Cell junctions attach cells to other cells or to the extracellular matrix (ECM), a fibrous system that supports cells. These junctions serve many purposes: they create a seal between cells, help to increase cell stability, and enable cells to communicate with each other. The junctions also help higher organisms organize their cells and ECM into tissues and organs. Most of these junctions are too small to be seen by a light microscope but can be visualized by an electron microscope.
Three types of junctions occur in animal tissues:
Occluding Junctions: Also known as Tight Junctions. These junctions form seals between lines of cells, making a "wall" that prevents small molecules and liquid from passing through. Therefore occluding junctions control what passes through these cellular "walls"; any molecules that want to pass through must directly enter the cells by diffusion or active transport. This allows the environment on one side of the "wall" to be kept distinct from the environment on the other side.
Occluding junctions are thought to be made by strands of transmembrane proteins (proteins that extend outside of the cell) that "stitch" the cells together. The tightness of the resulting seal depends on how many strands are present. Under an electron microscope the junctions look like a band that encircles the cell and fuses together the adjacent cell membranes. Occluding junctions are present in the intestine, bladder, liver, and pancreas and help prevent the bodily wastes in these organs from reentering the body. They also help separate the developing fetus from its mother.
Anchoring/Adhesive Junctions: These junctions hold cells together at a fixed position, enabling them to act as a structural unit. They bind the cell's cytoskeleton, a "skeleton" of supporting filaments, to other cells or the ECM. They are more common than occluding junctions and are predominantly present in heart muscle and the epithelium of the skin.
The junctions are composed of two different types of proteins, ones that are present inside the cell, called "intracellular attachment proteins" and ones that extend outside the cell, called "transmembrane linker proteins". Intracellular attachment proteins form a disc on the inside of the cell membrane and connect it to cytoskeleton elements like actin or intermediate filaments. Transmembrane linker proteins bind to the disc and extend outside the cell to bind to other transmembrane linker proteins on other cells or the extracellular matrix, joining them together. There are three different types of anchoring junctions:
- Adherens Junctions: These junctions connect the actin filaments of a cell's cytoskeleton to other cells or to the ECM. When the junction attaches to other cells it uses transmembrane linker proteins called cadherins. When the junction binds to the ECM at regions referred to as focal contacts it uses transmembrane linker proteins called integrins. Both types of connections use intracellular attachment proteins called catenins. When viewed with an electron microscope, adherens junctions can look like streaks connecting the cell or may appear as "adhesion belts" which form completely around the cell. These junctions are mainly present in the tissues that line body cavities and organs. They are also in heart muscle and help the heart beat.
- Desmosomes: These junctions are very similar to adherens junctions except they use intermediate filaments in the cytoskeleton instead of actin. Desmosomes create a more focused attachment than adherens junctions. They help distribute force through the cells, protecting it from stress. Therefore, they are present in tissues that undergo lots of mechanical stress, including the heart, skin, and uterus tissues. They also help maintain cell position during embryo development.
- Hemidesmosomes:These junctions resemble desmosomes but they connect the intermediate filaments to the ECM instead of other cells. Like with adherens junctions, integrin proteins are used to make this connection.
Communicating Junctions: Also known as Gap Junctions. This is the most common type of cell junction and it exists in almost all species of animal in most tissues. Here the junction forms a gap between two cells that allows ions and other small molecules such as amino acids, cAMP, and hormones to pass directly from one cell to the other. The junctions can be opened or closed by changes in cellular pH or calcium concentrations.
The communicating junctions are created from transmembrane proteins called connexins that combine to form a ring structure called a connexon. The connexons of two cells then align to form a continuous channel between them. The junctions play a role in heart action potential, which keeps the heart beating, and transmitting electrical synapses in the brain. They appear to be important for early embryo development and help contract the uterus during birth. Plants have communication junctions called plasmodesmata, which work similarly but have a different structure.
Keep in mind that a cell can have multiple types of junctions that occur simultaneously. This drastically increases the stability of the resulting cellular unit and is especially important in tissues such as heart muscles that are constantly exposed to mechanical stress.
Cell junctions also seem to deter the progression of cancer. Tumors often move from where they originally started growing to other areas of the body in a process called metastasis. This process can be aided through mutations in the cell junction mechanism. These mutations prevent the cancerous cells from remaining attached to the other cells around them, allowing them to move around the body. For example, catenin, the intercellular attachment protein seen in anchoring junctions, is thought to be inactivated in several type of cancer including colorectal cancer and gastric cancer.
Sources:
- Molecular Biology of the Cell, Alberts, 3rd edition, 1994.
- The World of the Cell, Becker et. al, 3rd edition, 1996.