Cell junction

cellular level of body organization.

1. Cell Junction


Devendra Kumar Rawat (Assistant Professor) B.M.S. College of Pharmacy 1



➢ Cell junctions are a class of cellular structures consisting of multiprotein complexes that provide
contact or adhesion between neighboring cells or between a cell and the extracellular matrix in
animals.
➢ They also maintain the paracellular barrier of epithelia and control paracellular transport.
➢ Cell junctions are especially abundant in epithelial tissues. Combined with cell adhesion
molecules and extracellular matrix, cell junctions help hold animal cells together.
➢ Cell junctions are also especially important in enabling communication between neighboring cells
via specialized protein complexes called communicating (gap) junctions.
➢ Cell junctions are also important in reducing stress placed upon cells.
➢ Cell junctions are of mainly five types, which are as follows:
1. Tight junction
2. Adherens junction
3. Desmosome
4. Hemidesmosome
5. Gap junction



➢ A tight junction is a watertight seal between two adjacent animal cells.
➢ The cells are held tightly against each other by proteins (predominantly two proteins called
claudins and occludins).
➢ Occludins, which maintain the barrier between adjacent cells.
➢ Claudins, which form the backbone of tight junction strands.
➢ Tight junctions are the closely associated areas of two cells whose membranes join together to
form a virtually impermeable barrier to fluid.
➢ Tight junctions perform vital functions—such as holding cells
together—and form protective and functional barriers.
➢ Each strand is formed from a row of transmembrane
proteins embedded in both plasma membranes, with
extracellular domains joining one another directly.
➢ the plasma membranes of adjacent cells essentially fuse
together tightly in order to limit the leakage of various
substances between the two cells.
➢ Tight junctions are located within our body’s epithelia. This
includes organs (such as skin), blood vessels, and cavities.
CELL JUNCTION


Tight Junction

2. Cell Junction


Devendra Kumar Rawat (Assistant Professor) B.M.S. College of Pharmacy 2


➢ Adherens junctions provide strong mechanical attachments between adjacent cells through the
linkage of cytoplasmic face with cytoskeleton.
➢ Adherens junctions are involved in a number of critical
functions, including providing additional structural support.
For example, they hold cardiac muscle cells tightly together
as the heart expands and contracts.
➢ Adherens junctions are built primarily from cadherins, whose
extracellular segments bind to each other and whose
intracellular segments bind to catenins.
➢ Catenins are connected to actin filaments.
➢ Cadherin: Any of a class of transmembrane proteins
important in maintaining tissue structure.
➢ Catenin: Any of a class of proteins that have a role in cell
adhesion.
➢ This type of cell junction is located right below tight junctions and provides a strong bond between
the sides of adjacent epithelial cell membranes.




➢ Desmosomes are intercellular junctions that provide strong
adhesion between cells.
➢ Desmosomes are intercellular junctions of epithelia and
cardiac muscle.
➢ They also link intracellularly to the intermediate
filament cytoskeleton they form the adhesive bonds in a
network that gives mechanical strength to tissues.
➢ Short proteins called cadherins in the plasma membrane
connect to intermediate filaments to create desmosomes.
➢ The cadherins join two adjacent cells together and maintain
the cells in a sheet-like formation in organs and tissues that
stretch, like the skin, heart, and muscles.
➢ Their failure can result in diseases of the skin and heart.



➢ Hemidesmosomes are very small stud-like structures found in keratinocytes of the epidermis of
skin that attach to the extracellular matrix.
➢ They are similar in form to desmosomes when visualized by electron microscopy, however,
desmosomes attach to adjacent cells.
Desmosome







Adherens Junction
Hemidesmosome

3. Cell Junction


Devendra Kumar Rawat (Assistant Professor) B.M.S. College of Pharmacy 3

➢ Hemidesmosomes, particularly those attaching epithelial cells to their basement membrane, are
the tightest adhesive interactions in an animal body.
➢ The hemidesmosome is a complex junction containing many proteins. The keratin cytoskeleton
attaches to its cytoplasmic plaque, while its transmembrane elements interact with components
of the extracellular matrix.
➢ Hemidesmosome assembly involves recruitment of integrin and laminin.


➢ A gap junction is a specialized cell junction that directly connects the cytoplasm of two cells.
➢ Gap junctions allow various molecules and ions to pass freely
between cells.
➢ A gap junction channel is composed of two connexons, also
known as hemichannels that line up across the intercellular
space.
➢ Most gap junction hemichannels are composed of a complex of
six connexin proteins, each characterized by four
transmembrane domains.
➢ Six connexin sub-units assemble to create one connexon, or
hemichannel.
➢ Gap junctions are expressed in virtually all tissues and cells, but
most notably in cell types that are involved in direct electrical
communication, such as neurons and cardiac muscle.
➢ Gap junctions are found in many places throughout the body.
This includes epithelia, which are the coverings of body surfaces, as well as nerves, cardiac (heart)
muscle, and smooth muscle (such as that of the intestines).



Gap Junction