Gap Junctions Gap junctions are intercellular channels some
1.5�2 nm in diameter.
These permit the free passage between the cells of ions and small molecules (up to a
molecular weight of about 1000 daltons).
They are cylinders constructed from 6 copies of transmembrane proteins called connexins.
Because ions can flow through them, gap junctions permit changes in membrane potential to pass
from cell to cell.
Examples: � The action potential in heart (cardiac)
muscle flows from cell to cell through the heart providing the rhythmic contraction of
the heartbeat.
� At some so-called electrical synapses in the brain, gap junctions permit the arrival
of an action potential at the synaptic terminals to be transmitted across to the postsynaptic
cell without the delay needed for release of a neurotransmitter.
� As the time of birth approaches, gap junctions between
the smooth muscle cells of the uterus enable coordinated, powerful contractions to begin.
Several inherited disorders of humans such as
� certain congenital heart defects and � certain cases of congenital deafness
have been found to be caused by mutant genes encoding connexins.
Tight Junctions Epithelia are sheets of cells that provide
the interface between masses of cells and a cavity or space (a lumen).
� The portion of the cell exposed to the lumen is called its apical surface.
� The rest of the cell (i.e., its sides and base) make up the basolateral surface.
Tight junctions perform two vital functions: � They limit the passage of molecules and
ions through the space between cells.
So most materials must actually enter the cells (by diffusion or active transport) in
order to pass through the tissue.
This pathway provides tighter control over what substances are allowed through.
� They block the movement of integral membrane proteins (red and green ovals) between the
apical and basolateral surfaces of the cell.
Thus the special functions of each surface, for example
o receptor-mediated endocytosis at the apical surface
o exocytosis at the basolateral surface can be preserved.
Desmosomes Desmosomes are localized patches that hold
two cells tightly together.
They are common in epithelia (e.g., the skin).
Desmosomes are attached to intermediate filaments of
keratin in the cytoplasm.
Pemphigus is
an autoimmune disease in which the patient has developed antibodies against
proteins (cadherins) in desmosomes.
The loosening of the adhesion between adjacent epithelial cells causes blistering.
Carcinomas are cancers of epithelia.
However, the cells of carcinomas no longer have desmosomes.
This may partially account for their ability to metastasize.
Hemidesmosomes These are similar to
desmosomes but attach epithelial cells to
the basal lamina ("basement membrane" � View) instead of to each other.
Pemphigoid is an autoimmune disease in which the patient develops antibodies against proteins
(integrins) in hemidesmosomes.
This, too, causes severe blistering of epithelia.