I know, I know, not another article on cell junctions. But bear with me for a moment because this one is different! As opposed to the other articles that only address certain types of junctions in specific tissues (gap junctions in cardiac tissue), this article will cover them all and put them into one easy-to-read list. Here they are: tight junctions, gap junctions, desmosomes, and hemidesmosomes. Take a look at the post to learn about each junction’s function and location in the body!
The three main types of cell junction are tight junctions, gap junctions, and desmosomes. However, it’s worth mentioning that hemidesmosomes are acquired in the development of some vertebrates (namely fishes) during fetal development and aid in the attachment of the epidermis to underlying dermis.
Let’s start with tight junctions. A type of occluding junction, tight junctions form a seal between cells with similar polarity. They are located at the basal laminae (basal faces) of epithelial cells and control paracellular transport across epithelia (as opposed to transcellular transport). Tight junctions can be described as thin strands that run from one cell to another, creating an extremely small pore for water and solutes. The tight junction is composed of zonula occludens proteins (zonula occludens complex). Zonula occludens proteins mediate the formation of tight junctions when they form microfilaments and actin-myosin filaments that bind together to form a circle. This pinches off the lumen of the adjacent cell (creating a barrier from the outside world) and allows molecules to pass between cells only through a narrow gap (the anterior face of each cell is now facing the exterior).
Zonula occludens proteins are also responsible for regulating epithelial cell adhesion to underlying connective tissue. For example, in the case of the dermis and epidermis, they are responsible for allowing cells to attach at specialized junctional zones like adherens junctions or desmosomes. Tight junctions are composed of zonula occludens proteins (zonula occludens complex).
Gap junctions are another type of cell junction that allows cells to communicate with each other. Gap junctions form cell-cell connections between epithelial cells and allows them to communicate with each other. They also allow water and solutes to move into or out of cells without traversing the aqueous layer (e.g., lamina lucida). The cell-cell connection is formed by a ring of proteins that form hemichannels. As the name suggests, this is due to each channel having a gap in the middle. In both epithelial and endothelial cells, gap junctions play an important role in regulating ion levels of cells by contributing to active transport across cell membranes (which is not passive diffusion). They are also responsible for controlling blood pressure, which can be regulated by closing or opening the channels (an increase or decrease in membrane areas make ion channels open or close). This channel is composed of connexins, a type of gap junction protein that forms complexes with two other connexin proteins. The channel is open and closing the channels allows ions to move freely through the membrane, causing blood pressure to be regulated.
Desmosomes are special adhesion junctions that form between skeletal muscle cells or between cardiac muscle cells. They function to attach the myofibrils (fibrous bands) of skeletal muscle cells and actin filaments of cardiac muscle cells. The binding together of the actin filaments and myofibrils allow for a strong contractile force in skeletal muscles and contraction in cardiac muscles. Desmosomes are formed by linking two adjacent molecules, called cadherins on one cell with another molecule called desmocollin1 on the neighboring cell, thus creating a bridge.