Cell junctions are complex protein structures that connect neighboring cells, serving functions from mechanical support to direct communication. These specialized regions of contact are fundamental to the organization and function of multicellular tissues throughout the body. Desmosomes are recognized as powerful anchoring structures whose primary function is to physically integrate cells, essentially fastening them together to form a cohesive tissue unit. The structural design of desmosomes clearly indicates their role as robust mechanical links between adjacent cells.
Anatomy and Components of Desmosomes
A desmosome, also known as a macula adherens, is a spot-like adhesion complex randomly distributed on the lateral membranes of cells. This intricate structure is composed of several distinct protein groups that work together to form a highly stable anchor point. The connection begins in the space between the two cells, which is spanned by transmembrane linker proteins. These proteins are desmoglein and desmocollin, which belong to the cadherin family of adhesion molecules.
Inside the cell, these transmembrane proteins are secured by a dense, layered network of cytoplasmic proteins known as the plaque. This plaque includes proteins such as plakoglobin, plakophilin, and desmoplakin. Plakoglobin and plakophilin act as intermediary connectors, linking the tails of the cadherins to the large desmoplakin protein.
Desmosomes connect deeply to the cell’s internal scaffolding, or cytoskeleton. Desmoplakin functions as the ultimate anchor, binding the complex to a meshwork of intermediate filaments within the cytoplasm. These filaments are typically keratin in epithelial cells and desmin in heart muscle cells, which are strong, rope-like structural proteins that extend throughout the cell. This arrangement creates an integrated network of mechanical stability that spans multiple cells within a tissue.
The Role of Desmosomes in Tissue Integrity
The primary biological role of the desmosome is to provide tensile strength and mechanical adhesion, integrating cells into a functional, supracellular unit. They function like “spot welds” that resist forces that would otherwise tear cells apart. By linking the internal intermediate filament cytoskeletons of adjacent cells, desmosomes ensure that mechanical stress is distributed across the entire tissue.
This function is particularly evident in tissues that experience intense physical forces, stretching, or abrasion. Desmosomes are abundant in the epidermis, the outer layer of the skin, which constantly endures mechanical stress and friction. They are also numerous in the cardiac muscle tissue of the heart, where they are essential for keeping muscle cells connected as the heart contracts and relaxes rhythmically. Without the robust adhesion provided by desmosomes, these tissues would quickly fail under normal physiological strain.
Desmosomes and Permeability: The Direct Answer
Desmosomes do not allow the passage of ions; they are structurally incapable of facilitating this function. Ions, such as sodium, potassium, or calcium, are charged particles that require a specialized, water-filled channel or pore to move across cell membranes or between cells. Desmosomes are not constructed as pores or channels.
Instead of creating a pathway, the desmosomal structure fills the intercellular space with dense adhesive material. The transmembrane cadherin proteins—desmoglein and desmocollin—interlock tightly in the narrow gap between the two cell membranes. This crowded adhesive region, along with the dense cytoplasmic plaque, acts as a physical barrier, preventing ions or other small molecules from passively diffusing from one cell’s cytoplasm to the next.
Desmosomes are built for adhesion and mechanical stability, not for communication or transport. Their design focuses on tethering the internal cytoskeleton to the cell surface to resist physical force. Any movement of ions or molecules must occur either through the cell membranes, utilizing specific transporters or channels, or around the desmosome through the remaining intercellular space.
Contrasting Desmosomes with Ion-Passing Junctions
To understand the non-permeable nature of desmosomes, they are often contrasted with gap junctions, which are specifically designed for ion passage. While desmosomes are mechanical anchors, gap junctions are dedicated communication channels. Gap junctions are formed by cylindrical protein assemblies called connexons. Each connexon is composed of six protein subunits called connexins, which align precisely with a connexon from the adjacent cell membrane to form a continuous, open channel. This hydrophilic channel allows small signaling molecules, water, and inorganic ions to pass directly from the cytoplasm of one cell into the cytoplasm of its neighbor.
The purpose of gap junctions is rapid communication, such as coordinating the synchronized contraction of heart muscle cells or transmitting electrical signals in some nerve tissues. The fundamental objective is different: desmosomes are designed to resist mechanical separation, while gap junctions are designed to permit the passage of substances for metabolic and electrical synchronization. The mechanical strength of the desmosome is achieved by filling the junctional space, which is the exact opposite of the gap junction’s function of creating a patent channel.