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Cell Junctions

Cell Junctions

We learned that extracellular matrices in animals are involved in cell communication by providing or receiving signals to and from cells. But what if a cell needs to communicate with the adjacent cell? Does the cell send a signal to the extracellular matrix and then this signal has to be transferred to the adjacent cell? That sounds too much trouble for being side to side. Here we discuss how adjacent cells can connect to communicate with each other or to form a tight tissue. Keep reading to learn more about the functions of cell junctions, different types, and more.

Cell junctions definition

Cells form tissues, tissues form organs, organs form organ systems. For all these body components to function properly, cells need to adhere to and communicate with other cells. Cell junctions are basically cell to cell or cell-extracellular matrix connection sites. Here we focus on junctions between two adjacent cells.

A cell junction is a region that connects two adjacent cells or a cell and the extracellular matrix for communication or adhesion.

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.

Cell junctions Figure 1A-B: Intercellular junctions, plasmodesmata, tight, and gap junctions | StudySmarterFigure 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.

A tight junction is a waterproof region that seals adjacent cells together to prevent the leaking of molecules and liquid from one side of a tissue to the other side.

For example, the skin is an impermeable epithelial tissue as well as the intestinal epithelium. The epithelial tissue lining the urinary bladder prevents the leaking of urine into the extracellular space.

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.

Common bacteria that affect tight junctions include several gastrointestinal pathogens like Samonella, Helicobacter pylori, and pathogenic Escherichia coli. These target the tight junctions of the intestinal epithelium in different ways, such as using the junction proteins for their attachment to the tissue, which facilitates their passage to the interior cells, or just destroys them to access the underlying cells. The alteration of tight junctions in the intestine often results in inflammation and diarrhea1.

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.

This function in cell movement could also have a role in the development of tumor and cancerous cells. A characteristic of cells becoming cancerous is that the E-cadherin protein (which forms other types of cell junctions) degrades, affecting epithelial adhesion. These cells change their behavior in cancerous tissues, from the normal coordinated movement of cells in a tissue to a more individual and disorganized movement3.

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).

Cell junctions Intercellular junctions, desmosome diagram and TEM image StudySmarterFigure 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.

A desmosome is an adherence site between adjacent cells that connects their intermediate filaments and maintains the integrity of a tissue providing resistance to mechanical stress.

The PKP1 gene encodes for proteins involved in desmosome structure and participates in the connection of cadherins to intermediate filaments. Mutations in this gene are associated with a syndrome called ectodermal dysplasia/skin fragility. This is a rare genetic disease, with only a few known carriers. The syndrome results in skin fragility and it blisters easily, patients can also present hair formation defects, epidermal thickening that causes cracking of the skin, and sometimes nail dystrophy5.

The importance of the cadherin group of proteins is highlighted by the fact that the genes responsible for their production are found in all animals including one species of sponge, (sponges are basal animals, that appeared early in the evolution of the group). Furthermore, they are also present in choanoflagellates (eukaryotes that are the closest living relatives or animals), suggesting that this family of proteins appeared in the common ancestor of choanoflagellates and animals6.

Interestingly, choanoflagellates are not multicellular (thus do not need cell junctions) and although some species form colonies, cadherin genes have been found in one unicellular species. the role of these proteins in single-cell organisms is unknown, but they might have played adhesion-related functions like the capture of bacterial prey, attachment to surfaces, and the detection of environmental cues6.

Cell Junctions - Key takeaways

  • The two main functions of intercellular junctions are to allow intercellular transportation and communication or to maintain tissue structure and integrity through adherence of adjacent cells.
  • Connections between cell plants are called plasmodesmata, while animal cells can connect through tight junctions, gap junctions, and desmosomes.
  • Plasmodesmata connect the cytoplasm of adjacent plant cells for intercellular communication and movement of material.
  • Gap junctions allow the intercellular movement of ions and small molecules and are essential for the transmission of electrical signals throughout a tissue.
  • Tight junctions and desmosomes are for adherence rather than connection, maintaining tissue integrity. Tight junctions provide a waterproof seal, while desmosomes give resistance to mechanical stress.

References

  1. Julian A. Guttman, B. Brett Finlay, Tight junctions as targets of infectious agents, Biochimica et Biophysica Acta - Biomembranes, Volume 1788, Issue 4, 2009. https://doi.org/10.1016/j.bbamem.2008.10.028
  2. University of Göttingen. “Tug of war” between cells: What happens when crucial connections are missing. Phys.org news. November 10, 2021. https://phys.org/news/2021-11-war-cells-crucial.html
  3. Universität Leipzig. Less “sticky” cells become more cancerous. Medical Xpress news. August 25, 2020. https://medicalxpress.com/news/2020-08-sticky-cells-cancerous.html
  4. Ao Dong et al., Gap Junctions in the Nervous System: Probing Functional Connections Using New Imaging Approaches. Frontiers in Cellular Neuroscience. 2018. https://www.frontiersin.org/articles/10.3389/fncel.2018.00320/full
  5. John McGrath and Jemima Mellerio. Ectodermal dysplasia-skin fragility syndrome. Dermatologic clinics. 2010. https://pubmed.ncbi.nlm.nih.gov/19945625/
  6. Robert Sanders. Genome of marine organism tells of animals’ one-celled ancestors. Press Release, UC Berkely News, 2008. https://www.berkeley.edu/news/media/releases/2008/02/14_choanos.shtml

Frequently Asked Questions about Cell Junctions

Tight junction are waterproof regions that seals adjacent animal cells together to prevent the leaking of molecules and liquid from one side of a tissue to the other side.

A cell junction is a region that connects two adjacent cells or a cell and the extracellular matrix for communication or adhesion.

Three types of cell junctions are unique to animals, tight junctions, gap junctions and desmosomes.

The type of cell junctions that allow cellular communication are gap junctions in animal cells and plasmodesmata in plant cells.

The function of a junction is to connect two adjacent cells or a cell and the extracellular matrix for communication or adhesion.

Final Cell Junctions Quiz

Question

Which of the following intercellular junctions are found in plants?

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Answer

plasmodesmata

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Question

Which of the following intercellular junctions are found in animals?

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Answer

plasmodesmata

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Question

Junctions whose function is Intercellular communication and material transport?

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Answer

plasmodesmata

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Question

Junctions whose function is adhesion between cells in a tissue:


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Answer

plasmodesmata

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Question

Which of the following intercellular junctions are more abundant in epithelial tissues?

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Answer

plasmodesmata

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Question

Junctions that prevents leaking from one side of the tissue to the other side:

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Answer

Plasmodesmata

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Question

Which of the following intercellular junctions are more abundant in tissues that stretch?

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Answer

plasmodesmata

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Question

The proteins found in desmosomes are called____ and connect to _____ inside the cell.

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Answer

Cadherins, intermediate filaments

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Question

What would happen to an epithelial tissue if desmosomes do not function properly?

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Answer

The epithelial cells would not be properly adhered to each other, and their intermediate filaments would not form a nearly continuous fibrous network throughout the tissue. The epithelium would not be able to withstand mechanical stress and cells would detach easily.

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Question

Why do plant cells do not have adherence junctions?

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Answer

Plant cells do not need adherence junctions because their cell walls and the lamella between them is what give mechanical and structural support to tissues and maintain the cells in their position.

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Question

Desmosomes are more abundant in the tissues that form:


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Answer

skin

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Tight junctions are more abundant in the tissues that form:


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skin

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Question

Why are gap junctions critical under certain situations where an extremely fast response is required?


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Answer

Because ions can pass with no delay through gap junctions, thus electrical signals travel faster than other signals (like chemical signals).

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Question

Which of the following statements are true about gap junctions?

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Answer

They are involved in the synchronous movement of cells in tissue that contract (heart, intestine)

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Question

Gap junctions are only present in cells that transmit electrical signals


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Answer

False

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