Cells in multicellular organisms have to assume specialized roles and must know when to carry out specific activities. Cells coordinate their actions through different types of cellular communication, also called cell signaling. Typical cell signals are chemical in nature and can be targeted locally or for the organism in general.
Cellular communication is a multistage process that includes the following:
- Sending the chemical signal.
- Receiving the signal at the target cell's outer membrane receptor.
- Relaying the signal into the target cell's interior.
- Changing the target cell's behavior.
The different types of cellular communication all follow the same steps but distinguish themselves by the speed of the signaling process and the distance at which it acts. Nerve cells signal rapidly but locally while glands releasing hormones work more slowly but throughout the organism.
The different types of cellular signaling have evolved to take into account the speed and distance requirements for various cell functions.
Cells Communicate with Four Types of Signals
Cells use different types of signaling depending on which other cells they want to reach. The four types of cell communication are:
- Paracrine: The signaling cell secretes a chemical that diffuses locally to target cells.
- Autocrine: Similar to paracrine signaling, but the target cell is the signaling cell. The cell is sending signals from one cell membrane area to another.
- Endocrine: Endocrine signaling produces a hormone that travels throughout the organism via the circulatory system.
- Synaptic: The sending and receiving cells have built a synaptic structure bringing their cell membranes in close contact for easy exchange of signals.
Cells release chemical signals to let other cells know what actions they are taking, and they receive signals informing them of the activities of other organism cells. Actions such as cell division, cell growth, cell death and the production of proteins is coordinated through the different types of cell signaling.
Paracrine Signals Keep Order in the Cell Neighborhood
During paracrine signaling, a cell secretes a chemical that eventually causes specific changes in the behavior of neighboring cells. The originating cell produces the chemical signal that diffuses throughout the tissue nearby. The chemical is not stable and deteriorates if it has to travel long distances.
As a result, paracrine signaling is used for local cell communication.
The chemical that the cell produces is targeted at other specific cells. The targeted cells have receptors on their cell membranes for the secreted chemical. Non-targeted cells don't have the required receptors and are not affected. The secreted chemical attaches itself to the receptors of targeted cells and triggers a reaction inside the cell. The reaction in turn influences targeted cell behavior.
For example, skin cells grow in layers with the top layer made up of dead cells. Cells of a different tissue lie underneath the bottom layer of skin cells. Local cell signaling ensures that the skin cells know in which layer they are located and whether they have to divide to replace dead cells.
Paracrine signaling is also used to communicate inside muscle tissue. A paracrine chemical signal from the nerve cells in the muscle causes the muscle cells to contract, allowing for muscle movement in the larger organism.
Autocrine Signaling Can Promote Growth
Autocrine signaling is similar to paracrine signaling but acts on the cell that initially secretes the signal. The original cell produces a chemical signal, but the receptors for the signal are on the same cell. As a result, the cell stimulates itself to change its behavior.
For example, a cell could secrete a chemical that promotes cell growth. The signal diffuses throughout the local tissue but is captured by receptors on the originating cell. The cell that secreted the signal is then stimulated to engage in more growth.
This feature is useful in embryos where growth is important, and it also promotes effective cell differentiation, when autocrine signaling reinforces a cell's identity. Autocrine self-stimulation is rare in adult healthy tissue but can be found in some cancers.
Endocrine Signaling Affects the Whole Organism
In endocrine signaling, the originating cell secretes a hormone that is stable over long distances. The hormone diffuses through the cell tissue into capillaries and travels through the circulatory system of the organism.
Endocrine hormones spread throughout the body and target cells in locations that are remote from the signaling cell. The targeted cells have receptors for the hormone and change their behavior when the receptors are activated.
For example, cells in the adrenal gland produce the hormone adrenaline, which causes the body to enter the "fight or flight" mode. The hormone spreads throughout the body in the blood and causes reactions in targeted cells. Blood vessels constrict to increase blood pressure for the muscles, the heart pumps more quickly and some sweat glands are activated. The whole organism is placed into a state of readiness for extra exertion.
The hormone is the same everywhere, but when it triggers receptors on cells, the cells change their behaviors in different ways.
Synaptic Signaling Links Two Cells
When two cells continuously have to exchange extensive signaling, it make sense to build special communication structures to facilitate the exchange of chemical signals. The synapse is a cell extension that brings the outer cell membranes of two cells into close proximity. The signaling across a synapse always links only two cells, but a cell can have such close associations with several cells at the same time.
Chemical signals released into the synaptic gap are immediately taken up by the partner cell receptors. For some cells, the gap is so small that the cells are effectively touching. In that case, chemical signals on the outer cell membrane of one cell can directly engage receptors on the membrane of the other cell, and communication is especially fast.
Typical synaptic communication takes place between neurons in the brain. The brain cells construct synapses to establish preferred communication channels with some neighboring cells. The cells can then communicate especially well with their synaptic communication partners, exchanging chemical signals rapidly and frequently.
The Signal Reception Process Is Similar for All Types of Cellular Communication
Sending a cellular communication signal is relatively straight forward as the cell secretes the chemical and the signal is distributed according to its type. Receiving a signal is more complicated because the signal chemical stays outside the target cell. Before the signal can change cell behavior, it has to enter the cell and trigger the change.
First, the target cell must have receptors corresponding to the chemical signal. The receptors are chemicals on the surface of the cell that can bind to certain chemical signals. When a receptor binds to a chemical signal, it releases a trigger on the inside of the cell membrane.
The trigger then engages a process of signal transduction in which the triggered chemical targets a part of the cell where the cell's behavior should change.
Gene Expression Is a Mechanism for Changes in Cell Behavior
Cells grow and divide as a result of signaling from other cells. Such a growth signal binds to the target cell receptors and triggers a signal transduction inside the cell. The transduction chemical enters the cell nucleus and causes the cell to initiate growth and subsequent cell division.
The transduction chemical accomplishes this by influencing gene expression. It activates the genes that are responsible for the production of additional cell proteins that make the cell grow and divide. The cell expresses a new set of genes and changes its behavior according to the signal that was received.
Cells can also change their behavior according to cell signals by changing the amount of energy they produce, changing the amounts of chemicals they secrete or engaging in cell apoptosis or controlled cell death. The cellular communication cycle remains the same, with cells originating signals, target cells receiving them and target cells then changing their behavior according to the signal received.