Cell junctions are specialized protein structures that mediate physical contact between neighboring cells or anchor cells to the surrounding extracellular matrix. These multiprotein complexes are fundamental to the organization and coordinated function of multicellular life. They enable cells to assemble into tissues and organs, allowing the body to maintain distinct internal environments and withstand external forces. The ability of cells to adhere, form barriers, and communicate directly is a prerequisite for complex physiology.
Creating Impermeable Seals Between Cells
One primary function of cell junctions is to establish selective barriers between fluid compartments, essentially acting as the grout between the tiles of an epithelial or endothelial layer. This task is performed by tight junctions (zonula occludens), which form a continuous, belt-like seal around the cell’s apex. These junctions are formed by a network of transmembrane proteins, primarily claudins and occludins, which fuse the outer leaflets of the adjacent plasma membranes.
This intricate molecular stitching prevents the uncontrolled passage of ions, water, and solutes through the space between cells, a process called paracellular transport. By sealing the gaps, tight junctions enforce a selective transport mechanism, forcing substances to pass through the cell’s cytoplasm, known as transcellular transport. This precise control is necessary for tissues that maintain distinct environments.
Tight junctions in the lining of the intestine prevent harmful bacteria and digestive enzymes from leaking into the bloodstream. They also form the highly restrictive barrier in the brain’s capillaries, controlling what substances can enter the nervous tissue. The complexity of the barrier is variable, with some tissues having “tighter” seals, while others are “leaky,” allowing the regulated passage of specific small ions or water.
Providing Structural Stability and Mechanical Strength
The second major function of cell junctions is to provide mechanical strength, allowing tissues to resist physical stress, such as in the skin or heart muscle. This structural role is fulfilled by anchoring junctions, which link the cytoskeletons of adjacent cells or connect a cell’s internal framework to the extracellular matrix. These junctions are categorized based on what they connect and which cytoskeletal filament they utilize.
Adherens junctions mediate cell-to-cell adhesion, typically forming a band just below the tight junctions in epithelial sheets. They link to the internal actin cytoskeleton via transmembrane proteins called cadherins and associated adapter proteins. This connection is essential for coordinating cell movement during development and for stabilizing tissue architecture.
Desmosomes (macula adherens) are spot-like attachments that provide robust mechanical resilience by linking cells through their intermediate filament networks. Transmembrane proteins called desmogleins and desmocollins anchor to an intracellular plaque, which in turn connects to intermediate filaments like keratin in skin cells. This strong connection acts like a rivet, distributing tension across the tissue and preventing cells from being pulled apart under stress.
For anchoring to the underlying support structure, cells rely on hemidesmosomes and focal adhesions. Hemidesmosomes, which resemble half of a desmosome, use integrin proteins to connect the cell’s intermediate filaments to the basal lamina of the extracellular matrix. Focal adhesions also use integrins but link the cell’s actin cytoskeleton to the matrix, playing a dynamic role in cell migration and sensing the external environment.
Direct Communication Between Adjacent Cells
A third distinct function involves establishing direct, rapid communication pathways between the cytoplasm of two adjacent cells. This is the role of gap junctions, which act as regulated tunnels for the quick exchange of small molecules and ions. The structure is formed by protein complexes called connexons, each composed of six connexin protein subunits.
A connexon from one cell aligns perfectly with a connexon from a neighboring cell, creating an aqueous pore that bridges the intercellular space. This channel allows for the passage of substances smaller than about 1,000 daltons, including ions, sugars, amino acids, and second messenger signaling molecules. The channels are not static but can open and close based on cellular needs.
This direct electrical and metabolic coupling is necessary for the coordinated activity of entire cell populations. In the heart, gap junctions enable the rapid spread of electrical signals, ensuring that all muscle cells contract in a synchronized rhythm. In non-excitable tissues, like bone, they facilitate metabolic coupling, allowing the sharing of nutrients and waste products among cells in an extensive network.
When Junctions Fail
The proper function of cell junctions is necessary for health, and their failure can lead to severe disease, typically linked to the loss of their specific function. A breakdown of the sealing function of tight junctions, for instance, results in increased tissue permeability. This loss of barrier integrity is implicated in various conditions, including inflammatory bowel disease, where the gut lining becomes “leaky,” allowing pathogens and toxins to enter the underlying tissue.
When anchoring junctions fail, the tissue loses its mechanical strength, leading to disorders where cells are easily separated. A classic example is the autoimmune blistering disease Pemphigus, where the body produces antibodies that attack the desmosomal proteins connecting skin cells. The resulting lack of cell-to-cell adhesion causes the outer layers of the skin to detach and form painful blisters.
The dysfunction of communicating junctions can disrupt the coordination of excitable tissues. Mutations in connexin proteins that form gap junctions are associated with conditions, including hereditary deafness and cardiac arrhythmias. In the heart, a failure of these channels to transmit electrical signals uniformly results in uncoordinated contraction, which can be life-threatening.