SECRETION (Latin secretio department) — process of education in a cell of a specific product (secret) of a certain functional purpose and the subsequent its allocation from a cell.
The page, at a cut a secret is allocated for the surface of skin, mucous membrane or went to a cavity. - kish. a path, call external (an ekzosekretion, an ekzokriniya), at allocation of a secret in internal environment of an organism of S. call internal (an inkretion, an endokriniya).
At the expense of S. a number of the vital functions is carried out: education and release of milk, saliva, gastric, pancreatic and intestinal juice, bile, sweat, urine, got down; education and release of hormones closed glands and diffusion endocrine system went. - kish. path; neurosecretion, etc.
Beginning of studying of S. as fiziol. process it is connected with a name of R. Geydengayn (1868), to-ry described a number of consecutive changes in cells of glands and formulated initial ideas of a secretory cycle in a stomach, i.e. about interface tsitol. pictures of glands of a stomach with contents in his mucous membrane of a pepsinogen. Identification of communication between microscopic changes in a structure of sialadens and their S. at irritation of the parasympathetic and sympathetic nerves innervating these glands J allowed R. Geydengayn. To Lengli and other researchers to draw a conclusion on existence of secretory and trophic components in activity of ferruterous cells, and also on separate nervous control of these components.
Use light (see. Microscopic methods of a research ) and submicroscopy (see), autoradiography (see), uljtratsentrifugirovaniye (see), electrophysiologic, gisto-and cytochemical methods (see. Electrophysiology , Histochemistry , Cytochemistry ), methods immunol. identifications of primary and subsequent secretory products and their predecessors, receiving secrets and their physical. - the chemical and biochemical analysis, fiziol. methods of a research of mechanisms of regulation of S., etc. expanded idea of mechanisms C.
Mechanisms of secretion
the Secretory cell can allocate products, various by the chemical nature: proteins, mukoproteida, mucopolysaccharides, lipids, solutions of salts, bases and acids. One secretory cell can synthesize and allocate one or several secretory products of one or different chemical nature.
The material emitted by a secretory cell can have various relation to intracellular processes. Across Hirsh (G. Hirsch, 1955), can be allocated: actually the secret (a product of intracellular anabolism), excretes (a product of a catabolism of this cell) also rekrt (the product absorbed by a cell and then in not changed look allocated with it). At the same time the main function of a secretory cell is synthesis and allocation of secrets. Not only inorganic matters, but also organic, including high-molecular can Rekretirovatsya (e.g., enzymes). Due to this property secretory cells can transport or allocate products of metabolism of other cells and fabrics from a blood-groove, to excrete these substances, participating thus in providing a homeostasis of all organism. Secretory cells can rekretirovat (to resekretirovat) from blood enzymes or their zimogenny predecessors, providing their gematoglandulyarny circulation in an organism.
In general the sharp border between various manifestations of functional activity of secretory cells cannot be carried out. So, external secretion (see) and incretion (see) have much in common. E.g., the enzymes synthesized by digestive glands not only ekzosekretirutsya, but also inkretiruyuts, and gastrointestinal hormones in nek-rum quantity can pass into a cavity went. - kish. a path as a part of secrets of digestive glands. As a part of nek-ry glands (e.g., the exocrine cells, endocrine cells and cells which are carrying out bidirectional (ekzo-and endosecretory) removal of the synthesized product are available a pancreas).
These phenomena find an explanation in the excretory theory of an origin of secretory processes offered A. M. U goal (1961). According to this theory both types of S. — external and internal — came as specialized functions of cells from function of nonspecific excretion inherent to all cells (i.e. allocations of metabolic products). Thus, according to A. M. Ugolev, specialized morfostatichesky S. (without essential morfol. changes of a cell) came not from morfokinetichesky or morfonekrotichesky S., at a cut in a cell occur rough morfol. shifts or their death, and from morfostatichesky excretion. Morfonekroticheskaya S. is an independent branch of evolution of glands.
The process of periodic change of a secretory cell connected with education, accumulation, allocation of a secret and recovery of a cell for further S. carries the name of a secretory cycle. In it several phases, border between allocate to-rymi it is usually expressed unsharply; there can be also an imposing of phases. Depending on the temporary relation of phases C. happens continuous and discontinuous. At continuous S. the secret is allocated in process of its synthesis. At the same time there are absorption by a cell of substances, initial for synthesis, the subsequent intracellular synthesis and allocation of a secret (e.g., secretion of cells of a superficial epithelium of a gullet and stomach, closed glands, a liver).
At discontinuous secretion the cycle is stretched in time, phases of a cycle in a cell follow in a certain sequence one after another and accumulation of a new portion of a secret begins only after removal from a cell of the last portion. Different cells can be in the same gland on different phases of a secretory cycle at present.
The specific condition of a cell in general and its intracellular organellas is characteristic of each of phases.
The cycle begins with the fact that water, inorganic matters and low-molecular organic compounds come to a cell from blood (all glands have intensive blood supply) (amino acids, fat to - you, carbohydrates, etc.). Have the leading value in intake of substances in a secretory cell pinocytic (see), active transport of ions (see) and diffusion (see). Transmembrane transport of substances is carried out with participation of ATP-ases and an alkaline phosphatase. The substances which came to a cell are used by it as mother substances not only for synthesis of a secretory product, but also for the intracellular power and plastic purposes.
The following phase of a cycle is synthesis of primary secretory product. This phase has essential distinctions depending on a type of the secret synthesized by a cell. Process of synthesis proteinaceous sekrety in acinous cells is fullestly investigated pancreas ((see). From the amino acids which came to a cell on ribosomes of an endoplasmic granular reticulum within 3 — 5 min. protein is synthesized, and then moves to Golgi's system (see. Golgi complex ), where collects in the condensing vacuoles. In them within 20 — 30 min. there is a maturing of a secret, and the condensing vacuoles turn into granules of zymogen. The role of system of Golgi in formation of secretory granules was for the first time shown by D. N. Nasonov (1923). Secretory granules move to an apical part of a cell, the cover of a granule merges with a plasmolemma, through an opening in a cut contents of a granule pass into a cavity of an acinus or secretory capillary. From the beginning of synthesis to an exit (extrusion) of a product from a cell there pass 40 — 90 min.
Existence of cytologic features of formation in granules of various pancreatic enzymes is supposed. In particular, Kramer and Purt (M. of F. Kramer, S. of Poort, 1968) pointed to a possibility of extrusion of enzymes passing a phase of condensation of a secret in granules, at a cut synthesis of a secret continues, and extrusion is carried out by diffusion of not granulated secret. At blockade of extrusion accumulation of the granulated secret (a regranulyarny stage) is recovered. In the subsequent lag phase of a granule fill apical and average speak rapidly cells. Synthesis of a secret, continuing, but insignificant on intensity, fills its insignificant extrusion in the form of the granulated and not molding grains. The possibility of intracellular circulation of granules and their inclusion from one organellas in others is postulated.
Ways of formation of a secret in a cell can differ depending on character of the allocated secret, specifics of a secretory cell and conditions of its functioning.
So, synthesis of primary product goes in granular endoplasmic reticulum (see) with participation ribosomes (see), material moves to Golgi's complex where there is his condensation and «packaging» in the granules collecting in an apical part of a cell. Mitochondrions (see) at the same time play, apparently, an indirect role, providing process of a sekretoobrazovaniye with energy. Synthesis of proteinaceous secrets is so carried out generally.
In the second, estimated, option of a sekretoobrazovaniye of S. occurs inside or on a surface of mitochondrions. The secretory product then moves to Golgi's complex where forms in granules. In the course of formation of a secret Golgi's complex can not participate. In such a way lipidic secrets, napr, steroid hormones of an adrenal gland can be synthesized.
In the third option formation of primary secretory product happens in tubules of an agranular endoplasmic reticulum, then the secret passes into Golgi's complex where there is his condensation. On such type some nonprotein secrets are synthesized.
Synthesis polisakharidny, muko-and glikoproteidny secrets is investigated insufficiently, but is established that the leader, a role in it is played by Golgi's complex and also that various intracellular organellas take part in synthesis of different secrets in a different measure.
Depending on a type of allocation: a secret from S.'s cell it is accepted to divide into several main types (golokrinovy, apocrenic and me-rokrinovy). At golokrinovy S. all cell as a result of its specialized degradation turns into a secret (e.g., S. of sebaceous glands).
Apokrinovy S., in turn, is divided into two main types — makroapokrinovy and mikroapokrinovy Page. At makroapokrinovy S. on a surface of a cell outgrowths are formed, to-rye in process of maturing of a secret separate from a cell therefore its height decreases. On such type cosecrete many glands (stalemate, milk, etc.). At mikroapokrinovy S., edges it is observed under a supermicroscope, from a cell small sites separate cytoplasms (see) or the expanded tops of microvillis containing a ready secret.
Merokrinovy secretion is also divided into two look — with an exit of a secret through formed at contact with a vacuole or a granule of an opening in a membrane and with an exit of a secret of a cell by diffusion through a membrane, edges at the same time, probably, does not change the structure. Merokrinovy S. is characteristic of digestive and closed glands.
There is no strict border between the above described types of secretion. E.g., allocation of a drop of fat secretory cells mammary gland (see) happens to a part of an apical cell membrane. Such type C. is called lemmokrinovy (E. A. Shubnikova, 1967). In the same cell there can be a change of types of extrusion of a secret. Existence of communication between synthesis and extrusion of a secret and its character are not finalized. One researchers consider that such communication is available, others deny, believing that processes in itself are autonomous. The sequence of data of dependence of extrusion ratio on the speed of synthesis of a secret is received, and also it is shown that accumulation in a cell of secretory granules exerts the braking impact on process of synthesis of a secret. Constant allocation of a small amount of a secret promotes its moderate synthesis. Stimulation of allocation of a secret increases also synthesis of a secretory product. It is revealed that in intracellular transport of a secret the large role is played by microtubules and microfilaments. Destruction of these structures, napr, by influence by colchicine or cytochalasin, significantly transforms mechanisms of education and extrusion of a secret. There are regulatory factors operating preferential on extrusion of a secret or on its synthesis as well as on both of these phases and receipt of initial products in a cell.
As showed E. Sh. Gerlovin (1974), in secretory cells in the course of an embryogenesis, and also at their regeneration is marked out (on the example of acinous cells of a pancreas) consecutive change of three main stages of their activity: the first stage — in kernels of cellular kernels goes synthesis of RNA, edges as a part of free ribosomes comes to cytoplasm; 2) the second stage — on ribosomes of cytoplasm is carried out synthesis of the structural proteins and enzymes participating then in formation of lipoproteidny membranes of an endoplasmic reticulum, mitochondrions and Golgi's complex; 3) the third stage — on ribosomes granular endoplasmic a reticulum in basal parts of cells occurs synthesis of secretory protein, to-ry is transported in tubules endoplasmic a reticulum, and then in Golgi's complex where it is made out in the form of secretory granules; granules collect in an apical part of cells, and at S.'s stimulation their contents are allocated outside.
Specifics of synthesis and allocation of secrets, different in structure, were the basis for a conclusion about existence of 4 types of the secretory cells possessing specific intracellular conveyors: the squirrels - synthesizing mucoid - a lipid - and mineralsekretiruyushchy.
Secretory cells have a number of features of bioelectric activity: low speed of fluctuations of membrane potential, various polarization of basal and apical membranes. For excitement of one types of secretory cells depolarization (e.g., of exocrine cells of a pancreas and channels of sialadens), of excitement of others — hyperpolarization is characteristic (e.g., of acinous cells of sialadens).
In transport of ions through basal and apical membranes of such secretory cells there are nek-ry distinctions: in the beginning polarization of a basal, then apical membrane changes, but at the same time the basal plasmolemma is more polarized. Discontinuous variations of polarization of membranes at S. are called secretory potentials. Their emergence is a condition of inclusion of secretory process. The optimum polarization of membranes necessary for emergence of secretory potentials makes apprx. 50 mV. Believe that distinction of polarization of basal and apical membranes (2 — 3 mV) creates rather strong electric field (20 — 30 in/cm). Its force approximately doubles at excitement of a secretory cell. It, according to B. I. Gutkina (1974), promotes movement of granules of a secret to an apical pole of a cell, circulation of contents of a granule, contact of granules with an apical membrane and an exit through it from a cell of the granulated and not granulated macromolecular secretory product.
Potential of a secretory cell is important also for S. of electrolytes, for the account a cut the osmotic pressure of cytoplasm and current of water playing an important role in secretory process are regulated.
Regulation of secretion
C. glands is under control of nervous, humoral and local mechanisms. The effect of these influences depends on a type of an innervation (sympathetic, parasympathetic), a type of gland and a secretory cell, on the mechanism of action of physiologically active agent on intracellular processes it.
According to I. P. Pavlov, S. is under control of three types of influences of c. N of page on glands: 1) functional influences, to-rye it is possible to divide on starting (transfer of gland from a condition of relative rest in a condition of secretory activity) and corrective (the stimulating and braking influences on the cosecreting glands); 2) vascular influences (change of level of blood supply of gland); 3) trophic influences — on intracellular metabolism (strengthening or weakening of synthesis of a secretory product). Began to refer to trophic influences as well proliferogenny effects of century of N of page and hormones.
In S.'s regulation various glands nervous and humoral factors correspond differently. E.g., S. of sialadens in connection with meal is regulated practically only by nervous (reflex) mechanisms; activity of gastric glands — nervous and humoral; The Page of a pancreas — is preferential by means of duodenal hormones of secretin (see) and cholecystokinin-sir-kreozimina.
Efferent nerve fibrils can form true synapses on ferruterous cells. At the same time it is proved that nerve terminations allocate a mediator in an interstitium, on Krom it diffuses already directly to secretory cells.
Physiologically active agents (mediators, hormones, metabolites) stimulate and brake S., affecting various phases of a secretory cycle through membrane receptors of a cell (see Receptors, cellular receptors) or getting into its cytoplasm. Efficiency of action of mediators is influenced by its quantity and a ratio with the enzyme hydrolyzing this mediator, the number of the membrane receptors reacting with a mediator and other factors.
S.'s braking can be result of inhibition of release of the stimulating agents. E.g., secretin brakes S. salt to - you glands of a stomach at the expense of inhibition of release of gastrin (see) — stimulator of this of Page.
Various substances of an endogenous origin influence activity of secretory cells differently. In particular, acetylcholine (see), interacting with cellular holinoretseptor, strengthens S. of a pepsinogen glands of a stomach, stimulating its extrusion from the main cells; synthesis of a pepsinogen stimulates also gastrin. Gistamin (see) interacts with H2 receptors of obkladochny cells of gastric glands and through system adenylatecyclase — tsAMF strengthens synthesis and extrusion salt to - you from a cell. Stimulation of obkladochny cells acetylcholine is mediated by its action on their holinoretseptor, strengthening of receipt in a cell of calcium ions, activation of system guanylate cyclase — tsGMF. Ability of acetylcholine to activate gastric Na, K-Atfazu and to strengthen intracellular transfer of calcium ions is important for S. These mechanisms of action of acetylcholine provide also release from G-cells of the gastrin which is S.'s stimulator of a pepsinogen and salt to - you with glands of a stomach. Acetylcholine and holetsisto-kinin-pancreozymin through systems adenylatecyclase — tsAMF and activation of current of calcium ions in acinous pancreatic cells strengthen in them synthesis of enzymes and their extrusion. Secretin in tsentroatsinozny cells and in cells of pancreat ducts also through system adenylatecyclase — tsAMF activates intracellular metabolism, transmembrane transfer of electrolytes and extrusion of bicarbonates.
In S. the large role is played and prostaglandins (see), being in some cases stimulators and inhibitors C.
Secretory cells react also to locally operating factors (pH of the environment, a number of metabolites, hydrolysates of nutrients and ingredients of secrets), value to-rykh is especially big in regulation of activity of digestive glands, endocrine cells went. - kish. path, endocrine system of providing homeostasis of an organism.
The cosecreted products can influence synthesis and extrusion of secrets a cell. So, S. of a pancreas is braked at hit of its secret in a duodenum. It connect with effect of pancreatic enzymes on release in blood of duodenal hormone cholecystokinin-pancreozymin. The enzymes circulating in blood also influence S., stimulating or oppressing in glandulotsita synthesis and extrusion of hydrolases.
In the conditions of a complete organism nervous and humoral factors and local regulatory mechanisms in the unity provide thin regulation by S. Primer there can be an adaptive character of S. of digestive glands which is expressed in a confinedness of quantity and structure of their secrets to a constant diet and type of food — in a secret the enzymes hydrolyzing the nutrients prevailing in a diet prevail. Besides, there is for the first time an urgent adaptation of S. to a type of the eaten food established by I. P. Pavlov — on this type of food at its reception and during digestive process the secret of digestive glands corresponding in the quantitative and qualitative relation with higher content in it of the enzymes hydrolyzing the prevailing type of nutrients is allocated. S.'s adaptation is carried out at the level of this or that digestive gland and all secretory device went. - kish. path. Participation in urgent adaptations of the central and peripheral nervous mechanisms, gastrointestinal hormones, physical is revealed. - chemical properties of nutrients and products of their hydrolysis. Cellular mechanisms of urgent adaptation of S. are investigated insufficiently.
Denervation of glands causes at first their increased secretory activity. This phenomenon was established by K. Bernard in 1864 on sialadens: their parasympathetic denervation temporarily caused the strengthened and continuous salivation — so-called paralytic secretion (its duration apprx. 5 — 6 weeks with a maximum on 6 — the 8th day after denervation). S.'s strengthening in the first days is connected with strengthening of allocation of acetylcholine owing to a degeneration of neurons (degenerative secretion), then with a hyperreactivity of denervated gland to its S.'s stimulators circulating with blood, to the Crimea this gland with an intact innervation was insensitive. At other digestive glands paralytic S.'s phenomenon is expressed more weakly.
Deprivation of gastric glands of a parasympathetic innervation increases quantity a holinoretsepto-ditch in membranes of secretory cells of these glands by 10 — 20 times. At the same time duration of a half-cycle of substitution of receptor protein making 10 days at innervated glands is reduced to 1 days and less at denervated glands. At the same time in denervated glands the content of cholinesterase decreases that explains the reason of increase in their reactivity.
S.'s strengthening the isolated loop of a small bowel after its denervation is noted that is explained by increase in permeability of its gistogema-tichesky barriers.
Early embryonic development is characteristic of digestive glands and their glandulotsit. The epithelium and ferruterous structures at the end of embryonic development are the functioning structures and interactions of a germ with the environment perform specific functions. In the course of further development and over all life there are characteristic changes of secretory function of digestive glands. In ontogenesis (see) S. forms and reaches thin differentiation — there is a formation of a secretory cycle characteristic of this type of cells, all links of nervous and humoral control, including cellular mechanisms of regulation and self-control of education and extrusion of a secret are improved.
The geterokhroniya of their age development is characteristic of closed glands (see. Endocrine system ). One glands reach a maturity in very early ontogenesis (an epiphysis, a thymus gland, the insulyarny device of a pancreas, a glucocorticoid zone of adrenal glands), others — in late youth and an early maturity (thyroid, parathyroid glands, a neurohypophysis, an adenohypophysis, incretory function of a hypothalamus). After all closed glands gonads develop. Geterokhroniya is characteristic also of transformation of functions of glands in the course of aging.
Age changes of exocrine and endocrine S. are connected not only with development of the corresponding glands and their glandulotsit, but also with a complex system of change of mechanisms of their nervous and humoral control, and also reactivity of target cells.
Disturbances of secretion
Disturbances of secretion can be shown by hyposecretion, i.e. reduction of allocation by glands of secretory products, and hypersecretion (increase in their allocation). These disturbances can be caused by the different reasons: hyperplasia of glands and their atrophic changes; change of reactivity of secretory cells and (or) forces of neurohumoral impacts on a cell; changes a secret - the producing and transport activity of a cell etc. Various mechanisms can be the cornerstone of these changes. In glands, complex on a set of cells, hypo - and hyper secretory frustration can concern activity of all gland, its part or a pool of the corresponding cells and to be followed by disturbances in ratios of ingredients of a secret. E.g., S.'s disturbance salt to - you is not necessarily combined by glands of a stomach with disturbance of allocation of pepsinogens by them. S.'s disturbance various enzymes and isozymes can be observed. Compensatory increase in S. of one glands at secretory insufficiency of others is quite often observed. One of such displays of pathology of S. is disturbance of its adaptive opportunities. Also changes of activity of glands are described, at to - ry secretory cells produce secrets or secrets unusual for them with the transformed properties.
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