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Cell Junctions
Although certain cell types – blood cells, and some immune system cells – move freely in the body, many others are packed into
tight communities. Typically, three factors act to bind cells together.

Glycoproteins in the glycocalyx act as an adhesive.

Contours in adjacent cells membranes fit together in a tight knit fashion.

Special cell junctions form.
Let’s take a closer look at the different types of cell junctions.
Tight Junctions
In a tight junction, a series of integral protein molecules in the plasma membranes of adjacent cells fuse together, forming an
impermeable junction that encircles the cell. Tight junctions help prevent molecules from passing through the extracellular space
between adjacent cells. For example, tight junctions between epithelial cells lining the digestive tract keep digestive enzymes and
microorganisms in the intestine from leaking into the bloodstream (Note: some tight junctions may leak and allow certain ions to
pass).
Desmosomes
An epithelial cell is shown joined to adjacent cells by three common types of cell junctions. (Note: Except for epithelia, it is unlikely
that a single cell will have all three junction types.)
Desmosomes are anchoring junctions – mechanical couplings scattered like rivets along the sides of adjacent cells to prevent their
separation. On the cytoplasmic face of each plasma membrane is a thickening called a plaque. Adjacent cells are held together by
thin linker protein filaments that extend from the plaques and fit together like the teeth of a zipper in the intercellular space. Thicker
keratin filaments extend from the cytoplasmic side of the plaque across the width of the cell to anchor to the plaque on the cell’s
opposite side. In this way, desmosomes bind neighboring cells together and also contribute to a continuous internal network of
strong wires.
This arrangement distributes tension throughout a cellular sheet and reduces the chance of tearing when it is subjected to pulling
forces. Desmosomes are abundant in tissues subjected to great mechanical stress, such as skin and heart muscle.
Gap Junctions
A gap junction is a communicating junction between adjacent cells. At gap junctions the adjacent plasma membranes are very close,
and the cells are connected by hollow cylinders called connexons, composed of transmembrane proteins. The many different types
of connexon proteins vary the selectivity of the gap junction channels. Ions, simple sugars, and other small molecules pass through
these water-filled channels from one cell to the next.
Gap junctions are present in electrically excitable tissues, such as smooth muscle and the heart, where ion passage from cell to cell
helps synchronize their electrical activity and contraction.
Review

Tight junctions: Impermeable junctions that prevent molecules from passing through the intercellular space.

Desmosomes: Anchoring junctions that bind adjacent cells together and help form an internal tension-reducing network of
fibers.

Gap junctions: Communicating junctions that allow ions and small molecules to pass for intercellular communication.
Check your understanding
1.
Which two types of cell junctions would you expect to find between muscle cells of the heart?
Cell to Cell Junctions
Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow
varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions (Figure).
Types of Cell Junctions
The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.
At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments. An anchoring
junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral
and basal surfaces of cells where they provide strong and flexible connections. There are three types of anchoring junctions:
desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are
structural proteins on the inner surface of the cell’s membrane. The adhesion molecule, cadherin, is embedded in these patches and
projects through the cell membrane to link with the cadherin molecules of adjacent cells. These connections are especially
important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for
example, the basal lamina. While similar in appearance to desmosomes, they include the adhesion proteins called integrins rather
than cadherins. Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix.
The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell
membrane. The actin can connect isolated patches or form a belt-like structure inside the cell. These junctions influence the shape
and folding of the epithelial tissue.
In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of
adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow
electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.