Intercellular junctions are specialized structures that connect cells, forming a complex network throughout the body. These connections allow cells to associate and interact, which is fundamental for the organization of cells into tissues and organs. They facilitate the coordinated activity necessary for multicellular organisms to function.
The Role of Intercellular Junctions
Intercellular junctions maintain the physical integrity of tissues. They bind cells together, providing structural support and allowing tissues to withstand various stresses. Beyond their role in physical cohesion, these junctions also enable communication and coordinated responses among cells.
These connections facilitate the transport of ions, nutrients, and signaling molecules, which allows cells to work in harmony. For instance, in muscle tissue, cell junctions enable the synchronized contraction of muscle fibers, allowing for movement.
Major Types and Functions
Tight junctions form impermeable seals between adjacent cells, preventing the leakage of fluids and molecules. They are found in epithelial tissues, such as the lining of the gut, where they prevent digestive enzymes and bacteria from entering the bloodstream. These junctions also maintain cell polarity by restricting the movement of membrane proteins between the apical and basolateral surfaces of cells, which is important for directional transport.
Gap junctions create direct channels between the cytoplasms of neighboring cells, allowing for the rapid passage of ions and small molecules. This direct communication is important in electrically active tissues like heart muscle, where it facilitates the synchronous contraction of cells. They also enable metabolic coupling, allowing cells to share nutrients and waste products.
Adherens junctions provide strong cell-to-cell adhesion by linking the actin cytoskeletons of adjacent cells. These junctions often form continuous belts around epithelial cells, contributing to the mechanical strength and integrity of tissues. They are particularly noticeable in cardiac muscle, where they help hold cells tightly together during contraction and relaxation.
Desmosomes are strong anchoring junctions that provide strong adhesion in tissues subjected to significant mechanical stress. They connect intermediate filaments to the plasma membrane, acting like “spot welds” that resist shearing forces. These junctions are abundant in the epidermis (outer layer of skin) and heart muscle, preventing cells from separating under tension.
Hemidesmosomes are distinct from desmosomes as they anchor cells to the extracellular matrix rather than to other cells. These structures are found at the basal surface of epithelial cells, connecting the cell’s intermediate filaments to the underlying basement membrane. They are important in tissues like the skin, where they ensure stable adhesion and resistance to mechanical forces that could otherwise lead to tissue detachment.
How Intercellular Junctions Are Built
The construction of intercellular junctions involves specific transmembrane proteins that span the cell membrane and interact with proteins from neighboring cells or the extracellular matrix.
Tight Junctions
Tight junctions are primarily formed by proteins called claudins and occludins, which create the sealing strands between cells. These proteins interact with intracellular scaffolding proteins that connect to the actin cytoskeleton, providing structural support.
Gap Junctions
Gap junctions are assembled from protein subunits called connexins. Six connexin proteins arrange to form a cylindrical structure known as a connexon or hemichannel in the membrane of one cell. This connexon then aligns with a connexon from an adjacent cell to form a complete channel, allowing direct passage of small molecules between the two cells. The specific types of connexins present can influence the permeability and selectivity of these channels.
Adherens Junctions
Adherens junctions rely on cadherin proteins for cell-cell adhesion. These transmembrane cadherins bind to each other in the extracellular space, while their intracellular tails connect to a complex of proteins called catenins. This complex links the cadherin-catenin complex to the actin cytoskeleton, providing mechanical stability.
Desmosomes
Desmosomes also utilize cadherin-family proteins for their adhesive function. These desmosomal cadherins interact with a cytoplasmic plaque composed of proteins. Desmoplakin then serves as a linker, connecting the entire complex to the intermediate filament network, which provides tensile strength characteristic of desmosomes.
Hemidesmosomes
Hemidesmosomes are anchored by integrin proteins, which bind to components of the extracellular matrix. On the intracellular side, integrins interact with adapter proteins that connect the complex to the keratin intermediate filaments of the cell’s cytoskeleton.
When Junctions Malfunction
Dysfunction in intercellular junctions can have consequences for human health.
Tight Junctions
When tight junctions in the intestine become compromised, it can result in increased intestinal permeability, often referred to as “leaky gut syndrome.” This allows undigested food particles, toxins, and bacteria to pass into the bloodstream, potentially triggering inflammation and contributing to inflammatory bowel disease and certain autoimmune disorders.
Desmosomes
Defects in desmosomes are linked to several skin disorders characterized by fragile skin and blistering. For instance, autoimmune conditions like pemphigus involve antibodies that attack desmosomal cadherins, leading to a loss of cell adhesion in the epidermis and the formation of blisters. Genetic mutations in desmosomal components can also cause inherited skin blistering diseases.
Gap Junctions
Malfunctions in gap junctions can disrupt the coordinated electrical activity in excitable tissues. In the heart, altered gap junction function or expression can lead to abnormal cardiac conduction and arrhythmias. These disruptions can impair the synchronized contraction of heart muscle.
Cancer
Changes in cell adhesion properties are also observed in cancer. In many solid tumors, the breakdown of normal cell-cell adhesions can facilitate cancer cell migration and invasion. This altered junctional integrity allows cancer cells to detach from the primary tumor and spread to distant sites, a process known as metastasis.