Hepatocyte Gap Junctions: Metabolic Regulation and Possible Role in Liver Metabolism
Over the past three decades, it has become clear that cells of most tissues communicate through specialized intercellular structures called gap junctions(1) which have also been termed nexus or maculae communicantes. One of the most exhaustively studied examples of this type of intercellular communication is that between hepatocytes, which is emphasized in this chapter. In electron micrographs of thin sections, gap junctions are seen as specialized regions of contact where apposed plasma membranes of adjacent cells are separated by a gap of 2–3 nm (Fig. 1A). In electron micrographs of freeze fracture replicas, hepatocyte gap junctions show arrays or plaques of 8.5–9.5 nm intramembrane particles cleaving with the P face (Fig. 1B); complementary pits appear on the E face. In the center of the fractured particles a dimple is commonly discernible that presumably represents the central aqueous lumen of the gap junction channel. Channels of isolated gap junctions can form a regular hexagonal array, and application of Fourier transform techniques reveal substructure in which each hemichannel or con-nexon is made up of six subunits.(2) The hemichannels or connexons crossing each plasma membrane protrude into the extracellular gap, where they connect to those in the other half of the junction to form the complete aqueous channel(3) (Fig. 1C).
KeywordsCouple Cell Junctional Conductance Freeze Fracture Replica Hepatocyte Plasma Membrane Appose Plasma Membrane
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