Cell junctions are specialized protein structures that physically connect cells to each other or to the surrounding support matrix. They hold tissues together, control what passes between cells, and allow cells to communicate. Without them, your skin would fall apart, your intestines would leak, and your heart couldn’t beat in rhythm. Cell junctions fall into three functional categories: occluding junctions that seal gaps between cells, anchoring junctions that provide mechanical strength, and communicating junctions that pass signals or small molecules directly from one cell to the next.
Occluding Junctions: The Seals
Occluding junctions, commonly called tight junctions, seal neighboring cells together so tightly that even small molecules cannot slip between them. Picture a row of cells lining your intestine. Without tight junctions, digestive acids and bacteria could leak through the gaps between those cells into your bloodstream. Tight junctions prevent that by forming a continuous, belt-like seal near the top of each cell, effectively turning a sheet of individual cells into a waterproof barrier.
This sealing function is critical in several places: the lining of the gut, the inner surface of blood vessels, and the barrier between blood and brain tissue. In each case, tight junctions force substances to pass through cells rather than between them, giving the body precise control over what gets absorbed or excluded.
Anchoring Junctions: The Rivets
Anchoring junctions are the mechanical fasteners of your tissues. They physically attach cells to their neighbors or anchor them to the structural scaffolding (called the extracellular matrix) that surrounds them. There are four main types, and they split into two pairs based on what they connect to.
Cell-to-Cell Anchoring
Adherens junctions and desmosomes both hold neighboring cells together using proteins from the cadherin family, but they connect to different internal frameworks inside the cell.
Adherens junctions link to the cell’s actin network, the same system of filaments that drives cell movement and shape changes. The connection works through a chain of adapter proteins: cadherin proteins span the cell membrane and grab onto the neighboring cell, while on the inside, beta-catenin and alpha-catenin bridge the gap between the cadherin’s tail and the actin filaments. This setup doesn’t just glue cells in place. It mechanically couples them so that when one cell contracts or moves, its neighbors feel the pull. That coupling drives much of the tissue remodeling that happens during embryonic development and wound healing.
Desmosomes connect instead to intermediate filaments, which in skin cells are made of the tough protein keratin. Think of desmosomes as spot welds scattered across the surface where two cells meet, with keratin cables radiating inward from each weld point. This architecture is built for absorbing stress. Studies using keratin-deficient skin cells showed that without keratin filaments attached, desmosomes fell apart under rotational force. When the keratin pair KRT5/KRT14 was added back, the desmosomes reformed and the cell sheet regained its resistance to shearing. Desmosomes are especially abundant in skin, the lining of the mouth, and heart muscle, all tissues that endure constant mechanical strain.
Cell-to-Matrix Anchoring
Focal adhesions and hemidesmosomes anchor cells to the extracellular matrix rather than to other cells. Both use a different family of transmembrane proteins called integrins, which reach through the cell membrane to grip structural proteins in the matrix below.
Hemidesmosomes are particularly important in skin. They pin the bottom of the outermost skin layer (the epidermis) to the basement membrane beneath it. The integrin pair alpha-6/beta-4 concentrates heavily in hemidesmosomes and connects, through internal adapter proteins, to the same keratin filament network that desmosomes use. This means the entire epidermis is essentially stitched together by a continuous cable system: hemidesmosomes anchor cells to the floor, desmosomes anchor cells to each other, and keratin filaments run between all of them like tensile cables in a suspension bridge.
Communicating Junctions: The Channels
Gap junctions are the primary communicating junctions in animal cells. Rather than sealing or fastening, they create tiny channels that allow small molecules and electrical signals to pass directly from the interior of one cell into its neighbor.
Each channel is built from two half-channels called connexons, one contributed by each cell. A connexon is a ring of six connexin protein subunits arranged around a central pore. When the connexons from two neighboring cells line up and dock, they form a complete channel spanning both cell membranes and the small gap between them. Molecules up to roughly 1,000 daltons in molecular weight can pass through. That includes ions (which carry electrical current), signaling molecules like calcium, and small metabolites like amino acids and sugars. It excludes proteins, DNA, and other large molecules.
Different connexin types create channels with slightly different properties. Some favor positively charged ions over negatively charged ones, some have larger or smaller pores, and some preferentially pass specific signaling molecules. This variety lets different tissues fine-tune what their gap junctions transmit. In heart muscle, gap junctions rapidly spread the electrical impulse that triggers each heartbeat, ensuring millions of cells contract in a coordinated wave. In the liver, they share metabolic signals. In the lens of the eye, they distribute nutrients to cells that lack their own blood supply.
Plants use a different structure called plasmodesmata to accomplish the same goal. Like gap junctions, plasmodesmata directly connect the interiors of adjacent cells, but their architecture is distinct, built around a tube of membrane rather than a ring of protein subunits.
What Happens When Junctions Fail
Because cell junctions are load-bearing and barrier-forming structures, defects in their component proteins cause serious disease. Desmosome disorders are among the best studied examples.
Pemphigus vulgaris is an autoimmune condition in which the immune system produces antibodies that attack desmosomal cadherin proteins (specifically desmoglein-3 and desmoglein-1) in the skin and mucous membranes. With the desmosomes disabled, skin cells detach from each other, a process called acantholysis, leading to painful blisters in the mouth, throat, and on the skin. A related condition, pemphigus foliaceus, targets only desmoglein-1 and causes more superficial blistering confined to the skin.
Genetic mutations in desmosome proteins cause a different set of problems. Mutations in desmoglein-2, desmocollin-2, or the adapter protein plakophilin-2 can cause arrhythmogenic right ventricular cardiomyopathy, a condition where heart muscle cells gradually lose their connections, are replaced by fat and scar tissue, and can trigger dangerous heart rhythm disturbances. Naxos disease, caused by a mutation in the desmosomal protein plakoglobin, produces a striking combination: the same type of heart disease along with thickened skin on the palms and soles and unusually woolly hair.
Even a partial loss of desmosome function matters. Having only one working copy of the gene for desmoglein-1 (instead of the usual two) causes striate palmoplantar keratoderma, a condition marked by ridged, thickened skin on the hands and feet where mechanical stress is highest.
Gap junction defects cause their own spectrum of disease. Mutations in connexin-26, one of the most common connexin types, are the leading genetic cause of nonsyndromic hearing loss. The cells of the inner ear rely on gap junctions to recycle potassium ions after sound stimulation, and when those channels malfunction, the delicate sensory cells gradually lose function.
How Junction Types Work Together
In a typical epithelial tissue like the lining of your intestine, all three junction categories appear in a predictable arrangement on each cell. Tight junctions sit nearest the top, forming the permeability seal. Just below them, a belt of adherens junctions provides the first layer of mechanical attachment and connects to the actin-based contractile network. Further down, desmosomes add reinforcement by linking to the keratin network. And scattered along the lateral surfaces, gap junctions allow the cells to share ions and signaling molecules so they can coordinate their behavior as a unified tissue.
This layered arrangement isn’t accidental. Each junction type handles a different aspect of tissue function: barrier control, mechanical resilience, and communication. Together, they turn what would otherwise be a loose collection of individual cells into a cohesive, functional tissue capable of withstanding mechanical stress, controlling molecular traffic, and responding to its environment as a coordinated unit.