Cell junction

Cell junctions, often called junctional complexes, are structures built from multiple proteins that link animal cells to one another or to the extracellular matrix. They form a seal between epithelial cells, regulate what passes between them, and are found in high numbers in epithelial tissue. Along with cell adhesion molecules and the extracellular matrix, these junctions keep animal cells connected. They also allow cells to communicate directly through specialized protein channels known as gap junctions, and they help cells withstand physical stress. In plants, similar communication channels are called plasmodesmata; in fungi, they are known as septal pores.

In vertebrates, three main types of cell junctions exist: anchoring junctions (which include adherens junctions, desmosomes, and hemidesmosomes), gap junctions (communicating junctions), and tight junctions (occluding junctions). Invertebrates have additional types, such as septate junctions (an occluding type) and the C. elegans apical junction. In multicellular plants, cell walls provide the structural support that cell junctions offer in animals, while plasmodesmata serve as the plant equivalent of communicative junctions.

Anchoring junctions tie cells together and attach them to the extracellular matrix, giving tissues structural cohesion. They are most common in tissues under constant mechanical stress, like skin and heart. These junctions use anchoring proteins that cross the plasma membrane to link the cytoskeleton of one cell to that of a neighbor or to matrix proteins. There are three types, distinguished by their cytoskeletal anchor and transmembrane linker protein.

Desmosomes, also called maculae adherentes, act like rivets between adjacent cells. Intermediate filaments made of keratin or desmin attach to a dense protein plaque on the inner side of the membrane. Cadherin molecules then extend through the membrane from each cell and bind tightly to one another, forming the actual anchor.

Hemidesmosomes form rivet-like connections between the cytoskeleton and the extracellular matrix, such as the basal lamina under epithelia. Like desmosomes, they attach to intermediate filaments, but their transmembrane anchors are integrins, not cadherins.

Adherens junctions anchor cells through their cytoplasmic actin filaments. Their transmembrane anchors are cadherins when linking cells together, and integrins (in focal adhesions) when attaching to the extracellular matrix. These junctions come in various shapes: isolated streaks or spots, or bands that encircle the cell (adhesion belts). The band type is linked to bundles of actin filaments that also ring the cell just under the membrane. Spot-like focal adhesions help cells stick to the matrix. Because the actin filaments are contractile, adherens junctions are thought to help fold and bend epithelial sheets—the actin bands act like drawstrings, and when they contract in a group of cells, the sheet deforms into patterns.

Gap junctions, or communicating junctions, let adjacent cells exchange chemicals directly through diffusion, bypassing the extracellular fluid. They are made of six connexin proteins that form a cylinder with a central pore, called a connexon. Each connexon spans the cell membrane, and when two connexons from neighboring cells align, they create a complete channel. The pores vary in size and charge selectivity depending on the specific connexin proteins involved. Despite this variation, the structure remains fairly standard, ensuring efficient communication without leaking molecules or ions into the extracellular space. Gap junctions are vital for the uniform contraction of heart muscle, signal transfer in the brain (their absence reduces brain cell density), and the differentiation and proliferation of retinal and skin cells.

Tight junctions, found in vertebrate epithelia, act as barriers that control the movement of water and solutes between epithelial layers. They form a paracellular barrier that does not discriminate by direction, but solute passage depends largely on size and charge. Evidence suggests that solutes move through pore-like structures, and physiological pH influences which solutes are allowed through.