Cell junctions are specialized structures within the tissues of multicellular organisms that govern the interaction and organization of individual cells. These complexes coordinate biological function at the tissue level, acting as physical barriers and communication hubs. Without these connections, cells could not assemble into the cohesive sheets and robust structures necessary for organ function. Cell junctions allow tissues to maintain structural integrity while simultaneously regulating the passage of substances and transmitting signals between cellular neighbors. Their diverse functions are fundamental to processes like nutrient absorption, synchronized muscle contraction, and protection against pathogens.
Creating Impermeable Barriers
One function of specific cell junctions is to create a highly selective seal between adjacent cells, forming an impermeable barrier that dictates what passes through a tissue. These junctions, known as tight junctions, form a continuous, belt-like network around the apical perimeter of epithelial cells, such as those lining the gut or skin. The role of this structure is to block the movement of molecules and ions through the paracellular pathway, the space situated between the cells. This sealing action forces necessary substances to pass directly through the cell itself, where transport is controlled by specialized membrane proteins.
The physical seal is accomplished by rows of transmembrane proteins, primarily claudins and occludins, extending from one cell’s membrane and interacting with identical proteins on the neighboring cell. These protein strands interlock like stitches in a seam, fusing the outer leaflets of the two plasma membranes. The number of these sealing strands directly correlates with the tightness of the barrier; more strands result in a more impermeable seal. This regulated diffusion barrier is important for establishing and maintaining distinct body compartments with different fluid compositions.
The composition of claudin proteins expressed by a tissue determines the permeability characteristics of the tight junction, allowing for fine control over the barrier. For example, the epithelial lining of the gastrointestinal tract uses these junctions to prevent the leakage of digestive enzymes and block pathogens from entering the underlying tissue and bloodstream. The blood-brain barrier relies on exceptionally tight junctions in its endothelial cells to restrict the passage of substances from the blood into the neural tissue. In tissues like the kidney, specific claudins create selective channels for the regulated passage of certain ions or water, necessary for the efficient reabsorption of nutrients and waste clearance.
Providing Mechanical Strength
Another set of cell junctions provides tissues with mechanical strength, anchoring cells to one another or to the surrounding extracellular matrix. These anchoring junctions function like molecular rivets, ensuring that tissues can withstand significant physical stress, such as stretching, pulling, or shearing forces. The structures that connect cells to one another are primarily desmosomes and adherens junctions, which link the internal cytoskeletons of adjacent cells into a cohesive network.
Desmosomes serve as strong, localized patches that connect the intermediate filaments of neighboring cells, which are ropelike fibers that provide tensile strength to the cell. In the skin, this system links keratin filaments, creating the tissue’s resistance to tearing and abrasion. A similar arrangement links desmin filaments in heart muscle cells, ensuring the tissue can endure the continual, rhythmic mechanical stress of contractions. The adhesion is mediated by specialized cadherin proteins, which extend into the intercellular space to connect the cells, while adaptor proteins anchor them to the internal intermediate filament network.
Adherens junctions operate slightly differently, connecting the actin filament networks of cells, often forming a continuous belt just below the tight junctions in epithelial sheets. This belt of actin filaments provides a mechanism for coordinated shape changes and folding within the cell sheet during tissue development. Hemidesmosomes are the anchors that connect the cell’s internal structure to the basal lamina, the specialized sheet of extracellular matrix material that underlies epithelial tissue. These structures utilize integrin proteins to link the intermediate filaments inside the cell to the laminin and collagen fibers outside the cell.
Facilitating Direct Signaling
A third category of cell junctions focuses on communication, establishing direct channels between the cytoplasm of two adjacent cells. These channels, known as gap junctions, allow cells to share small molecules and ions without having to release signals into the extracellular space. This direct exchange mechanism is highly efficient and tightly regulated, enabling a group of cells to function as a single, coordinated unit.
The structure of a gap junction involves a cluster of protein assemblies called connexons, each of which is a hollow cylinder formed by six connexin protein subunits. A connexon in one cell membrane docks precisely with a connexon in the adjacent cell membrane, creating a continuous, open channel that bridges the two cytoplasms. The size of the pore formed by the docked connexons limits the passage to small molecules, typically those with a molecular weight under 1,000 daltons.
The substances that pass through these channels include inorganic ions, simple sugars, amino acids, and small intracellular signaling molecules like cyclic AMP and inositol triphosphate. This immediate sharing of electrical current and metabolic resources allows for two functions: electrical coupling and metabolic coupling. Electrical coupling is displayed in the heart, where the rapid, low-resistance passage of ions through gap junctions ensures that all muscle cells contract almost simultaneously. Metabolic coupling allows non-excitable cells, such as those in the lens of the eye or in developing embryos, to share nutrients and waste products, coordinating cellular activities across the tissue.