Membrane specializations: Junctions
 The
drawing is of a polarized cell. The top is specialized for
absorption and the bottom for transfer of materials to the blood stream. The sides have
specialized junctions that keep the nutrients from entering the space between the cells.
For more information about the thick filaments associated with these junctions, please
consult the intermediate filaments web page. This figure was modified from Bloom and
Fawcett, A Textbook of Histology, Chapman and Hall, N.Y., Twelfth Edition, 1994, Figure
2-10
Tight Junctions
One of these is called a tight junction or "occluding junction" (zonula
occludens). This is shown as the top junction in the above drawing. At this site,
membrane glycoproteins and associated "glue" bind the cells together like
double-sided "strapping tape" The freeze-fracture/freeze etch view of this
junction (shown below) illustrates the ridges in the plane of the exposed leaflet. These
are the proteins that bind to the proteins from the adjacent cell. This figure was
modified from Bloom and Fawcett, A Textbook of Histology, Chapman and Hall, N.Y., Twelfth
Edition, 1994, Figure 2-11.
Another type of junction allows communication between cells. This type is called a gap
junction. Small molecules or ions can pass through, as we will see by the following
figures.
The above freeze-fracture /freeze etch image shows the internal view of the gap
junction on the left. The proteins look like little donuts which reflects the fact
that they are actually a channel. These proteins are "connexon" molecules. The
side facing the cytoplasm (called the P face) is shown in the center panel. The region
looks like aggregated lumps. Finally, the typical electron microscopic view is seen in the
third panel. This shows a thin line between the two plasma membranes indicating a
"gap junction". This figure was modified from Bloom and Fawcett, A Textbook
of Histology, Chapman and Hall, N.Y., Twelfth Edition, 1994, Figure 2-14.
There are several ways to prove the cells are communicating by gap junctions. First,
one can identify the connexon molecules by immunocytochemical labeling. Second, one can
identify the actual junctional complex with freeze-fracture/freeze etch. To see if they
are functional, however, one needs to inject one cell with a dye and watch to see if it is
transferred to another cell.  This cartoon diagrams a view of a gap junction showing molecules that can freely pass. Ions pass and
in this way the cells can be electrically coupled together. Other small molecules that
pass through include cyclic AMP (a second messenger) and the dye marker fluorescein. This
last compound enables the scientist to study transport throught the gap junction.
This figure was modified from Bloom and Fawcett, A Textbook of Histology, Chapman and
Hall, N.Y., Twelfth Edition, 1994, Figure 2-15.
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In
certain areas of the cell, one also sees protrusions or bumps. These are colored red in
the cartoon. Sometimes one can see structure within the bumps themselves. These are the transmembrane proteins.
Note
that the microvilli are lined with the 
