secretion (redirected from secreted)

secretion, in biology, substance elaborated by the living material of an animal or plant. Secretions in humans can be produced by a single cell or by a group of cells commonly called a gland. Some secretions perform special functions in the body (true secretions); others are eliminated as waste products (excretions). Digestive secretions include saliva, gastric juice, intestinal juice, pancreatic juice, and bile. Certain secretions serve as lubricants, e.g., the synovial fluid in joints or the secretions from mucous membranes and from the lachrymal (tear) glands. The mammary glands secrete milk. The endocrine (ductless) glands secrete hormones that enter directly into the bloodstream (see gland). Among the excretions from the body are urine (from the kidneys), perspiration (from the sweat glands), and bile pigments (from the gall bladder). Plant secretions include nectar and various enzymes concerned with the digestion of nutrients within the plant cells.

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secretion

1. a substance that is released from a cell, esp a glandular cell, and is synthesized in the cell

2. the process involved in producing and releasing such a substance from the cell

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secretion [si′krē·shən]

(geology)

A secondary structure formed of material deposited (from solution) within an empty cavity in any rock, especially a deposit formed on or parallel to the walls of the cavity, the first layer being the outer one.

(physiology)

The act or process of producing a substance which is specialized to perform a certain function within the organism or is excreted from the body.

The material produced by such a process.

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Secretion

The export of proteins by cells. With few exceptions, in eukaryotic cells proteins are exported via the secretory pathway, which includes the endoplasmic reticulum and the Golgi apparatus. Secreted proteins are important in many physiological processes, from the transport of lipids and nutrients in the blood, to the digestion of food in the intestine, to the regulation of metabolic processes by hormones. See Cell (biology), Cell organization

Proteins destined for export are synthesized on ribosomes attached to the outside of the rough endoplasmic reticulum, a portion of the endoplasmic reticulum that is specialized for the synthesis of secretory proteins and most of the cell's membrane proteins. After they are folded, the proteins enter small vesicles in which they are transported to the Golgi apparatus. When the proteins reach the last cisterna of the Golgi, a highly tubulated region known as the trans-Golgi network, they are sorted and packaged again into transport vesicles, some of which are in the form of elongated tubules. From here, there are two pathways that proteins can take to the cell surface, depending on the cell type. Proteins can be transported directly to the plasma membrane (constitutive secretion) or to secretory granules (regulated secretion). See Endoplasmic reticulum, Golgi apparatus

In all cells, there exists a constitutive secretion pathway whereby vesicles and tubules emerging from the trans-Golgi network fuse rapidly with the plasma membrane. The emerging vesicles and tubules attach to microtubules, cytoskeletal elements emanating from the Golgi region, that accelerate their transport to the plasma membrane. See Absorption (biology), Cell membranes

In cells that secrete large amounts of hormones or digestive enzymes, most secretory and membrane proteins emerging from the trans-Golgi network are not immediately secreted, but are stored in membrane-bounded secretory granules. Secretory granules release their contents into the extracellular space in a process known as exocytosis, when their membranes fuse with the plasma membrane. Exocytosis occurs only after the cell receives a signal, usually initiated by the binding of a hormone or neurotransmitter to a receptor on the cell surface. The receptor triggers a signal transduction cascade that results in increased concentrations of second messengers such as cyclic adenosine 3^′, 5^′-monophosphate and phosphatidylinositol triphosphate. In most secretory cells, the second messengers or the hormone receptors themselves trigger the opening of calcium channels through which calcium ions stream into the cytoplasm. Calcium initiates the docking of the secretory granules with the plasma membrane and the activation of the fusion apparatus. See Enzyme, Hormone, Signal transduction

In exceptional cases, proteins can be exported directly from the cytoplasm without using the secretory pathway. One such protein is fibroblast growth factor, a hormone involved in the growth and development of tissues such as bone and endothelium. Several interleukins, proteins that regulate the immune response, are also released via an unconventional route that may involve transport across the plasma membrane through channel proteins. These channels have adenosine 5^′-triphosphatase (ATPase) enzyme activity and use the energy derived from the hydrolysis of ATP to catalyze transport. See Cellular immunology