346
Cell Junctions, Structure, Function, and Regulation
Microvilli
Hemidesmosome
Basal lamina
Tight
junction
Adherens
junction
Gap
junction
Desmosome
(a)
(b)
Fig. 9
Junctional complexes in epithelial cell monolayer. (a) Schematic showing the
cellular location of tight junctions, adherens junctions, desmosomes, and gap junctions in
an epithelial cell layer. (b) Electron micrograph of tight junctions (TJ), adherens junctions
(AJ), and desmosomes (DS). Note the electron-dense regions associated with the plasma
membrane in each junction. This is due to the localization of plaque proteins in these
areas. (Reprinted with permission from Tsukita, S., Furuse, M., Itoh, M. (2001)
Multifunctional strands in tight junctions,
Nat. Rev. Mol. Cell Biol.
2
, 285–293).
junctions (AJs). Desmosomes are found
primarily in tissues that experience me-
chanical stress, such as epithelial cells
of the skin and myocardial cells of the
heart. These junctions are connected to
intermediate Flaments and are thought
to have mainly a structural function. AJs
are connected to the actin cytoskeleton
and also serve a structural function. In-
deed, in certain epithelial cells such as
those in the intestine, the AJ acts together
with the actin cytoskeleton to allow con-
traction of the epithelial sheet. Occlusive
junctions, also termed
tight junctions
(TJ),
form restrictive barriers that regulate the
paracellular flux of molecules across ep-
ithelial and endothelial monolayers, and
act as a fence to prevent the movement of
membrane components from the apical to
the basal surface of a cell layer. Gap junc-
tions allow for communication between
cells. These junctions are formed by pro-
teins called
connexins
. Connexins form a
hexameric multimer that create a pore in
theplasmamembraneofthecell.Thehex-
americ pore binds to a similar pore on
an adjacent cell, thereby forming a con-
tinuous pore between cells that allows for
the passage of ions and small molecules
between neighboring cells. Gap junctions
provide electrical and chemical coupling
between cells whereas AJs, desmosomes,
and TJs provide mechanical coupling be-
tween cells. The following discussion will
focus on the junctions that provide me-
chanical coupling. A number of recent
reviews provide further information on
gap junctions and their role in cell–cell
communication.
A common theme exists with respect
to the structure of cell–cell junctions. All
contain transmembrane proteins whose
extracellular domains mediate binding to
similar proteins on neighboring cells. The
transmemebrane proteins of desmosomes
and AJs are members of the cadherin
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