Cell Junctions

The function of cell junctions

The main function of some junctions is to connect the cytoplasm between adjacent cells and allow intercellular transportation and communication. Other junctions mostly function as adherence sites that maintain tissue structure and integrity. All types of tissues in animals have junctions, however, junctions that serve for adherence are more abundant in epithelial tissues (tissues that line internal organs and cavities).

Types of cell junctions

There are different ways two cells can connect, these are classified according to their main function and the molecules involved. Connections between cell plants are called plasmodesmata, while animal cells can connect through tight junctions, gap junctions, and desmosomes.

Plasmodesmata

Plant cells have walls surrounding the plasma membrane and need some kind of connection to communicate with adjacent cells. Plasmodesmata (singular plasmodesma) are channels going through adjacent cell walls enabling the transportation of material between cells. These are direct channels, not even the plasma membrane interrupts the channel, as the membranes of adjacent cells are continuous and line the interior of the plasmodesma. Plasmodesmata differ in size and large molecules may be too big to pass through a plasmodesma though.

Figure 1: Intercellular junctions. A) plasmodesmata in plant cells, B) tight junctions, and C) gap junctions.

Tight cell junctions

Some junctions are for adherence rather than connection. In animals, epithelial tissues have to prevent the leaking of substances from one side of the tissue to the other. These cells have tight junctions at several sites between adjacent cells that serve as waterproof seals. Tight junctions are formed by groups of proteins in a cell membrane that connects to a partner group of proteins in the membrane of the adjacent cell.

The integrity and tightness of epithelial barriers not only prevent the leaking of liquids and other components but also block the entrance of microbes into a host. Because of this, tight junctions are a target for pathogenic microbes (bacteria and viruses), who use several tactics to disrupt these structures as part of the infection.

Tight junctions not only serve to form tight tissues though, as these connections between cells are also important in the synchronized movement of cells needed for processes like wound healing and fetus development. The proteins that form the junctions do so by transferring information between adjacent cells, allowing the tissue to be mobile. If the junctions are lost, the cells do not move and the tissue is not mobile anymore, hindering the proper healing of wounds for example².

Gap cell junctions

Gap junctions are channels between adjacent animal cells that allow the movement of material. They consist of a group of proteins (called connexins in vertebrates) that form a type of elongated cylinder called connexon, that connects two neighboring cell membranes.

Ions and small molecules can freely pass through gap junctions. The electric signal in cardiac muscle is transmitted as ions pass through gap junctions and result in the synchronous movement of all the cells allowing the cardiac tissue to contract. Gap junctions are also present in tissues that do not produce electrical signals though.

Junction between two nerve cells

Gap junctions are also critical under certain situations where an extremely fast response is required. A neuron communicates with another neuron through a narrow gap between the two cell membranes called a synapse. Neuron communication through a synapse is chemical, via the release of a neurotransmitter from a neuron that goes to the receiving neuron. However, they also have electrical communication across gap junctions. Since ions can pass with no delay through gap junctions, electrical signals between neurons travel faster this way than chemical transmissions. The difference is on the order of milliseconds, but that can result in death for prey that does not escape fast enough!⁴

Desmosomes

In animal tissues that stretch (for example skin, muscle, and heart tissues), junction areas called desmosomes keep the cells together. They are like a weld between two cells. A group of transmembrane proteins called cadherins present in both adjacent cell membranes connect to each other. Cadherins are anchored to the membrane by a plaque on the cytoplasmic side of each membrane. The plaque is a complex of several proteins and connects to the cell intermediate filaments (components of the cytoskeleton).

Figure 2: Desmosomes are cell junctions in animal cells. The diagram shows desmosome structure (left), and transmission electron microscope images of desmosomes (indicated with red arrows), where the protein plaque at each side of the plasma membrane is clear (middle), and two desmosomes where the intermediate filaments are clear (right).

Desmosomes provide tissues with resistance to mechanical stress. The connection with intermediate filaments inside the cells provides a nearly continuous fibrous network throughout an epithelial sheet, distributing the mechanical stress throughout the tissue. Mutations in desmosomal genes result in several skin and heart diseases.