Cell junctionsA cell junction is a structure within a tissue of a multicellular organism. Cell junctions are

especially abundant in epithelial tissues. They consist of protein complexes and provide contact

between neighbouring cells, between a cell and the extracellular matrix, or they built up the

paracellular barrier of epithelia and control the paracellular transport.

In vertebrates, there are three major types of cell junctions:

Adherens junctions (or zonula adherens) are protein complexes that occur at cell-cell junctions in

epithelial tissues.

They can appear as bands encircling the cell (zonula adherens) or as spots of attachment to the

extracellular matrix (adhesion plaques).

They are composed of three major proteins:

cadherins. The cadherins are a family of transmembrane proteins that form homodimers in a

calcium-dependent manner with other cadherin molecules on adjacent cells.

-catenin and a-catenin. The catenins both form a complex with the intracellular portion of

the cadherin molecule.

A desmosome, also known as macula adherens (Latin: adhering spot), is a cell structure

specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like

adhesions randomly arranged on the lateral sides of plasma membranes.

Desmosomes help to resist shearing forces and are found in simple and stratified squamous

epithelium. The intercellular space is very wide (about 30nm).

They are molecular complexes of cell adhesion proteins and linking proteins that attach the cell

surface adhesion proteins to intracellular keratin cytoskeletal filaments.

The cell adhesion proteins of the desmosome are members of the cadherin family of cell adhesion

molecules.

They are transmembrane proteins that bridge the space between adjacent epithelial cells by way of

homophilic binding of their extracellular domains to other desmosomal cadherins on the adjacent

cell.

On the cytoplasmic side of the plasma membrane is a disk-like structure (attachment plaque)

composed of very dense materials.

The main desmosomal linking proteins, desmoplakins and plakoglobins, bind to the intracellular

domain of cadherins and form a connecting bridge to the cytoskeleton.

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A gap junction is a junction between certain animal cell-types that allows different molecules and

ions to pass freely between cells. The junction connects the cytoplasm of cells. One Gap junction is

composed of two connexons (or hemichannels) which connect across the intercellular space.

1. Allows for direct electrical transmission between cells

2. Allows for chemical transmission between cells, through the transmission of small second

messengers, such as IP3 and Ca2+[1]

3. Allows any molecule smaller than 1,000 Dalton to pass through. The large biomolecules, for

example, nucleic acid and protein, are not allowed to be shared.

Gap junctions are particularly important in the cardiac muscle: the signal to contract is passed

efficiently through the gap junctions, allowing the heart muscle cells to contract in tandem.

However, gap junctions are now known to be expressed in virtually all tissues of the body, with the

exception of motile cell types such as sperm or erythrocytes. Several human genetic disorders are

now associated with mutations in gap junction genes. Many of those affect the skin, because this

tissue is heavily dependent upon gap junction communication for the regulation of differentiation

and proliferation.

Tight junctions, or zonula occludens, are the closely associated areas of two cells whose

membranes join together forming a virtual impermeable barrier to fluid. It is a type of junctional

complex.

They are formed by claudin and occludin proteins, joining the cytoskeletons of the adjacent cells.

They perform three vital functions:

They hold cells together

They block the movement of integral membrane proteins between the apical and basolateral

surfaces of the cell, allowing the specialized functions of each surface (for example

receptor-mediated endocytosis at the apical surface and exocytosis at the basolateral surface)

to be preserved. This aims to preserve the transcellular transport.

They prevent the passage of molecules and ions through the space between cells. So

materials must actually enter the cells (by diffusion or active transport) in order to pass

through the tissue. This pathway provides control over what substances are allowed through.

(Tight junctions play this role in maintaining the blood-brain barrier.)

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