Cell Signaling
- All cells receive and acknowledge signals from their surroundings.
- This is competent by a variety of signal molecules that are secreted or expressed on the surface of one cell and bind to receptors expressed by other cells, thereby integrating and coordinating the functions of the many individual cells that make up organisms.
- Each cell is programmed to acknowledge specific extracellular signal molecules.
Extracellular signaling usually involves the following steps:
- Synthesis and deliverance of the signaling molecules by the signaling cell;
- Transport of the signal to the target cell;
- Binding of the signal by a specific receptor leading to its activation;
- Initiation of signal-transduction pathways.
Endocrine Signaling:
- In endocrine signaling, the signaling molecules act on the target cells distantly located from their site of synthesis.
- It is long-range signaling in which the signal molecule is transported by the bloodstream.
Fig: Endocrine signaling
Paracrine Signaling:
- In paracrine signaling, the signaling molecules released by a cell affect target cells only nearby. An example of this is the action of neurotransmitters in carrying signals between nerve cells at a synapse.
Fig: Paracrine signaling
Autocrine Signaling:
- In autocrine signaling, the signaling molecules produce an effect on the same cell that produces it.
- One important example of such is the response of cells of the vertebrate immune system to foreign antigens.
- Certain types of T-lymphocytes respond to antigenic stimulation by synthesizing a growth factor that derives their own proliferation, thereby increasing the number of responsive T-lymphocytes and amplifying the immune response.
Fig: Autocrine signaling
Juxtacrine Signaling: Signaling across gap junctions.
- In juxtacrine signaling, signal molecules do not diffuse from the cell producing it, and cell bearing signal molecules interact with the receptor proteins of adjacent responding cells.
- Unlike another mode of cell signaling, it requires physical contact between the cells involved.
- Notch signaling and classical cadherin signaling are examples of juxtacrine signaling.
Fig: Juxtacrine signaling
