In multicellular organisms, cells are organized into tissues whose integrity is maintained by cell junctions. These multiprotein complexes hold cells together, connect them to the extracellular matrix, and provide both structural support and a means for communication. The presence and type of cell junctions are determined by the needs of the tissue, such as withstanding mechanical stress or controlling the passage of substances between cells.
Anchoring Junctions for Structural Support
Anchoring junctions provide mechanical strength to tissues by linking the cytoskeletons of adjacent cells. They are abundant in tissues subjected to significant physical forces, such as the skin and heart muscle. The main types include desmosomes and adherens junctions, which act like rivets to hold cells together and prevent them from being pulled apart.
Adherens junctions form a continuous belt-like structure around the cell, often located just below tight junctions. These junctions are mediated by cadherin proteins, which are dependent on calcium to function correctly. The cadherins of neighboring cells link together in the space between them, while inside the cell, they connect to the actin cytoskeleton. This connection provides a contractile force that can influence cell shape and tissue folding during development.
Desmosomes are spot-like adhesions that connect the intermediate filaments of adjacent cells. Like adherens junctions, desmosomes utilize cadherin proteins to link cells together. The intracellular portion of these cadherins connects to a dense plaque of proteins, which in turn anchors intermediate filaments like keratin. This arrangement creates a resilient network that distributes mechanical stress across the tissue. Hemidesmosomes are a related junction that anchors a cell’s intermediate filaments to the underlying extracellular matrix.
Tight Junctions as Cellular Barriers
Tight junctions, also known as occluding junctions, seal the space between adjacent cells to create a barrier. This seal is selectively permeable, controlling the passage of water, ions, and other molecules through the paracellular pathway, the space between cells. These junctions are formed by a network of transmembrane proteins, like claudins and occludins, that stitch the membranes of neighboring cells together.
The effectiveness of this barrier is evident in the lining of the small intestine. Here, tight junctions prevent the contents of the gut, including digestive enzymes and microbes, from leaking into the bloodstream while selectively allowing the absorption of nutrients. The permeability of these junctions can be regulated to meet the physiological needs of the tissue.
Another example of the barrier function of tight junctions is the blood-brain barrier. This highly selective barrier protects the brain from potentially harmful substances circulating in the blood. The tight junctions between the endothelial cells lining the brain’s capillaries are exceptionally restrictive, limiting the passage of most molecules and regulating the brain’s environment.
Gap Junctions for Direct Communication
Gap junctions are specialized channels that directly connect the cytoplasm of two adjacent cells, allowing for rapid communication. These channels are formed by proteins called connexins, where six connexins assemble to form a connexon. When the connexons of two neighboring cells align, they create a continuous pore that allows for the passage of small molecules, ions, and electrical signals.
This form of direct communication is important in tissues where cells must act in a coordinated manner. In the heart muscle, for instance, gap junctions allow electrical impulses to spread rapidly from one cell to the next. This results in the synchronized contraction of the heart, which is needed to pump blood effectively.
Gap junctions also play a role in the coordinated activities of neurons in certain areas of the brain. The rapid transfer of electrical signals through these junctions allows for faster communication than is possible at chemical synapses.
Cell Junctions in Health and Disease
The proper functioning of cell junctions is necessary for maintaining tissue health, and their disruption can lead to a variety of diseases. When anchoring junctions are compromised, the structural integrity of tissues can be weakened. For example, certain genetic defects in the proteins that form desmosomes can lead to blistering skin diseases, where the layers of the skin separate easily under minor physical stress.
Defects in tight junctions can compromise their barrier function, leading to conditions often described as a “leaky gut.” In inflammatory bowel disease, the increased permeability of the intestinal lining allows bacteria and other luminal contents to enter the bloodstream, triggering an inflammatory response. This breakdown of the barrier contributes to the chronic inflammation characteristic of the disease.
The malfunction of gap junctions can disrupt the coordinated activity of cells, with significant consequences in electrically active tissues. In the heart, faulty gap junctions can impair the propagation of electrical signals, leading to cardiac arrhythmias. The breakdown of cell junctions is also a feature of cancer progression, as the loss of cell-cell adhesion can allow cancer cells to detach from the primary tumor and metastasize.