- Cell junctions are also called as intercellular bridge.
- Cell junctions are contact points between the plasma membranes of tissue cells.
- Cell junctions consist of multi-protein complexes that provide contact between neighboring cells or between a cell and the extracellular matrix.
- There are five different types of cell junctions:
- Tight junctions
- Adherens junctions
- Gap junctions
- It acts as a barrier that regulates the movement of water and solutes between the epithelial layers.
- The cells of epithelial tissues that line the stomach, intestines, and urinary bladder have many tight junctions to retard the passage of substances between cells and prevent the leaking of contents into the blood or surrounding tissues.
- Tight junctions present in different types of epithelia are selective for solutes of differing size, charge, and polarity.
- These are composed of a branching network of sealing strands acting independently from the others.
- Each strand is formed from a row of transmembrane proteins embedded in both plasma membranes, with extracellular domains joining one another directly.
- The major proteins present are claudins and occludins.
- These are associated with different peripheral membrane proteins such as ZO-1 located on the plasma membrane, which anchors the strands to the actin component of the cytoskeleton.
- They hold the cells together.
- They help to maintain the polarity of cells by preventing the lateral diffusion of proteins between the apical and lateral/basal surfaces.
- They prevent the passage of molecules and ions through the space between the plasma membranes of adjacent cells.
- These are also called intermediate junction or belt desmosome.
- These are protein complexes that occur at cell-cell junctions in epithelial and endothelial tissues.
- Adherens junctions contain plaque, a dense layer of proteins on the inside of the plasma membrane that attaches both to membrane proteins and microfilaments of the cytoskeleton.
- The trans-membrane glycoproteins present in adherens junctions are called cadherins that join the cells.
- In epithelial cells, adherens junctions form the extensive zones called adhesion belts.
- Adherens junctions help epithelial surfaces to resist separation during various contractile activities such as the movement of food through the intestines.
- Desmosomes contain plaque and trans-membrane glycoproteins such as cadherins that extend into the intercellular space between adjacent cell membranes and attach cells to one another.
- A desmosome plaque attaches to a cytoskeleton known as intermediate filaments that consist of keratin protein.
- The intermediate filaments extend from desmosomes on one side of the cell across the cytosol to desmosomes on the opposite side of the cell.
- Such a type of structural arrangement helps in the stability of cells and tissues.
- These types of junctions are more common in the epidermis (the outermost layer of the skin) and cardiac muscle cells of the heart.
- Hemidesmosomes resemble desmosomes but they do not link the adjacent cells.
- The structure of hemidesmosomes is like half of a desmosome
- The trans-membrane glycoproteins present in hemidesmosomes are integrins.
- To the inner side of the plasma membrane, integrins attach to the intermediate filaments made up of keratin protein.
- To the outer side of the plasma membrane, the integrins attach to the protein laminin present in the basement membrane.
- Hence, it plays an important role in anchoring cells to the basement membrane.
- The membrane proteins present in gap junctions are called connexins, form tiny fluid-filled tunnels called connexons that connect neighboring cells.
- The plasma membranes of gap junctions are separated by a very narrow intercellular gap (2 to 4 nm).
- Through the connexions, the ions and small molecules can diffuse from the cytoplasm of one cell to another cell.
- A gap junction allows the communication of cells with one another.
- Gap junctions enable nerve or muscle impulses to spread rapidly among nerve cells.
- Dissolved substances such as ions or glucose can pass through the gap junctions.