ABSTRACT
Gap junctions are strongly adhesive junctions of porous construction that enable cells to communicate/signal directly with each other. Gap junctions consist of closely packed arrays of hexameric protein channels that provide direct inter-cytoplasmic continuity in cells (Figure I.1). The term “gap” is a historic misnomer and derives from observations that lanthanum salts revealed a narrow 2 nm extracellular space between attached cells now known to be bridged by the component channels; at the time, these studies identiŸed important morphological and functional differences between gap and tight junctions. In reality, gap junctions underpin cell−cell cooperation that is essential for orchestrating metabolic, electrical, and mechanical cell behavior in tissues and organs. A large number of channel units are compressed into a plaque in the membrane, but this does not necessarily mean that all channels are open and functional. Thus, small gap junction plaques size found in components of immune and other systems need not compromise the extent, and the importance of intercellular communication for electrophysiological evidence shows that as few as 20% of the channel structures in a large gap junction are active, with these being mainly those newly recruited to the edges of junctional plaques (Gaietta et al., 2002). Gap junction channels are weakly selective and allow small molecules and ions (generally up to 1200 Da) to pass from cell to cell while restricting the passage of larger molecules that might compromise cellular individuality.
