Cell-cell interactions: How cells communicate with each other
Can you imagine what it would be like if your brain couldn’t tell your leg muscles to contract so you could walk? Or your bladder couldn’t tell your brain that you had to use the bathroom? Or what if you had contracted an infection and your immune system had to deal with it? Cells need to be able to communicate with each other to do these things, and so much more!
There are a few different types of cell-cell interactions. Some of these interactions are meant for big molecules that enter and exit the cell called, endocytosis (entering the cell) and exocytosis (exiting the cell). For smaller particles like amino acids, water, ions and other solutes there are different types of direct contact between the cells called gap junctions.
Exocytosis - exiting the cell
Exocytosis is a process used by the cell to take out its trash and to incorporate proteins into the cell membrane. During exocytosis, the phospholipid bilayer of the cell membrane surrounds the waste proteins, creating a bubble-like structure called a vesicle. Vesicles are frequently used in the cell for transportation of molecules across the cell membrane.
Diagram showing vesicle fusing with cell membrane to expel its contents via exocytosis.
A slightly different process occurs for waste products being ejected out of the cell, instead of proteins being incorporated into the cell membrane. Once the vesicle has enclosed the waste proteins on the inside of the cell, it moves towards the cell membrane. The vesicle merges with the cell membrane, opening the bubble-like structure and ejecting the contents in the environment surrounding the cell.
Diagram showing waste being expelled from the cell via exocytosis.
Proteins destined for the cell membrane
Exocytosis is also used to integrate new proteins into the cell membrane. In this process, the new protein is formed inside the cell, and migrates to phospholipid bilayer of the vesicle. The vesicle, containing the new protein as a part of the phospholipid bilayer, fuses with the cell membrane. This allows the protein to be directly integrated into the cell membrane when the vesicle, in the same way as with waste proteins, fuses and opens with the cell membrane.
Diagram showing a vesicle fusing with cell membrane to incorporate a new membrane protein.
Endocytosis - bringing in the goods
Endocytosis is the opposite process of exocytosis. Endocytosis brings molecules into the cell. These molecules are important for the survival of the cell, such as glucose. There are three different styles of endocytosis: 1) phagocytosis, 2) pinocytosis, and 3) receptor-mediated endocytosis.
Phagocytosis is the process similar to eating, where the cell engulfs a molecule in order to move it to the interior of the cell. The process starts by the molecule binding to specific receptors on the surface of the cell membrane, triggering the cell membrane to reshape, surrounding the molecule. The receptors allow this process to be specific, controlling what can enter the cell. Then, the two ends of the cell fuse, creating a vesicle that surrounds the molecule. Eventually the membrane around the molecule will be digested and its contents will be used! For example, white blood cells recognize pathogens, such as viruses or bacterial cells, outside of the cell and will use phagocytosis to bring it in to destroy it!
Diagram showing how a cell takes up a virus via phagocytosis.
If phagocytosis is how the cell eats, then pinocytosis is how the cell drinks. Pinocytosis engulfs dissolved ions and other solutes in the liquid medium surrounding the cell. This is different than phagocytosis, which brings full, undissolved or insoluble molecules into the cell. The distortion of the cell membrane to engulf the dissolved solutes is similar to that of phagocytosis. Another important distinction is that pinocytosis is not specific to what is carried into the cell, whereas phagocytosis can be highly specific. The liquid medium outside the cell is always filled with dissolved particles and solutes that are handy for the cell, so the cell doesn’t need this process to be specific.
Diagram showing how a cell takes up liquid via pinocytosis.
Receptor-mediated endocytosis is very specific with respect to what is imported into the cell. It’s actually a bit like a lock-and-key system. There are receptors embedded in the cell membrane that, when bound by molecules with an exact match in shape, size, or other physical attribute, will allow the molecule to enter into the cell through the same engulfment process as phagocytosis or pinocytosis.
Diagram showing how certain substances must bind a cell membrane protein to be taken up by that cell.
There are many different ways that cells can connect to each other. The three main ways for cells to connect with each other are: gap junctions, tight junctions, and desmosomes. These types of junctions have different purposes, and are found in different places.
Gap junctions are essentially tubes that join two cells together. These tubes create a connection that allows for the transport of water and ions to and from the connecting cells. The tubes also help to spread electrochemical signals that are produced by action potentials that occur in the nervous system (neurons) and in cardiac cells that make your heart beat. Gap junctions have a pretty important job!
Diagram showing how cells can exchange substances in their cytoplasms via gap junctions.
Tight junctions are different from gap junctions because they are the connections that form when cells are squished up against one another. In this case, the cell membranes are connected, but the contents of each cell are not connected in any way. There are no tubes here, but there is an impermeable layer in between the cells. These types of cell connections are useful in places that need to contain certain fluids, like in the bladder, the intestines or the kidneys. Imagine if you didn’t have a watertight seal in those connections! Fluids like your urine would be circulating through your body! Yuck!
Diagram showing a tight junction between two cells.
Finally, desmosomes are quite different from gap junctions and tight junctions. With desmosomes, cell membranes are connected by thread like substances that connect the cells across the space in between cells. Much like tight junctions, desmosomes physically hold the cells together, but do not allow fluids or materials to pass from the inside of one cell to the next. These connections are also attached to the scaffolding of the cell, called the cytoskeleton, to help with structural support. The space in between the cells allows for water and solutes to flow freely between each cell without compromising the connection. This is convenient for areas of our body that experience high stress like in our skin or our intestines because the space in between the cells offer flexibility that the other junctions can’t.
Diagram showing desmosomes between two cells.
Consider the following:
What happens when cell junctions don’t work properly? Sometimes there are mistakes in the genetic blueprints of our body. Gap junctions are most commonly found in the skin, so mistakes in their functions can lead to a variety of diseases that make up ectodermal dysplasia, a series of genetic disorders affecting the development or function of the teeth, hair, nails and sweat glands. Additionally, errors in specific gap junction genes called, Cx43 and Cx56.6, can lead to the breakdown of some of our brain tissue called white matter which makes up 60% of our brain. Diseases that include the breakdown of white matter include multiple sclerosis and Huntington’s disease! Mistakes in our genes that produce desmosomes cause skin blistering.