Cellular junctions are specialized structures that allow cells to connect and interact, which is fundamental to the formation and function of tissues within the body. These protein complexes govern how cells associate with one another or with the surrounding extracellular matrix. They help maintain the structural integrity of tissues, enabling cells to form continuous sheets that act as protective and selective barriers, such as in the lining of the gut or the surface of the skin. Desmosomes represent a class of these cell-to-cell connections, playing a distinct role in maintaining tissue cohesion.
Anchoring Junctions and Mechanical Stress
Desmosomes are formally classified as a type of anchoring junction, a category of cellular connection designed specifically to provide robust mechanical strength to tissues. Within this group, a desmosome is more precisely known as a macula adherens, a term that refers to its spot-like, localized adhesion structure. The primary function of all anchoring junctions is to mechanically couple the cytoskeletons of adjacent cells, allowing them to withstand significant physical forces without pulling apart.
These junctions are distinct from other types, such as tight junctions, which create sealing barriers to block the passage of molecules between cells, or gap junctions, which form channels for direct communication. Desmosomes act like spot-welds distributed across the cell surface, ensuring cells remain tightly bound together even under extreme duress. By linking the internal scaffolding of two neighboring cells, the desmosome effectively distributes any applied mechanical stress across a wider cellular network, preventing damage from concentrating in one area.
The Molecular Architecture of Desmosomes
The ability of a desmosome to withstand mechanical stress is rooted in its highly organized, multi-component structure. At the center of the connection are transmembrane linker proteins belonging to the cadherin family, specifically desmoglein and desmocollin. These proteins extend from the plasma membrane of one cell, cross the narrow extracellular space, and bind with their counterparts on the adjacent cell in a calcium-dependent manner, forming the adhesive interface.
On the inside of each cell, the cytoplasmic tails of these cadherin proteins connect to a dense, disc-shaped structure known as the cytoplasmic plaque. This plaque is a layered assembly of various linker proteins, including plakoglobin and plakophilins, which are members of the armadillo family. These proteins act as adaptors, organizing the junction and recruiting the most abundant component of the plaque, desmoplakin.
Desmoplakin functions as the primary mediator, extending from the inner plaque to link the entire desmosome to the cell’s internal structural network. Desmosomes are specifically characterized by their attachment to the intermediate filaments of the cytoskeleton, which are typically keratin filaments in epithelial cells. This connection to the strong, ropelike intermediate filaments allows the desmosome to anchor the adhesion deep within the cell, enabling the tissue to absorb and dissipate powerful mechanical tension.
Key Tissues and Functional Importance
The functional consequence of the desmosome’s robust structure is seen most clearly in tissues that are constantly subjected to high levels of physical force. Desmosomes are highly concentrated in the stratified squamous epithelial tissues, such as the epidermis of the skin, where they are essential for resisting abrasion and shear forces. The extensive network of desmosomes and their associated keratin filaments gives the skin its resilience, allowing it to stretch and withstand external friction without tearing or blistering.
These powerful junctions are also abundant in the myocardium, the muscle tissue of the heart. Cardiac muscle cells must contract forcefully and rhythmically over a lifetime, generating significant internal mechanical stress. Desmosomes ensure that the individual heart cells remain tightly linked together during these powerful, repetitive contractions, preventing the tissue from being pulled apart. Disruptions to the proteins that form the desmosome often lead to diseases characterized by fragile tissues, such as blistering skin disorders or certain types of cardiomyopathy.