as microtubules and microfilaments presents a mechanism for directed trafficking within the cell. The extent to which signals spread in the liver depends on the number of open gap junction channels.