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 is 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 prevent 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.