Cell Junctions and Their Functions

Cells are the small parts of human body. They are organized to form tissues. They are linked together by cell junctions and are supported by a matrix which they themselves secrete.

The connection between cells is called cell junction. Cell junctions have any various kinds of junctions. Generally, it can be divided into two types : those that link cells together and those that link cells to the extracellular matrix.

Cell junctions can be important part of human body. They can be important to another live creatures. For example, they maintain tissue integrity, act as barriers to permeability and allow intercellular transport. Cell junctions have many functions in our human body and another live creatures.

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Cell junctions are one of important parts of our body. They linked together and because of that, cells can form into tissues, tissues can form into organs, organs can form into systems and finally systems can form into organism. They have three general types of junctions. The types of junction have each functions in human body and another live creatures. Therefore, cell junctions can be important.

Cell junctions have many kind of junctions. There are three general kind of junctions. On the book “Molecular Biology of the Cell” (Alberts, et al, 2002) states :

“Specialized cell junctions occur at points of cell-cell and cell-matrix contact in all tissues, and they are particularly plentiful in epithelia. Cell junctions are best visualizedusing either conventional or freeze-fracture electron microscopy, which reveals that the interacting plasma membranes (and often the underlying cytoplasm and the intervening intercellular space as well) are highly specialized in these regions.

Cell junctions can be classified into three functional groups:

• Occluding junctions seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other.
• Anchoring junctions mechanically attach cells (and their cytoskletons) to their neighbors or to the extracellular matrix.
• Communicating junctions mediate the passage of chemical or electrical signals from one interacting cell to its partner.

OCCLUDING JUNCTIONS

• Tight junctions (vertebrates only)
• Septate junctions (inverterates mainly)

ANCHORING JUNCTIONS

• Actin filament attachment sites
• Cell-cell junctions (adherens junctions)
• Cell-matrix junctions (focal adhesions)
• Intermediate filament attachment sites
• Cell-cell junctions (desmosomes)
• Cell-matrix junctions (hemidesmosomes)

COMMUNICATING JUNCTIONS

• Gap junctions
• Chemical junctions
• Plasmodesmata (plants only)

In animals, there are three common kinds of junctions.There are adhesive junctions, tight junctions, and gap junctions. On the book “The World of the Cell”(Becker, et al, 2009) states : 422

ADHESIVE JUNCTIONS:

“One of the three mainly junctions in animal cells is the adhesive (or anchoring) junction. Adhesive junctions link cells together into tissues, therby enabling the cells to function as a unit. All junctions in this category anchor the cytoskeleton to the cell surface. The resulting interconnected cytoskeletal network helps to maintain tissue integrity and to withstand mechanical stress. The two main kinds of cell-cell adhesive junctions are adherens junction and desmosomes.” (Becker, et al, 2009)

ADHERENS JUNCTIONS:

“Adherens junctions connect bundles of actin filaments from cell to cell. Adherens junctions occur in various forms. In many nonepithelial tissues, they take the form of small punctate or streaklike attachments that indirectly connect the cortical actin filaments beneath the plasma membranes of two interacting cells. But the prototypical examples of adherens junctions occur in epithelia, where they often form a continuous adhesion belt (or zonula adherens) just below the tight junctions, encircling each of the interacting cells in the sheet. The adhesion belts are directly apposed in adjacent epithelial cells, with the interacting plasma membranes held together by the cadherins that serve here as transmembrane adhesion proteins.” (Alberts, et al, 2002)

DESMOSOMES

“Desmosomes are button-like points of strong adhesion between adjacent cells in a tissue. Desmosomes give the tissue structural integrity, enabling cells to function as a unit and to resist stress. Desmosomes are found in many tissues but are especially abundant in skin, heart muscle, and the neck of the uterus.” (Becker, et al, 2009)

TIGHT JUNCTIONS

“Although tight junctions seal the membranes of adjacent cells together very effectively, the membranes are not actually in close contact over broad areas. Rather, they are connected along sharply defined ridges. Tight junctions can be seen especially well by freeze-fracture microscopy, which reveals the inner faces of membranes. Each junction appears as a series of ridges that form an interconnected network extending across the junction. Each ridge consists of a continuous row of tightly packed transmembrane junctional proteins about 3-4 nm in diameter. The result is rather like placing two pieces of corrugated metal together so that their ridges are aligned and then fusing the two pieces lengthwise along each ridge of contact.” (Alberts, et al, 2002)

GAP JUNCTIONS

“With the exception of few terminally differentiated cells such as skeletal muscle cells and blood cells in animal tissues are in communication with their neighbors via gap junctions. Each gap junction appears in conventional electron micrographs as a patch where the membranes of two adjacent cells are separated by a uniform narrow gap of about 2-4 nm. The gap is spanned by channel-forming proteins (connexins). The channel they form (connexons) allow inorganic ions and other small water-soluble molecules to pass direcly from the cytoplasm of one cell to the cytoplasm of the other, therby coupling the cells both electically and metabolically.” (Becker, et al, 2009)

Cell Junctions are linked to one another together. Cells junctions fall into three functional classes : occluding junctions, anchoring junctions, and communicating junctions. In animal, there are three kind types of junctions : adhesive junctions, tight junctions and gap junctions. In plants, cell junctions is called plasmodesmata. There are two main kind of adhesive junctions : adheren junctions and desmosomes. Adhesive junctions are anchored to the cytoskeleton by linker proteins that attach to adherens junctions and desmosomes. Desmosomes are particularly prominent in tissues that must withstand considerable mechanical stress. Tight junctions form a permeability barrier between epithelial cells and they prevent the lateral movement of membrane proteins. Gap junctions form open channels between cells, allowing direct chemical and electical communication between cells. Their connexons allow inorganic ions.