Cells must be able to receive and process signals from outside their borders in order to respond to changes in their immediate environment. Signals are often sent to cells in chemical form via signaling molecules or ligands. When a signal is recognized and bound by a receptor protein, the receptor experiences a conformational change, which triggers a sequence of metabolic processes within the cell. This binding sets in motion a series of processes that activate intracellular signaling pathways, also known as signal transduction cascades, which often amplify the message or signal, resulting in multiple intracellular messages for each bound receptor.
Chemical signaling in multicellular organisms is divided into four categories:
- Paracrine signaling
- Autocrine signaling
- Endocrine signaling
- Contact-dependent signaling (Juxtacrine)
The distance that the signal travels through the body to reach the target cell is the key distinction between the various types of signaling.
- Chemical messengers (ligands that can diffuse through the gap between cells) are frequently released by cells that are close to one another.
- Paracrine signaling is a type of signaling in which cells communicate over very short distances.
- During development, paracrine signals are extremely crucial because they allow one group of cells to notify another group of cells what cellular identity to adopt.
- Synaptic signaling is an example of paracrine signaling. This mechanism is named after synapse, the interface between two nerve cells where signal transmission happens. When a transmitting neuron fires, an electrical impulse rapidly travels down a long, fiber-like extension called an axon and through the cell.
- A cell signals to itself by releasing a ligand that binds to receptors on its own surface in autocrine signaling (or, depending on the type of signal, to receptors inside of the cell).
- Autocrine signaling is vital in a variety of processes.
- For example, it is critical throughout development, as it aids cells in adopting and reinforcing their correct identities.
- Autocrine signaling is significant in cancer and is hypothesized to play a role in metastasis from a medical standpoint.
- In immune system macrophages, autocrine signaling is a critical cell communication mechanism. Macrophage secretes the cytokine Interleukin-1 (IL-1) and has IL-1 receptors on its cells. More IL-1 is produced by the intracellular cascade triggered by the bound receptors. As a result, autocrine signaling regulates IL-1 secretion, creating a forward-feedback loop.
- Signals produced by specialized cells and released into the bloodstream are carried to target cells in remote sections of the body in long-distance endocrine signaling.
- Hormones produced in one region of the body move via the circulation to reach distant targets, which are known as signals.
- The thyroid, hypothalamus, and pituitary, as well as the gonads (testes and ovaries) and the pancreas, are endocrine glands that release hormones in humans.
- Many hormones are master regulators of growth and physiology, and each endocrine gland produces one or more types of hormones.
- The pituitary gland, for example, secretes growth hormone (GH), which promotes skeletal and cartilage growth. GH, like most hormones, influences a wide range of cell types throughout the body.
Contact-dependent signaling (Juxtacrine)
- The gap junctions in animal and plant plasmodesmata are small tubes that connect neighboring cells directly.
- Small signaling chemicals termed intracellular mediators can diffuse between the two cells through these water-filled channels.
- Small molecules, such as Ca2+, can flow between cells, while huge molecules, such as proteins and DNA, require particular assistance to pass through the channels.
- The current condition of one cell is communicated to its neighbor through the transfer of signaling molecules.
- This enables a group of cells to coordinate their responses to a signal that may have been received by only one of them.
- Plants have plasmodesmata that connect practically all of their cells, forming a massive network.
- Two cells may bind to each other in another type of direct signaling because they have complementary proteins on their surfaces.
- When two proteins attach to each other, the interaction alters the structure of one or both proteins, resulting in the transmission of a signal.
- Immune cells use cell-surface markers to distinguish between “self” cells (the body’s own cells) and cells infected by pathogens, and this type of signaling is very crucial in the immune system.
- The Delta-Notch pathway, which is involved in embryonic development, is an example of contact-dependent signaling. Delta, a signaling cell transmembrane protein, binds to Notch, a receptor on the receiving cell. Notch changes its shape, allowing a secretase enzyme to cut off its cytoplasmic domain.