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4 Types of Cell Junctions

In this 4 Types of Cell Junctions post we have briefly explained about different types of cell junctions Tight Cell Junctions, Adherens Cell Junctions and Gap Cell Junctions composition, locations and functions.

In all tissues, specialised cell junctions can be found at areas of cell-cell and cell-matrix interaction, although epithelia have the most. Cell junctions are best viewed using conventional or freeze-fracture electron microscopy, which reveals that the connecting plasma membranes (as well as the underlying cytoplasm and intercellular space) are highly specialised in these areas.

Different Types of Cell Junctions

1. Tight Cell Junctions

Tight junctions serve as selectively permeable seals in our body’s internal and external surfaces. Imagine a largely waterproof zipper connecting the sides of two different jackets. That zipper is like a tight junction (TJ), also called an occluding junction. A TJ creates a small zone that occludes the extracellular space (the space between cells). This is why tight junctions are also called zonula occludens. The word zonula comes from words that mean small zone or encircling belt, while occludens comes from the Latin word occludere, which means to close up.

Composition

A tight junction a kind of symmetrical cell junction is composed of numerous important proteins that are either directly involved in its composition or intimately involved with connecting the tight junction to and between the cells in one way or another. These proteins include:

(i) Occludins, which maintain the barrier between adjacent cells. (ii) Claudins, which form the backbone of tight junction strands. (iii) Junctional adhesion molecules (JAMs) are immunoglobulin (antibody) proteins that help seal the intercellular space between two cells. (iv) Zonula occludens (ZO) are proteins that help link the tight junction to each cell’s internal skeleton (cytoskeleton).

The occludins and claudins are the major components of tight junction strands. When fully formed, a tight junction is not one, long, continuous seal. Instead, it looks like a series of local seals joined together in a maze-like fashion.

cell junctions

Tight junction: Diagram of tight junction components.

Function

Tight junctions are virtually (but also partly selectively) impermeable seals that encircle cells and bind them together into leak-proof sheets. In other words, the plasma membranes of adjacent cells essentially fuse together tightly in order to limit the leakage of various substances between the two cells.

What can and cannot go through all depends on the substance’s size, charge, as well as the location and precise composition of the tight junctions in the part of the body in question. Tight junctions are located within our body’s epithelia. Epithelia are the plural of epithelium. Epithelium is a word that refers to the covering of the body’s internal and external surfaces. This includes organs (such as skin), blood vessels, and cavities.

Thus, these tight junctions serve various functions, depending on what epithelium is in question. In the skin, they keep us somewhat watertight and help keep allergens out of our body. In the digestive system, they help prevent the leakage of digestive enzymes into our bloodstream. Tight junctions also serve as a structural support mechanism that helps keep the epithelium together.

Different Types of Cell Junctions: 4 Types of Cell Junctions

2. Adherens Cell Junctions

Adherens junctions provide strong mechanical attachments between adjacent cells through the linkage of cytoplasmic face with cytoskeleton.  Adherens junctions are also referred to as zonula adherens, intermediate junction, or as belt desmosomes. Zonula means small zone or belt-like, and adherens refers to adhesion (sticking together). As a result, the zonula adherens often runs like a belt around the entire cell in a continuous fashion, and it acts as an adhesion belt.

Cell Junctions

The structural proteins in an adherens junction: These are the principal interactions of structural proteins at a cadherin-based plasma membrane adherens junction. Actin filaments are associated with adherens junctions in addition to several other actin-binding proteins.

Composition

The zonula adherens is composed of several different proteins: The actin microfilaments of the cytoskeleton (the internal skeleton of the cell). Anchor proteins, found inside each cell. These are called alpha-catenin, beta-catenin, gamma-catenin (aka plakoglobin), vinculin, and alpha-actinin. They link the actin microfilaments to the cadherins. Cadherins, namely E-cadherin. These are transmembrane adhesion proteins, whose main portions are located in the extracellular space.

The extracellular part of one cell’s cadherin binds to the extracellular part of the adjacent cell’s cadherin in the space between the two cells. Each cell’s cadherin molecule also contains a tail that inserts itself inside its respective cell.

This intracellular (within the cell) tail then links up to catenin proteins to form the cadherin–catenin complex. This complex binds to vinculin and alpha-actinin; these two proteins are what link the cadherin–catenin complex to the cell’s internal skeletal framework (the actin microfilaments).

The extracellular portions of the cadherin molecules of adjacent cells are bonded together by calcium ions (or another protein in some cases). This means that the functional as well as morphological integrity of the adherens junctions are calcium dependent. If you were to remove calcium from the equation, this type of cell junction would disintegrate as a result.

Function

This type of cell junction is located right below tight junctions and provides a strong bond between the sides of adjacent epithelial cell membranes. While other junctions, like tight junctions, provide some support for and fusion of adjacent cells, their resistance to mechanical stress is relatively small compared to the much stronger adherens junctions.

3. Gap Cell Junctions

A gap junction is a specialized cell junction that directly connects the cytoplasm of two cells. Gap junctions are also called communicating junctions, macula communicans, or nexuses. These are connections that allow for the direct passage of molecules between two cells.

Gap junctions consist of a number of transmembrane channels called pores that are found in a closely packed arrangement. The number of gap junctions shared between two cells can vary as well.

Location

Gap junctions are found in many places throughout the body. This includes epithelia, which are the coverings of body surfaces, as well as nerves, cardiac (heart) muscle, and smooth muscle (such as that of the intestines).

Their primary role is to coordinate the activity of adjacent cells. For instance, when heart cells need to beat in unison, gap junctions allow for the transmission of electrical signals between the cells.

Cell Junctions

Gap junction: The major molecular components of the gap junction.

Structure

Each gap junction channel is made up of two half channels (hemichannels), one in each cell’s membrane. These half channels join together, bridge the extracellular space in the process, and form the entire channel that spans both cell membranes.

Each of these half channels is called a connexon. Each connexon is made up of six symmetrical integral membrane protein units called connexins. This means each channel is made up of 12 circularly arranged protein units.

Function

The molecules that may cross this channel include the likes of ions, regulatory proteins, and metabolites (products of metabolism).  Examples of this include calcium ions and cAMP (cyclic adenosine monophosphate).

Depending on the type of gap junction in question, molecules can pass evenly in both directions, or asymmetrically, so in some gap junctions the molecules will move in one direction faster than in the other direction.

The channels in a gap junction aren’t always open. They fluctuate between being open and closed. The ability of the channel to open or close is made possible in part to calcium ions, which induce a reversible conformational change in the connexin molecules, which leads to the closure of a channel at its extracellular surface. The cytoplasmic end of each connexon can also be closed, if necessary.

Further Readings

Reference