Intercellular junctions

Intercellular junctions are regions of contact between adjacent cells, made of multiprotein complexes that allow communication between cells. They are abundant in epithelial cells.

There are three types, and each answers a different question.

Barrier, or impermeable, junctions, the tight or occluding junctions, also called zonula occludens: do they seal? Yes, completely.

Adherent junctions: do they hold? Yes, but they do not seal.

Communicating junctions, the gap junction or nexus: do they let things through? Yes, deliberately.

Intercellular junctions are regions of contact between adjacent cells.

They consist of multiprotein complexes, which allow communication between cells.

They are abundant in epithelial cells, which is where cells are most tightly packed and where a sheet has to behave as a barrier.

There are three types.

Barrier, or impermeable, junctions are the tight or occluding junctions, also called zonula occludens.

• They form a band completely encircling the cell.
• They hold cells together really tightly.
• They act as a fluid barrier: water and ions are not able to go through between the cells, so everything is blocked.
• They are found in epithelial cells.
• About 40 proteins are involved in tight junctions. They can be classified into scaffolding proteins, signalling proteins, regulation proteins and transmembrane proteins.

The consequence is the point of the whole structure: they ensure that molecules crossing an epithelium go through the cell rather than between them. That is what makes an epithelium a controlled surface rather than a leaky one.

Adherent junctions connect neighbouring cells in particular spots, but do not seal them together.

• They are attached to the cytoskeleton.
• They are strong but flexible, for example in the stomach or bladder, both organs that change shape constantly.
• Strong intermediate filaments couple adjacent cells, binding in different ways depending on the type of anchoring junction:

The distinction is in the names: a zonula is a belt encircling the cell, a macula is a spot.

The communicating junction is the gap junction, also called the nexus.

• It provides channels for communication between adjacent cells.
• 6 transmembrane gap junction proteins, called connexins, make 1 connexon.
• 2 connexons make 1 gap junction channel.
• It is important for signal transfer in the brain and the contraction of the heart muscle.

The arithmetic is worth memorising as a chain: 6 connexins → 1 connexon → and 2 connexons → 1 channel. Two connexons because the channel has to be built from both cells, one half each.