Secretion(redirected from external secretion)
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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
a substance elaborated and released by animal and human glands. According to their physiological function, secretory products are grouped as secretions proper, excretions, or incretions (hormones). In the narrow sense of the word, a secretion is a substance needed for the performance of an organism’s normal vital activities. Some secretions, for example, digestive enzymes, cause the chemical alteration of the environment, whereas other secretions, including mucus and cutaneous sebum, cover the external surface of the body with a layer that protects the body against the external environment. Hormones, a third group of secretions, are released into the body, where they are conveyed by the bloodstream and exert a regulatory influence (excitatory or inhibitory) on other bodily functions or systems. A fourth group of secretions consists of food products, including milk.
Secretions may also include pheromones, which are substances released by the glands of some animals into the external environment, where they serve to regulate or sexually attract other individuals of the same species and sometimes of other species. Excretions are substances formed as the end products of dissimilation and must be removed from the organism as quickly as possible. Chemically, most secretions are proteins, polysaccharides, or glycoproteins. Some glands secrete lipids, including steroids.
B. V. ALESHIN
the elaboration and release of secretions by glandular cells. While performing its vital activities, every cell of an organism forms several metabolic products, releasing them either into the external or internal environment. When the secretory function is the basic function of a group of specialized glandular cells it is called secretion. External, or exocrine, secretion is distinguished from internal, or endocrine, secretion. In external secretion the products elaborated by a gland are released into the external environment; the secretion first enters the glandular duct, from which it is discharged onto the surface of the body or into hollow organs. In internal secretion (incretion) synthesized substances are released into the blood or lymph.
The secretory cycle of any gland has both a production (biosynthesis) phase and a release phase. The term “secretion” is sometimes applied only to the latter phase. In some glands the phases occur simultaneously; the phases occur at different times in glands whose phases are regulated by different specific mechanisms. The secretion process resembles an intracellular conveyor system, in which the synthesized product gradually matures and steadily moves with the cell from one organoid to another. The initial products, including amino acids, monosaccharides, fatty acids, and salts, are absorbed from the blood and tissue fluid by a glandular cell.
The biosynthesis of the secretion (especially of protein products) starts in the endoplasmic reticulum, where the amino acids, which have been adsorbed on the cellular membranes, are joined together in a sequence determined by the messenger RNA of ribosomes. The synthesized initial product accumulates in the fissures and lacunae of the endoplasmic reticulum, from which it shifts to the area of the laminar Golgi complex, where the maturation of a secretion is completed. In the area of the Golgi complex in some glandular cells the synthesized protein combines with carbohydrates and the secretion is converted into a glycoprotein. Mitochondria, which are numerous in glandular cells, produce the energy needed to synthesize and release a secretion. The synthesis of secretions of a lipidic (steroid) nature is completed on the mitochondria.
In the phase of secretion release increases are observed in oxygen consumption by the glandular cells, intracellular osmotic pressure, and the amount of water entering the cells. The result is the establishment in a glandular cell of a stream of water, which enters through the base of the cell and emerges through the apical membrane. Flowing through the cytoplasm, the water picks up the accumulated secretion and releases it from the cell, either in the form of a solution that is diffused through the apical membrane or in the form of drops that are passed through membrane pores. During this type of secretion, which is called merocrine secretion, glandular cells do not suffer any damage. If, however, the secretion is insoluble in water or for some other reason is incapable of passing through the apical membrane, the intensified passing of water into a swelling glandular cell causes the apex of the cell with its accumulated granules or drops of secretion to swell clavately and then rupture or be detached.
The liberation of secretion by the detachment of the apex of a glandular cell without the cell’s destruction is called apocrine secretion. Sometimes this type of secretion is limited to the swelling and detachment of microvilli from a glandular cell (microapocrine secretion). Occasionally, while a glandular cell is degenerating, it is completely transformed into a drop of secretion and is ejected from the epithelial layer into the lumen of the gland; this type of secretion is called holocrine secretion. In the course of evolution, holocrine secretion, which is a primitive type of secretion, has been replaced by merocrine secretion, which is more effective.
Both phases of the secretory cycle are regulated by the combined or successive influences of several neural and humoral factors. The nerve fibers that carry impulses stimulating secretion to the glands are called secretory fibers. The neural effects that are manifested by the intensified elaboration of secretion during the production phase are called trophic effects. There is no clear distinction between secretory and trophic nerves because the stimulation of a fiber that innervates a gland causes both secretory and trophic effects. Glandular activity is also influenced by humoral agents, including some hormones (especially those involved in regulating the functions of the endocrine glands). For example, the thyrotropic, gonadotropic, and adrenocorticotropic hormones of the anterior pituitary excite, respectively, the thyroid gland, the ovaries and testes, and the adrenal cortex (the glucocorticoid function). Secretin, which is produced in the duodenal mucosa, stimulates the release of pancreatic juice by the acinar cells of the pancreas.
Besides hormones, other substances formed in the body may also affect glandular function. Histamine, for example, sharply intensifies the secretion of the fundic glands of the stomach. The effect of humoral stimulants is manifested in both phases of the secretory cycle. Certain ions directly affect the secretion of many glands; an excess of monovalent cations (K+ or Na+) usually intensifies secretion, whereas bivalent ions (Ca2+ and Mg2+) weaken secretion. The stimulation of a glandular cell is based on the activation of adenyl cyclase, an enzyme localized in the cell’s surface membrane. Adenyl cyclase acts as a stimulus in the formation of cyclic adenosine monophosphate, which regulates the chain of intracellular reactions resulting in the increased activity of the specific enzyme systems causing secretion. The large number of factors influencing secretion is explained by the fact that they are all equally capable of activating the adenyl-cyclase mechanism of the glandular cell. Nerve cells are also characterized by secretory activity; they all elaborate and release mediators, and in neurosecretory cells the production of physiologically active substances called neurohormones reaches a high level of intensity.
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Babkin, B. P. Sekretornyi mekhanizm pishchevaritel’nykh zhelez. Leningrad, 1960. (Translated from English.)
Khirsh, G. “O printsipe ‘konveiera’ v vyrabotke fermentov ekzokrinnymi kletkami podzheludochnoi zhelezy.” In Funktsional’naia morfologiia kletki. Moscow, 1963.
Brodskii, V. Ia. Trofika kletki. Moscow, 1966.
Shubnikova, E. A.“Sekretornaia deiatel’nost’.” In Rukovodstvo po tsitologii, vol. 2. Moscow-Leningrad, 1966.
Shubnikova, E. A. Tsilologiia i tsitofiziologiia sekretornogo protsessa. Moscow, 1967.
DeRobertis, E., W. Nowinski, and F. Saez. Biologiia kletki. Moscow, 1973. (Translated from English.)
Yost, H. Fiziologiia kletki. Moscow, 1975. (Translated from English.)
Caro, L. G., and G. E. Palade. “Protein Synthesis, Storage and Discharge in the Pancreatic Exocrine Cell.” Journal of Cell Biology, 1964, vol. 20. no. 3.
Kurosumi, K. “Electron Microscopic Analysis of the Secretion Mechanism.” International Review of Cytology, 1961, vol. 11.
B. V. ALESHIN
a round mineral aggregate that forms when a crystalline or colloidal substance fills cavities in rocks. The characteristic feature of many secretions is a sequential, concentrically layered deposition of the mineral substance from the walls of the cavity toward the center. The separate layers frequently differ in color or composition. Small cavities are usually completely filled with the mineral substance. Sometimes the central part of a secretion has radial-filamented aggregates of some mineral, such as zeolite. In the center of large cavities a small empty space is often found with walls covered by druses of crystals or sinter formations. Small secretions, measuring up to 10 mm across, are called amygdules, and large ones, geodes. The formation of secretions is usually linked with hydrothermal or supergene processes.