Cellular communication is a complex process involving various biochemical steps and many different messenger molecules between cells and organs. Cells in the human body are highly specialized, and they use various signaling mechanisms to perform different functions. Paracrine signaling is a mechanism in which one cell secretes a molecule that acts on a second cell in close proximity. The signaling molecule may never enter the bloodstream. In contrast, endocrine signaling involves the secretion of a molecule by one cell into the bloodstream. The signaling molecule can travel in the blood and bind to the receptor on the effector cell. Autocrine pathway functions by the secretion and reception of a messenger molecule by a single cell. Juxtacrine signaling is a form of cell communication by direct contact. All these signals influence the behavior of the effector cells. These behaviors include regulating physiologic processes such as metabolism, transport, motility, division, and growth.
The interaction between a messenger molecule and the target cell is just the beginning of a complex cascade of events that happens intracellularly. Most cellular messengers exert their effect through the interaction with a specific receptor coupled to the lipid membrane. There are also intracellular receptors that interact with lipophilic molecules that diffuse through the lipid membrane in both directions without the help of transport proteins.
Examples of cellular messengers are:
Extracellular messengers: cytokines, autacoids, hormones, growth factors, catecholamines, histamine, serotonin, neurotransmitters, eicosanoids, nucleotides, and extracellular vesicles
Intracellular messengers: cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate, calcium, phosphatidylinositols, nitric oxide (NO), and diacylglycerol
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