Figure 5: Cx36 mediates the direct coupling of the insulin-producing β-cells. (a): transmission electron microscopy shows that the membranes of adjacent β-cells, identified by their content in typical insulin-containing granules, are usually separated by an electronlucent extracellular space, which is markedly narrowed at a gap junction plaque (arrow heads). (b): freeze-fracture electron microscopy of such a site reveals clusters of Cx36 channels (arrow heads) within the bilayer of a β-cell membrane. The arrows point to short tight junction fibrils, which are closely associated to the connexons. Bar: 250 nm in (a), and 120 nm in (b).