Epithelium: Cell Junctions
There are four main types of cell-cell junctions:
Three are different types of connecting junctions, that bind the cells together.
- occluding junctions (zonula occludens or tight junctions)
- adhering junctions (zonula adherens).
- desmosomes (macula adherens). There are also 'hemidesmosomes' that lie on the basal membrane, to help stick the cells to the underlying basal lamina.
- Gap junctions. These are communicating junctions. (also known as nexus, septate junction)
These types of cell junctions are found between epithelial cells, but can also between other types of cells.
The borders of two cells are fused together, often around the whole perimeter of each cell, forming a continuous belt like junction known as a tight junction or zonula occludens (zonula = latin for belt). These regions of the cells are very tightly connected together, such that the adjacent plasma membranes are sealed together. Proteins in the membrane of adjacent cells called occludin interact with each other to produce this tight seal. In the cytoplasm of the cell, occludin interacts with the actin cytoskeleton via another proteins called ZO-1. Many pathogens act on the proteins that form this tight junction, making it permeable.
This type of junction greatly restricts the passage of water, electrolytes and other small molecules across the epithelium. Transmembrane proteins from each cell membrane interlock across the intercellular space, all around the cell, in this belt (black lines in the diagram).
The permeability of tight junctions varies from site to site, and are often can be selectively leaky. For example, these junctions are important in the gut, in acting as a selective diffusion barrier, preventing diffusion of water soluble molecules. They also act to restrict the localisation of membrane bound proteins. (For more information, see the section on the gut).
Epithelial cells are held together by strong anchoring (zonula adherens) junctions.
The adherens junction lies below the tight junction (occluding junction). In the gap (about 15-20nm) between the two cells, there is a protein called cadherin - a cell membrane glycoprotein. (The type of cadherin found here is E-cadherin). The cadherins from adjacent cells interact to 'zipper' up the two cells together.
Inside the cell, actin filaments (microfilaments, shown here in red) join up the the adhesion junctions. These filaments tend to be arranged circumferentially around the cell, as a 'marginal' band.
This marginal band can contract, and deform the shape of cells held together in this way - this is thought to be key in the morphogenesis of epithelial cells, in forming ducts for example.
Desmosomes and Hemidesmosomes
from Molecular Biology of the Cell
Desmosomes connect two cells together. A desmosome is also known as a spot desmosome or macula adherens (macula = latin for spot), because it is circular or spot like in outline, and not belt- or band shaped like adherens junctions.
Desmosomes are particularly common in epithelia that need to withstand abrasion (see skin). Desmosomes are also found in cardiac cells, but the intermediate filament in this case is desmin, not keratin (which is found in epithelial cells).
The picture shows an EM of a desmosome formed between two cells. Notice the phase dense material between the two cell membranes, which is mad up of transmembrane linker glycoproteins (e.g. demosgleins and desmocollins - which are cadherin proteins). Also notice the intermediate filaments running from the desmosome into the cytoplasm.
Other proteins run across the membrane into the intracellular space, to connect the two cells together. These 'transmembrane linker' proteins are called cadherins (the type of cadherins found here are called desmoglein and desmocollin).
The diagram shows a desmosome. It is made up of a dense cytoplasmic plaque, to which the intermediate filaments attach.
These look similar to desmosomes, but are different functionally, and in their content. They connect the basal surface of epithelial cells via intermediate filaments to the underlying basal lamina. The transmembrane proteins of hemidesmosomes are not cadherins, but another type of protein called integrin.
A group of protein molecules called connexins form a structure called a connexon (each particle in B above is a connexon). When connexons (blue in the diagram to the right) from two adjacent cells (red and yellow in the diagram) align, they form a continuous channel between them.
This channel is big enough to allow small molecules such as inorganic ions, and other small water soluble molecules (smaller than 1000kDa) to pass between the cells. However the channel is too small for proteins, nucleic acids or sugars to pass through.