Cell junctions are structures that allow neighboring cells to associate with each other. The three main types of cell junctions are tight junctions, adhesive junctions, and gap junctions. Adhesive junctions like desmosomes and adherens junctions link cells together and to the extracellular matrix. These junctions contain intracellular attachment proteins and transmembrane linker proteins that anchor the cells. Gap junctions allow direct communication between cells by forming channels that let small molecules pass between cells. Cell junctions play important roles in cell polarization, barrier function, and coordinated cell behavior.

1. CELL JUNCTIONS
Department of Natural Sciences
University of St. La Salle
Bacolod City

3.  Cell junctions are the
CELL JUNCTIONS structures where long term
association between
neighboring cells are
established.
 The 3 most common kinds of
cell junctions are tight
junctions, adhesive/anchoring
junctions, and gap/
communicating junctions.
 Adhesive junctions
(desmosomes,
hemidesmosomes and
adherens junctions) link
adjoining cells to each other
and to the ECM.
 Although adhesive junction
types are similar in structure
and function, they contain
distinct intracellular
attachment proteins and
transmembrane linker proteins.

4.  The intracellular attachment proteins form a thick layer of
fibrous material on the cytoplasmic side of the plasma
membrane called a plaque which binds actin microfilaments in
adherens junctions and intermediate filaments in desmosomes
and hemidesmosomes.
 The transmembrane linker protein is anchored to the plaque by
the cytoplasmic domain and binds the ECM or to the same
proteins on other cells.

5. Distribution
of cell
junctions in
3 domains
of epithelial
cells.

6. ZONULA
OCCLUDENS
extends
around the
entire
perimeter of
the cell, but
typically
located near
the apex.
Also known as
terminal bars, tight or
occluding junctions

7. Tight junctions consist of fused
ridges of tightly packed
transmembrane junctional proteins.
They regulate formation of the
barriers by modulating cell
proliferation, differentiation and
polarization, and control barrier
function by restricting paracellular
diffusion. The above mechanisms
may pave way for new therapeutic
strategies in drug delivery across
epithelial barriers.

8. Tight junctions block
lateral movement of
lipids and membrane
proteins to keep a cell
polarized. They leave
no space between
plasma membranes of
adjacent cells to
prevent the movement
of molecules across
cell layers.
Sodium/glucose
symport proteins and
export by glucose
transport proteins on
the basolateral surface
and tight junctions
prevent the lateral
movement of these
transport proteins.

9. ZONULA ADHERENS (intermediate junction,
belt desmosomes) is basal to the zonula
ocludens. The adjacent plasma membranes
are separated by a gap of 15-20 nm, filled with
an electron dense plaque containing a
glycoprotein localized only in the membrane,
(adherens junction-specific cell adhesion
molecule or A-CAM or E-cadherin).

10. Myosin, tropom
yosin,
α-actinin, and
vinculin, actin-
containing
microfilaments
insert into the
plaque to
stabilize the
junction
between
epithelial
cells, fibroblast
s, smooth
muscle cells
and at
intercalated
discs.

11. MACULA ADHERENS or
DESMOSOMES are bipartite
structures of apposing cell
membranes. An attachment
plaque on the cytoplasmic
side anchors tonofilaments
which are intermediate
filaments.
Desmosomes form strong
points of adhesion between
cells in a tissue such that
two adjoining cells are
separated by a thin space
of 25-35 nm, the
desmosome core, in which
cadherin molecules
mediate cell-cell adhesion.

12. The plaques on the inner surfaces of cells joined
by desmosomes have a mixture of intracellular
attachment proteins (desmoplakins and
plakoglobin) which interact with the tonofilament
intermediate filaments.

13. Adherens junctions called FOCAL ADHESION can join
a cell to the ECM, primarily through fibronectin
receptors.

14. HEMIDESMOSOMES connect a cell, through a plaque, to
the basal lamina (ECM) by integrins. As in desmosomes,
hemidesmosomes interact with tonofilament intermediate
filaments. Adherens junctions resemble desmosomes
except two adjoining cells are
separated by a thin space of
20-25 nm and connect to actin
microfilaments
in the cytoplasm.
Some of the
transmembrane
glycoproteins are
cadherins.

15. Hemidesmosomes
occur at most basal
surface of stratified
squamous epithelia
where the
superficial layer lack
junctional
complexes, and the
basal cells are
exposed to the
underlying CT.
They serve mainly as sites of
attachment for the actin-based
contractile system of the
cytoplasm.

16. GAP JUNCTIONS (NEXUS)
separate cells by 2-3 nm and
allow direct electrical and
chemical communication.

17. The nexus is a site where there is no actual fusion of
membranes, and the gap is bridged by a connexon. These are
tightly packed 7 nm wide hollow cylinders in two adjacent cell
membranes that form a 3 nm thin hydrophilic channel that
allows the passage of small molecules and ions.

18.  The connexons of each membrane link to form
continuous pores that bridge the intercellular gap,
allowing passage of ions, cyclic AMP, amino acids
and other small molecules.
 As sites of electronic coupling (reduced resistance to
ion flow), it is the only type of junction mediating flow
of current between cells important in intercellular
communication and coordination.
 An influx of Ca+2 ions results in the closure of their
channels, preventing spread of damage to other cells.
 Also found between osteocytes, astrocytes, cardiac
muscle cells, smooth muscle cells, & endocrine cells.
 Cancer cells generally do not have gap junctions, so
that cells fail to communicate their mitotic activity to
each other, which may explain their uncontrolled
growth.

19.  When they were originally discovered cell
junctions were considered to be relatively static
structures. This was likely because they appeared
to have a consistent, unchanging structure when
viewed with the electron microscope.
 New techniques have revealed that proteins can
move in and out of these junctions allowing the
cell to sense the status of its intercellular
adhesions.
 For example, occludin and ZO1, two proteins from
adherens junctions have been shown to move into
the nucleus to regulate gene activity.
 The interaction of junctional adhesion molecules
with the cytoskeleton has also been shown to be a
dynamic process that is still being elucidated.

20. (A) In a single cell, a subset of E-cadherin is found in a complex with Nectin-2α
and components of the Exocyst complex. (B) Upon cell-cell adhesion, E-
cadherin and Nectin-2α homodimers form trans-interactions with E-cadherin
and Nectin-2α homodimers from the opposing cell, respectively. This interaction
initiates the recruitment of microtubules, the Exocyst complex and the basal-
lateral SNARE complex to the forming cell-cell contact. (C) Following cell-cell
contact formation, microtubules extend into the contact, and post-Golgi
carriers carrying basal-lateral cargo travel via microtubules to the forming
contact. At the forming contact, the Exocyst and SNARE complexes are fully
functional in mediating docking and fusion of basal-lateral post-Golgi carriers.

21. http://www.mhhe.com/biosci/gen
bio/biolink/j_explorations/explor
ations.html