Cell-cell junctions

Definitions & Key takeaways

Cell-cell junctions are protein structures that physically link cells together. They are essential for cells to interact with each other and coordinate their activities. There are several types of cell-cell junctions, but the three most common are tight junctions, anchoring junctions, and gap junctions.

Tight junctions block the passages between cells and selectively prevent molecules from passing between them. This is important for maintaining the separation between cells and preventing them from becoming mixed. Anchoring junctions anchor cells to each other and provide them with structural support. Anchoring junctions include adherens junctions, desmosomes, and hemidesmosomes subtypes. Finally, gap junctions connect cells and allow substances such as ions to pass freely.

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Cell-cell junctions are protein structures that physically connect cells to one another. Cell-cell junctions facilitate for cellular communication, boost tissue structure, help with transport of materials between cells, or create an impermeable barrier for certain substances.
Cell-cell junctions are only found between immobile cells of organs and tissues-- so mobile cells like sperm and macrophages don’t have these structures.
Cell-cell junctions are most abundant in epithelial tissue, which is found in skin and the innermost layer of the gastrointestinal tract.
However, these structures are also found in other organs like the heart, kidneys, and liver. The three types of cell-cell junctions are adherens junctions, tight junctions, and gap junctions.
Adherens junctions are formed by groups of proteins that anchor cells together side by side and prevent their separation - making them “adhere” to one another.
Adherens junctions have three major components. The first component are long filamentous proteins called actin filaments that are part of the cytoskeleton and help give a cell its shape.
The second component are protein plaques, which are protein structures within the cytoplasm that are anchored to the plasma membrane that bind to the actin filaments.
Third, are transmembrane proteins called cadherins which attach to the protein plaques on one side and transverse the plasma membrane and connect to cadherins of an adjacent cell, linking the two together.
In this way, adherens junctions create a continuous network of interconnected cells via actin, which ultimately ties all these cells together to prevents their separation and provides extra strength.
This is particularly important in tissues that are exposed to constant shearing or abrasive forces like the skin or gastrointestinal tract.
It’s a bit like the reinforcing steel bar or rebar that sits within cement blocks to give a wall extra strength. Now, tight junctions, also known as occluding junctions, are protein structures that seal two plasma membranes of adjacent cells together.
As a result, they prevent water, small proteins, and bacteria from passing in between two adjacent cells. Tight junctions are formed by proteins called claudins and occludins that are embedded in the plasma membranes of both cells.
The proteins on one membrane can interlink with the proteins on the membrane of an adjacent cell and seal them together.
Typically, this seal happens only in one area near the apical surface of the cells and not across the entire length of the membrane.
Tight junctions are especially important in places like the gastrointestinal tract, where they help to prevent bacteria or digestive enzymes from entering the deeper tissues or water from seeping out of the intestine.
In the brain, the endothelial cells that line the inside of blood vessels also use tight junctions to form the blood brain barrier, preventing harmful materials in the blood from reaching the central nervous system.
Now, having said that, some tight junctions called leaky junctions do allow certain molecules to pass. For example, the tight junctions in the proximal tubule of the kidney allow potassium, sodium, and chloride to slip by them.
This is because the claudins and occludins in leaky junctions have ion pores that allow these ions to pass through. Next are gap junctions, which are sort of like sky bridges that might connect two adjacent buildings.
A single gap junction is made up of several connexin proteins, which are transmembrane proteins. These connexins create a tubular structure that allows charged particles like ions and small molecules like cytokines to pass between cells.
This allows cells to rapidly communicate with one another. For example, in cardiac myocytes, or heart muscle cells, ions from one cell can move through a gap junction to an adjacent cell, and that can help create a coordinated heart contractions.
Another example, is when an infected cell sends molecules called cytokines through gap junctions to initiate apoptosis or programmed cell death in a neighboring cell.
This is called the bystander effect, and it’s an effective way to prevent the spread of an infection to neighboring cells - which can’t host the pathogen if they’ve already died.
Alright, as a quick recap, cell-cell junctions are found in non-mobile cells. Adherens junctions provide extra resistance in tissue under a lot of force, tight junctions selectively block the passage of water, proteins, and bacteria, and gap junctions allow cells to communicate with each other by allowing signal molecules to pass through.