RE: Composition of intercellular junctions

	Gap junctions, tight junctions, and desmosomes are all examples of
structures involved in intercellular adhesion processes.  Desmosomes
and tight junctions appear to serve a more or less exclusively
adhesive function, while gap junctions are intimately involved with
intracellular communications.

	Desmosomes are primarily associated with epithelial cells.  They have
a characteristic appearance under the electron microscope -- the
adjacent cell membranes show an area of $BEU(Jhickening$BG
(J called adesmosomal plaque, about 300 nanometers in length, with the interior
portion of the plaque having a sort of $BEG(Juzzy$BG
(Jappearance.  The
desmosome is made up of several specialized proteins.  Desmocollins,
which are large proteins (molecular weight 130,000) embedded in the
cell membrane, are the proteins responsible for the actual adhesion of
the cells -- desmocollins from one cell reach out and attach to
desmocollins in the adjacent cell.  Other membrane-embedded proteins,
desmogleins (molecular weight 165,000) and plakoglobins (molecular
weight 83,000), extend into the cytoplasm.  Another set of proteins,
desmoplakins, are entirely cytoplasmic and connect the desmosome with
intermediate filament proteins.  These give the desmosome its $BEG(Juzzy$BGï(J
appearance.  Desmosomes form a tight, impenetrable barrier anchoring
cells to each other or to a substrate.  A disease known as pemphigus,
which is a rather painful condition characterized by the formation of
large blisters on the skin, is caused by a breakdown of desmosomal
structures.

	Tight junctions are areas of cell contact where the cells come into
such close contact that the adjacent membranes actually appear to
fuse.  This isn$BCU(J the case, however -- instead, the adhesion is
mediated again by proteins embedded in the cell membrane.  There are a
number of proteins that appear to be associated with tight junctions,
but only a few have been identified so far.  Two proteins, called
cingulin and ZO1, are found in the area of tight junctions, but it$BCT(J
not known yet if they actually participate in the adhesion process. 
Tight junctions serve to form barriers between cells, helping to
maintain tissue compartmentalization as well as direct the flow of
nutrients and waste products through cells rather than around cells in
the extracellular spaces. 

	Gap junctions can be thought of as channels connecting cells to each
other.  The cell membranes come close together, but are separated by a
$BEH(Jap$BG
(Jof about 2-4 nanometers.  These junctions again are mediated by 
proteins in the cell membrane.  A protein called connexin appears to
be the main gap junction component.  Connexins come together in groups
of 6 to form hexagonal ring-shaped channels called connexons in the
cell membrane.  These come together with connexons in the membranes of
adjacent cells to form the gap junction.  The ring-like structure of
the gap junction allows for communication between cells.  Ions and
small molecules such as amino acids and nucleotides can pass freely
between the cytoplasms of cells connected by gap junctions.  This was
demonstrated in the laboratory by injecting cells with fluorescent
dyes and watching the dye spread from the cell into which it was
injected to neighboring cells.  By using dye molecules of increasing
size, the size of the channels connecting the cells could be
determined (about 2 nanometers in diameter).

	Gap junctions have been proposed to play important roles in
intracellular communication and signaling during embryonic
development.  Antibodies that bind to gap junction proteins can be
injected into cells to prevent transfer of molecules through the
channels.  When researchers have injected such antibodies into
developing embryos (usually frog embryos because of their large size),
the normal development of these embryos is disrupted, showing the
importance of gap junction proteins in embryonic development.