Cellular signalling
Cellular signalling is a complex system of information signals. The communication is made for:
- Perceiving of cell integrity
- Maintenance of optimal tissue homeostasis
- Coordination of cell actions
There are four categories of chemical signalling found in multicellular organisms:
1. Autocrine
Auto – self; is a form of chemical signalling where the cell targets itself. Autocrine signals are produced by signalling cells that can also bind to the ligand that is released; the signalling cell and the target cell can be the same or a similar cell.
2. Paracrine
Paracrine signals act locally between cells that are close together Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually cause quick responses that last only a short amount of time. To keep the response localized, paracrine ligand molecules are normally quickly degraded by enzymes or removed by neighbouring cells. Removing the signals will re-establish the concentration gradient for the signal, allowing them to quickly diffuse through the intracellular space if released again. Local concentrations of paracrine ligands can be very high.
3. Endocrine (hormone)
Endocrine signals are from distant cells, they originate from endocrine cells. Endocrine signals produce slower but long lasting effects. The ligands released in endocrine signalling are called hormones. Hormones are signalling molecules that are produced in one part of the body, but affect other body regions some distance away.Hormones travel the large distances between endocrine cells and their target cells via the bloodstream, which is a relatively slow way to move throughout the body. Because of their form of transport, hormones get diluted and are present in low concentrations when they act on their target cells. This is different from paracrine signalling in which local concentrations of ligands can be very high.
4. Direct signalling across Gap junctions
Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of neighboring cells. These water-filled channels allow small signalling molecules, intracellular mediators, to diffuse between the two cells. Small molecules like calcium ions can move between cells but large ones like proteins can’t. The specificity of the channels ensures that the cells remain independent, but can quickly and easily transmit signals. The transfer of signalling molecules communicates the current state of the cell to its neighbour target cell; this allows a group of cells to coordinate their response to a signal that only one of them may have received.