Question

What are the different forms of extracellular signaling

Answer 1

Signal molecules are of different types and the process of transferring the signal into the cell is called signal transduction. There are two types of cells in signal transduction- the sender cell where the signal originates and the target cell that receives the signal. The signal changes or modulates the cellular metabolism, function or development. There are four basic forms of extracellular signaling found in multicellular organism named as: Autocrine signaling, Paracrine signaling, Endocrine signaling and Signaling by direct contact.

(i) Autocrine signaling: In autocrine signaling, a cell signals to itself by releasing a ligand that binds to receptors on its own surface. This means that the same cell acts as sender and recipient, e.g growth, differentiation, immune and inflammatory response. Many growth factors act in this fashion by stimulating their own growth and proliferation. Autocrine signaling is particularly common in cancer cells and is thought to play a key role in metastasis (spread of cancer cells to distant sites from its original site).

(ii) Paracrine signaling: It is effected by local mediators which have their effect to target cells in close proximity to it without entering the circulation. The effect is rapid and transient. The conduction of an electric impulse from one nerve cell to another or from a nerve cell to a muscle cell occurs via paracrine signaling. The role of this type of signaling is mediated by synapse, the junction between two nerve cell where signal transmission occurs. When the impulse reaches the synapse, it triggers the release of neurotransmitters that conduct nerve impulses.

(iii) Endocrine signaling: Endocrine signaling take place between cells which are located at a distance from each other and the signaling molecules include hormones or chemical messengers secreted into circulation. In humans, endocrine glands that release hormones includes pituitary, hypothalamus, thyroid, gonads, pancreas, etc which possess specific set of functions. Once the hormones reach the target cell, they bind to specific target cell receptors with high affinity.

(iv) Signaling through cell-cell contact: This type of signaling is also called juxtacrine signaling which occurs when two type of cells are adjacent to each other so that contact is established through gap junctions or through protein molecules on the surface of the two cells. These channels allow small signaling molecules called intracellular mediators to diffuse between two cells, but large molecules like sugars, amino acids, etc cannot fit through the channels and need special carrier protein for assistance.