Cell junction is the connection between the neighboring cells or the contact between the cell and extracellular matrix. It is also called membrane junction.
Cell junctions are
classified into three types:
1. Occluding junctions
2. Communicating junctions
3. Anchoring junctions.
OCCLUDING JUNCTIONS
Cell junctions which
prevent intercellular exchange of substances are called occluding junctions,
i.e. these junctions prevent the movement of ions and molecules from one cell to
another cell. Tight junctions belong to this category.
TIGHT JUNCTION
Tight junction is the
intercellular occluding junction that prevents the passage of large molecules.
It is also
called zonula occludens. It is the region
where the cell membranes of the adjacent cells fuse together
firmly. This
type of junction is present in the apical margins of epithelial
and endothelial cells in intestinal mucosa, wall of renal tubule, capillary
wall and choroid plexus.
Structure of Tight
Junction
Tight junction is made
up of a ridge
which has two halves. One half of the ridge is from one cell and
another half is from the other cell. Both halves of the ridge fuse with each
other very tightly and occupy the space between the two cells. Each half of the
ridge consists of tight
junction strands.
Proteins of tight
junction
Proteins involved in
the formation of tight junctions are classified into two types:
1. Tight junction membrane proteins or integral membrane
proteins, such as occludin, claudin and junctional adhesion molecules
(JAMs)
CELL ADHESION MOLECULES
TYPES
2. Scaffold (framework
or platform) proteins or peripheral membrane proteins or cytoplasmic plaque
proteins such as
cingulin, symplekin and ZO1, 2, 3. Tight junction membrane protein molecules
are
anchored in the
strands of the ridge and attach with their counterparts of neighboring cell, so
that both the cells are held together. The scaffold (platform) proteins are attached
with the tight junction membrane proteins and strengthen the anchoring in the
ridges.
Functions of Tight
Junction
1. Strength and stability: The
tight junction holds the neighboring cells of the tissues firmly and thus
provides strength and
stability to the tissues.
2. Selective permeability (gate function): The
tight junction forms a selective barrier for small molecules
and a total barrier
for large molecules. In the epithelial and endothelial cells, tight junction is
the most apical intercellular junction, which functions as selective
(semipermeable) diffusion barriers between the neighboring cells. This function
is called barrier or gate function. Barrier function of
tight junction regulates the interchange of ions, water and varieties of
macromolecules between the cells. The magnitude of this function varies in
different tissues. In some epithelial cells, few substances pass through the
tight junction (by diffusion or active transport). In other cells, no substance
passes through the tight junction.
3. Fencing function: Tight
junction prevents the lateral movement of proteins (integral membrane proteins)
and lipids in cell
membrane and thus acts as a fence. The fencing function maintains the different
composition of proteins
and lipids between the apical and basolateral plasma membrane domains.
Because of this
function, the tight junction is sometimes referred as impermeable junction.
4. Maintenance of cell polarity: Fencing
function of the tight junction maintains the cell polarity by
keep ing the proteins
in the apical region of the cell membrane.
5. Blood-brain barrier: Tight
junction in the brain capillaries forms the bloodbrain barrier, which prevents
the entrance of many substances
from capillary blood into brain tissues. Only lipidsoluble substances like drugs
and steroid hormones can pass through the bloodbrain barrier.
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