Cell junctions are specialized multiprotein complexes located at the surface of cells, particularly abundant in epithelial tissues that line organs and cavities. They are indispensable for the structural integrity and coordinated function of multicellular organisms. Cell junctions serve a dual purpose: physically binding neighboring cells together and establishing pathways for direct communication. These connections ensure that tissues, such as skin and the lining of the digestive tract, maintain their shape and perform synchronized functions.
Junctions That Create Physical Barriers
The function of creating an impermeable seal between adjacent cells is carried out by tight junctions (zonula occludens). These junctions form a continuous, belt-like network around the cell that physically restricts the movement of molecules through the intercellular space, known as paracellular transport. To cross a sheet of cells protected by tight junctions, substances must be actively transported through the cells themselves, allowing precise control over what enters the body.
Tight junctions are formed by interlocking strands of transmembrane proteins, mainly from the claudin and occludin families, which span the membranes of two neighboring cells. Claudins are considered the backbone of the strands and influence the junction’s ability to seal the space. They sometimes form selective channels for small ions and water. Occludins also contribute to the barrier function and regulate cell structure.
This sealing function is important in forming the blood-brain barrier, where tight junctions in the endothelial cells of blood vessels prevent large or harmful molecules from entering the central nervous system. They also establish a “fence function” within the cell membrane, preventing the movement of proteins and lipids between the apical and basolateral surfaces of the cell. This segregation ensures that each surface maintains the unique receptors and transport mechanisms required for polarized cellular function.
Junctions That Anchor Cells Together
Anchoring junctions provide the mechanical strength and physical cohesion necessary for tissues to withstand stress and strain. They securely link the cytoskeleton of one cell to the cytoskeleton of its neighbor, functioning like rivets or belts. Two main types provide this structural support: adherens junctions and desmosomes, each connecting to a different component of the cellular skeleton.
Adherens junctions (zonula adherens) are protein complexes that connect to the actin microfilaments, which are thin, contractile cables beneath the cell membrane. These junctions often form a continuous band around the cell and are crucial in initiating cell-cell contacts and regulating cell shape. The transmembrane adhesion proteins are classical cadherins, which link the cells together in a calcium-dependent manner.
Desmosomes (macula adherens) function like spot-welds, providing strong localized adhesion between cells. Unlike adherens junctions, desmosomes connect to the intermediate filaments of the cytoskeleton, which are thicker and more stable. This connection grants tissues, such as the skin and heart muscle, tensile strength and resistance to tearing. The adhesion molecules are desmosomal cadherins, which link to a dense cytoplasmic plaque that ultimately tethers the intermediate filaments.
Junctions That Facilitate Direct Communication
Gap junctions are specialized channels that provide a direct pathway for communication and metabolic coupling between adjacent cells. They enable the rapid passage of small, water-soluble molecules, ions, and electrical signals between cytoplasms. This direct connection is formed by protein structures called connexons, which are assemblies of six protein subunits known as connexins.
A connexon in one cell membrane aligns precisely with a connexon in the neighboring cell membrane, creating a complete intercellular channel that bridges the narrow 2–4 nanometer gap. The size selectivity of the channel allows molecules up to about 1 kilodalton in size to pass through. These include ions, second messengers like cAMP, and small metabolites like glucose. This exchange allows cells to share nutrients and coordinate their responses.
The swift movement of ions through gap junctions is important in tissues requiring synchronized activity. In cardiac muscle, the rapid transfer of electrical signals ensures that cells contract almost simultaneously for a coordinated heartbeat. Gap junctions also coordinate metabolic functions in non-excitable tissues, such as glial cells in the brain, helping maintain tissue homeostasis.