Molecular structure of the macula adherens (desmosome). a. Electron micrograph of a macula adherens, showing the intermediate filaments (arrows) attaching into a dense intracellular attachment plaque located on the cytoplasmic side of the plasma membrane. The intercellular space is also occupied by electron-dense material (arrowheads) containing desmocollins and desmogleins. The intercellular space above and below the macula adherens is not well defined because of extraction of the plasma membrane to show components of this structure, x40,000. (Courtesy of Dr. Ernst Kallenbach.) b. Schematic diagram showing the structure of a macula adherens. Note the intracellular attachment plaque with anchored intermediate filaments. The extracellular portions of desmocollins and desmogleins from opposing cells interact with each other in the localized area of the desmosome, forming the Cadherin "zipper."
fluorescent dye is injected with a micropipette into one cell of an epithelial sheet. The readily visualized dye can be seen to pass to the immediately adjacent cells. These experiments confirm that adjacent cells share communicating channels that allow small molecules and ions to pass directly between cells without entering the extracellular space.
Gap junctions reduce resistance to passage of electric current between adjacent cells
Electrical conductance studies of gap junctions involve the introduction of microelectrodes into neighboring cells and the establishment of a voltage difference between the electrodes. Current flow between the cells is then measured. If no gap junctions are present between the neighboring cells, the current flow is low, primarily because of the high electrical resistance of the plasma membranes. In contrast, if neighboring cells are joined by gap junctions, there is little electrical resistance between them, and current flow is high. The low resistance reflects the direct cytoplasmic continuity between the two cells, resulting from the presence of the gap junctions. Therefore, gap junctions are also called low-resistance junctions.
Gap junctions can be visualized in TEM sections and freeze fracture preparations
When viewed with the TEM, the gap junction appears as an area of contact between the plasma membranes of adjacent cells (Fig. 4.14a). When uranyl acetate is applied as a "stain" before embedding the tissue (en bloc staining), a gap junction appears as two parallel, closely apposed plasma membranes separated by a gap of 2 nra.
Freeze fracture images of gap junctions reveal groups of channels formed by the apposition of identical structures in the facing membranes. Known as connexons, these structures consist of six integral membrane proteins called connexins, configured in a circular arrangement (Fig. 4.14b). Each channel is composed of two connexons, one belonging to the plasma membrane of each cell. Channels in gap junctions can fluctuate rapidly between an open 2-nm-diameter channel and a closed state through reversible changes in the confirmation of the individual connexins (Fig. 4.14c). The molecular mechanism of channel regulation is not yet fully understood. Like many other cellular organelles whose electron microscopic appearance suggests a static structure, gap junctions are actually dynamic.
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