Epithelial cells are held together by strong anchoring (adherens) junctions.
There are two types of adherens junctions:
zonula adherens - which contain actin filaments
macula adherens (desmosomes) which contain intermediate filaments.
The zonula adherens junction lies below the tight junction (occluding junction). In the gap between the two cells, there is a protein called E-cadherin - a cell membrane glycoprotein. The cadherins from adjacent cells interact to 'zipper' up the two cells together.
Inside the cell, E-cadherin binds to catenin, which in turn binds to other proteins (vinculin, alpha actinin) in a protein complex with actin filaments (microfilaments, shown here in red).
The actin filaments tend to be arranged circumferentially around the cell, into what is called 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.
These are regions in which two cells are very tightly connected together, and they will prevent some molecules from passing across an epithelium. 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). 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, see the section on the gut)
from Molecular Biology of the Cell
Desmosomes and Hemidesmosomes
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 desmoglein and desmocollin, which are types of cadherin.
This 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. The 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.
This electron micrograph shows a Hemidesmosome (H), and two of the three layers of the underlying basal lamina. LL - lamina densa, LD - lamina lucida. Integrins in the plasma membrane link the cell to the extra-cellular matrix.
from Wheater's Functional Histology.
Gap junctions are the most widespread of all cell junctions in animal tissues. Gap junctions thus couple cells electrically and metabolocially, enabling cells to communicate with each other directly. Gap junctions can open and close in response to changes in calcium levels, and pH. Gap junctions form in a narrow gap of 2-4nm, between two adjacent cells.
The picture shows an EM (A) of two gap junctions between two cells, and a freeze fracture EM (B) of the particles on the cytoplasmic face of the plasma membrane.
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.