By morphofunctional features The intestines are divided into thin and thick sections.
Small intestine(intestinum tenue) is located between the stomach and the caecum. The length of the small intestine is 4-5 m, the diameter is about 5 cm. There are three sections: the duodenum, jejunum and ileum. In the small intestine, all kinds of nutrients - proteins, fats and carbohydrates - undergo chemical processing. The enzymes enterokinase, kinasogen and trypsin, which break down simple proteins, are involved in the digestion of proteins; erepsin, which breaks down peptides into amino acids, nuclease digests complex nucleoprotein proteins. Carbohydrates are digested by amylase, maltase, sucrase, lactase and phosphatase, while fats are digested by lipase. In the small intestine, the process of absorption of the breakdown products of proteins, fats and carbohydrates into the blood and lymphatic vessels takes place. The intestine performs a mechanical (evacuation) function - it pushes food particles (chyme) towards the large intestine. For small intestine also characteristic is the endocrine function performed by special secretory cells and consists in the development of biologically active substances- serotonin, histamine, motilin, secretin, enteroglucogon, cholecystokinin, pancreozymin, gastrin.
The wall of the small intestine consists of four membranes: mucous (tunica mucosa), submucosa (tunica submcosa), muscular (tunica muscularis), serous (tunica serosa).
mucous membrane represented by epithelium (single-layered cylindrical border), lamina propria (loose fibrous connective tissue), muscular lamina (smooth muscle cells). A feature of the relief of the mucous membrane of the small intestine is the presence of circular folds, villi and crypts.
Circular folds composed of mucosa and submucosa.
intestinal villus- this is a finger-shaped outgrowth of the mucous membrane 5-1.5 mm high, directed into the lumen of the small intestine. The villus is based on the connective tissue of the lamina propria, in which there are separate smooth myocytes. The surface of the villus is covered with a single layer of cylindrical epithelium, in which three types of cells are distinguished: columnar epitheliocytes, goblet cells and intestinal endocrinocytes.
Columnar epithelial cells of the villi(lepitheliocyti columnares) make up the bulk of the epithelial layer of the villus. These are tall cylindrical cells measuring 25 µm. On the apical surface, they have microvilli, which under a light microscope look like a striated border. The microvilli are about 1 µm high and 0.1 µm in diameter. The presence of villi in the small intestine, as well as microvilli of columnar cells, the absorptive surface of the mucous membrane of the small intestine increases tenfold. Columnar epithelial cells have an oval nucleus, a well-developed endoplasmic reticulum, and lysosomes. The apical part of the cell contains tonofilaments (terminal layer), with the participation of which the end plates and tight junctions are formed, which are impermeable to substances from the lumen of the small intestine.
Columnar epithelial cells of the villi - the main functional element processes of digestion and absorption in the small intestine. The microvilli of these cells adsorb enzymes on their surface and break down nutrients with them. This process is called parietal digestion, unlike abdominal and intracellular, occurring in the lumen of the intestinal tube. On the surface of the microvilli is a glycocalyx, represented by lipoproteins and glycosaminoglycans. The breakdown products of proteins and carbohydrates - amino acids and monosaccharides - are transported from the apical surface of the cell to the basal one, from where they enter the capillaries of the connective tissue base of the villi through the basal membrane. This absorption path is also characteristic of water dissolved in it. mineral salts and vitamins. Fats are absorbed either by phagocytosis of emulsified fat droplets by columnar epithelial cells, or by absorption of glycerol and fatty acids, followed by resynthesis of neutral fat in the cytoplasm of the cell. Lipids through the basal surface of the plasmolemma of columnar epithelial cells enter the lymphatic capillaries.
goblet exocrinocytes(exocrinocyti caliciformes) are unicellular glands that produce a mucous secretion. In the expanded apical part, the cell accumulates a secret, and in the narrowed basal part, the nucleus, the endoplasmic reticulum, and the Goldki apparatus are located. Goblet cells are located on the surface of the villi singly, surrounded by columnar epitheliocytes. The secret of goblet cells serves to moisten the surface of the intestinal mucosa and thus promote the movement of food particles.
endocrinocytes(endocrinocyti dastrointestinales) are scattered singly among columnar epithelial cells with a border. Among the endocrinocytes of the small intestine, there are EC-, A-, S-, I-, G-, D-cells. The products of their synthetic activity are a number of biologically active substances that have a local effect on the secretion, absorption and motility of the intestine.
Intestinal crypts- these are tubular recesses of the epithelium in the own plate of the intestinal mucosa. The entrance to the crypt opens between the bases of neighboring villi. The depth of the crypts is 0.3-0.5 mm, the diameter is about 0.07 mm. There are about 150 million crypts in the small intestine, together with the villi they significantly increase the functionally active area of the small intestine. Among the epithelial cells of the crypts, in addition to columnar cells with a border, goblet cells and endocrinocytes, there are also columnar epitheliocytes without a border and exocrinocytes with acidophilic granularity (Paneth cells).
Exocrinocytes with acidophilic granules or Paneth cells (endocrinocyti cumgranulis acidophilis) are located in groups near the bottom of the crypts. Cells of a prismatic shape, in the apical part of which there are large acidophilic secretory granules. The nucleus, endoplasmic reticulum, Golgi complex are displaced to the basal part of the cell. The cytoplasm of Paneth cells stains basophilically. Paneth cells secrete dipeptidases (erepsin), which break down dipeptides into amino acids, and also produce enzymes that neutralize hydrochloric acid which enters the small intestine with food particles.
Columnar epitheliocytes borderless or undifferentiated epitheliocytes (endocrinocyti nondilferentitati) are poorly differentiated cells that are the source of physiological regeneration of the epithelium of the crypts and villi of the small intestine. In structure, they resemble border cells, but their apical surface lacks microvilli.
own record The mucous membrane of the small intestine is formed mainly by loose fibrous connective tissue, where elements of the reticular connective tissue occur. In the lamina propria, accumulations of lymphocytes form single (solitary) follicles, as well as grouped lymphoid follicles. Large accumulations of follicles penetrate through the muscular plate of the mucous membrane into the submucosa of the intestine.
muscularis lamina The mucous membrane is formed by two layers of smooth myocytes - inner circular and outer longitudinal.
Submucosa The walls of the small intestine are formed by loose fibrous connective tissue, in which there are a large number of blood and lymphatic vessels, and nerve plexuses. In the duodenum in the submucosa are the terminal secretory sections of the duodenal (Bruner's) glands. By structure, these are complex branched tubular glands with a mucous-protein secret. The terminal sections of the glands are composed of mucocytes, Paneth cells, and endocrinocytes (S-cells). The excretory ducts open into the intestinal lumen at the base of the crypts or between adjacent villi. The excretory ducts are built by cubic-shaped mucocytes, which at the surface of the mucous membrane are replaced by columnar cells with a border. The secret of the duodenal glands protects the duodenal mucosa from harmful effects gastric juice. Dipeptidases - products of the duodenal glands - break down dipeptides to amino acids, amylase breaks down carbohydrates. In addition, the secret of the duodenal glands is involved in the neutralization of acidic compounds of gastric juice.
Muscular membrane The small intestine is formed by two layers of smooth myocytes: internal oblique circular and external oblique longitudinal. Between them lie layers of loose fibrous connective tissue, rich in neurovascular plexuses. Function muscular membrane: mixing and promotion of digestion products (chyme).
Serous membrane The small intestine is formed by loose fibrous connective tissue, which is covered with mesothelium. It covers the outside of the small intestine from all sides, with the exception of the duodenum, which is covered by the peritoneum only in front, and in other parts has a connective tissue membrane.
Colon(intestinum crassum) department digestive tube, which ensures the formation and conduction of feces. Metabolic products, salts of heavy metals and others are released into the lumen of the colon. The bacterial flora of the large intestine produces vitamins B and K, and also ensures the digestion of fiber.
Anatomically, the large intestine is divided into the following sections: caecum, appendix, colon (its ascending, transverse and descending sections), sigmoid and rectum. The length of the large intestine is 1.2-1.5 m, the diameter is 10 mm. Four membranes are distinguished in the wall of the large intestine: mucous, submucosal, muscular and external - serous or adventitious.
mucous membrane The large intestine is formed by a single layer of prismatic epithelium, a connective tissue lamina propria, and a muscular lamina. The relief of the mucous membrane of the colon is determined by the presence of a large number of circular folds, crypts and the absence of villi. Circular folds are formed on the inner surface of the intestine from the mucous membrane and submucosa. They are located across and have a crescent shape. Most of the epithelial cells of the large intestine are represented by goblet cells, there are fewer columnar cells with a striated border and endocrinocytes. At the base of the crypts are undifferentiated cells. These cells do not differ significantly from similar cells of the small intestine. Mucus covers the epithelium and promotes the sliding and formation of feces.
In the lamina propria of the mucous membrane there are significant accumulations of lymphocytes that form large single lymphatic follicles that can penetrate the muscular lamina of the mucous membrane and merge with similar formations of the submucosa. Accumulations of dissociated lymphocytes and lymphatic follicles in the wall of the digestive tube are considered an analogue of the bursa (bag) of Fabricius of birds, responsible for the maturation and acquisition of immune competence by B-lymphocytes.
There are especially many lymphatic follicles in the wall of the appendix. The epithelium of the mucous membrane of the appendix is a single-layer prismatic, infiltrated by lymphocytes, with a small content of goblet cells. It contains Paneth cells and intestinal endocrinocytes. In the endocrinocytes of the appendix, the main part of the serotonin and melatonin of the body is synthesized. The lamina propria without a sharp border (due to the weak development of the muscular mucosal lamina) passes into the submucosa. In the lamina propria and in the submucosa there are numerous large locally confluent accumulations of lymphoid tissue. Appendix performs a protective function, lymphoid accumulations are part of the peripheral parts of the immune system of the tissue in it
The muscular plate of the mucous membrane of the large intestine is formed by two layers of smooth myocytes: internal circular and external oblique-longitudinal.
submucosa The large intestine is formed by loose fibrous connective tissue, in which there are accumulations of fat cells, as well as a significant number of lymphatic follicles. In the submucosa are the neurovascular plexus.
The muscular coat of the large intestine is formed by two layers of smooth myocytes: inner circular and outer longitudinal, between them are layers of loose fibrous connective tissue. IN colon the outer layer of smooth myocytes is not continuous, but forms three longitudinal bands. The shortening of individual segments of the inner layer of smooth myocytes of the muscular membrane contributes to the formation of transverse folds of the colon wall.
The outer shell of most of the large intestine is serous, adventitial in the caudal part of the rectum.
Rectum- has a number of structural features. It distinguishes between the upper (pelvic) and lower (anal) parts, which are separated from each other by transverse folds.
The mucous membrane of the upper part of the rectum is covered with a single layer of cubic epithelium, which forms deep crypts.
The mucous membrane of the anal part of the rectum is formed by three zones of different structure: columnar, intermediate and skin.
The columnar zone is covered with stratified cuboidal epithelium, the intermediate zone is covered with stratified squamous non-keratinized epithelium, and the dermal zone is covered with stratified squamous keratinized epithelium.
The lamina propria of the columnar zone forms 10-12 longitudinal folds, contains blood lacunae, single lymphatic follicles, rudiments: rudimentary anal glands. The lamina propria of the intermediate and zone is rich in elastic fibers, sebaceous jelly is located here, and there are dissociated lymphocytes. In the own plate of the rectum of the rectum in its skin part appear hair follicles, end sections of apocrine sweat glands, sebaceous glands.
The muscular plate of the mucous membrane of the rectum is formed by the inner circular and outer longitudinal layers of smooth myocytes.
The submucosa of the rectum is formed by loose fibrous connective tissue, in which the nerve and vascular plexuses are located.
The muscular layer of the rectum is formed by the inner circular outer longitudinal layers of smooth myocytes. The muscular membrane forms two sphincters, which play an important role in the act of defecation. The internal sphincter of the rectum is formed by a thickening of smooth myocytes of the inner layer of the muscular membrane, the outer - by bundles of fibers of the striated muscle tissue.
The upper part of the rectum is covered on the outside with a serous membrane, the anal part is covered with an adventitious membrane.
The wall of the small intestine is built from the mucous membrane, submucosa, muscular and serous membranes.
The inner surface of the small intestine has a characteristic relief due to the presence of a number of formations - circular folds, villi and crypts (Lieberkün's intestinal glands). These structures increase common surface small intestine, which contributes to the performance of its basic functions of digestion. Intestinal villi and crypts are the main structural and functional units of the mucous membrane of the small intestine.
The mucous membrane of the small intestine consists of a single-layer prismatic border epithelium of its own layer of the mucous membrane and the muscular layer of the mucous membrane.
The epithelial layer of the small intestine contains four main populations of cells:
- * columnar epitheliocytes,
- * goblet exocrinocytes,
- * Paneth cells, or exocrinocytes with acidophilic granules,
- * endocrinocytes, or K-cells (Kulchitsky cells),
- * as well as M-cells (with microfolds), which are a modification of columnar epitheliocytes.
The small intestine includes three sections: the duodenum, jejunum, and ileum.
In the small intestine, all kinds of nutrients - proteins, fats and carbohydrates - undergo chemical processing.
The enzymes of pancreatic juice (trypsin, chymotrypsin, collagenase, elastase, carboxylase) and intestinal juice (aminopeptidase, leucine aminopeptidase, alanine aminopeptidase, tripeptidase, dipeptidase, enterokinase) are involved in the digestion of proteins.
Enterokinase is produced by the cells of the intestinal mucosa in an inactive form (kinasogen), ensures the conversion of the inactive trypsinogen enzyme into active trypsin. Peptidases provide further sequential hydrolysis of peptides, which began in the stomach, to free amino acids, which are absorbed by intestinal epithelial cells and enter the bloodstream.
In the small intestine, the process of absorption of the breakdown products of proteins, fats and carbohydrates into the blood and lymphatic vessels takes place. In addition, the intestine performs a mechanical function: it pushes the chyme in the caudal direction. This function is carried out due to peristaltic contractions of the muscular membrane of the intestine. The endocrine function, performed by special secretory cells, consists in the production of biologically active substances - serotonin, histamine, motilin, secretin, enteroglucagon, cholecystokinin, pancreozymin, gastrin and gastrin inhibitor.
Intestinal juice is a cloudy, viscous liquid, is a product of the activity of the entire mucous membrane of the small intestine, has a complex composition and different origins. Up to 2.5 liters of intestinal juice is excreted per day in a person. (Potyrev S.S.)
In the crypts of the mucous membrane of the upper part of the duodenum, duodenal, or Brunner's, glands are laid. The cells of these glands contain secretory granules of mucin and zymogen. The structure and function of the Brunner glands are similar to those of the pyloric glands. The juice of the Brunner glands is a thick, colorless liquid of slightly alkaline reaction, which has little proteolytic, amylolytic and lipolytic activity. Intestinal crypts, or Lieberkün's glands, are embedded in the mucous membrane of the duodenum and the entire small intestine and surround each villus.
Many epithelial cells of the crypts of the small intestine have a secretory ability. Mature intestinal epitheliocytes develop from undifferentiated borderless enterocytes that predominate in crypts. These cells have proliferative activity and replenish intestinal cells that are desquamated from the tops of the villi. As they move toward the apex, the borderless enterocytes differentiate into absorptive villus cells and goblet cells.
Intestinal epithelial cells with a striated border, or absorbent cells, cover the villus. Their apical surface is formed by microvilli with outgrowths of the cell wall, thin filaments that form the glycocalyx, and also contains many intestinal enzymes translocated from the cell where they were synthesized. Enzymes are also rich in lysosomes located in the apical part of the cells.
Goblet cells are called unicellular glands. The mucus-filled cell has characteristic appearance glasses. The secretion of mucus occurs through breaks in the apical plasma membrane. The secret has enzymatic, including proteolytic, activity. (Potyrev S.S.)
Enterocytes with acidophilic granules, or Paneth cells, in a mature state also have morphological features secretions. Their granules are heterogeneous and are excreted into the lumen of the crypts by the type of merocrine and apocrine secretion. The secret contains hydrolytic enzymes. The crypts also contain Argentaffin cells that perform endocrine functions.
The contents of the small intestine loop, even isolated from the rest of the intestine, are the product of many processes (including desquamation of enterocytes) and bilateral transport of high- and low-molecular substances. This, in fact, is intestinal juice.
Properties and composition of intestinal juice. Centrifugation separates the intestinal juice into liquid and solid parts. The ratio between them varies depending on the strength and type of irritation of the mucous membrane of the small intestine.
The liquid part of the juice is formed by a secret, solutions of inorganic and organic substances transported from the blood, and partially by the contents of the destroyed cells of the intestinal epithelium. The liquid part of the juice contains about 20 g/l of dry matter. Among the inorganic substances (about 10 g/l) are chlorides, bicarbonates and phosphates of sodium, potassium, and calcium. The pH of the juice is 7.2-7.5, with increased secretion it reaches 8.6. The organic substances of the liquid part of the juice are represented by mucus, proteins, amino acids, urea and other metabolic products.
The dense part of the juice is a yellowish-gray mass that looks like mucous lumps and includes undestroyed epithelial cells, their fragments and mucus - the secret of goblet cells has a higher enzymatic activity than the liquid part of the juice (G.K. Shlygin).
In the mucous membrane of the small intestine, there is a continuous change in the layer of cells of the surface epithelium. They are formed in the crypts, then move along the villi and exfoliate from their tops (morphokinetic, or morphonecrotic, secretion). Complete renewal of these cells in humans takes 1-4-6 days. Such a high rate of formation and rejection of cells provides a sufficiently large number of them in the intestinal juice (in humans, about 250 g of epitheliocytes are rejected per day).
Mucus forms a protective layer that prevents excessive mechanical and chemical effects of chyme on the intestinal mucosa. In mucus, the activity of digestive enzymes is high.
The dense part of the juice has a much greater enzymatic activity than the liquid part. The main part of the enzymes is synthesized in the intestinal mucosa, but some of them are transported from the blood. There are more than 20 different enzymes in the intestinal juice that are involved in digestion.
The main part of intestinal enzymes takes part in parietal digestion. Carbohydrates are hydrolyzed by β-glucosidases, β-galactazidase (lactase), glucoamylase (g-amylase). β-glucosidases include maltase and trehalase. Maltase hydrolyzes maltose, and trehalase hydrolyzes trehalose by 2 molecules of glucose. b-Glucosidases are represented by another group of disaccharidases, which includes 2-3 enzymes with isomaltase activity and invertase, or sucrase; with their participation, monosaccharides are formed. (Briefly T.F.)
The high substrate specificity of intestinal disaccharidases in their deficiency causes intolerance to the corresponding disaccharide. Genetically fixed and acquired lactase, trehalase, sucrase and combined deficiencies are known. A significant population of people, especially the peoples of Asia and Africa, has been diagnosed with lactase deficiency.
In the small intestine, the hydrolysis of peptides continues and ends. Aminopeptidases make up the bulk of the peptidase activity of the enterocyte brush border and cleave the peptide bond between two specific amino acids. Aminopeptidases complete the membrane hydrolysis of peptides, resulting in the formation of amino acids - the main absorbable monomers.
Intestinal juice has lipolytic activity. In the parietal hydrolysis of lipids, intestinal monoglyceride lipase is of particular importance. It hydrolyzes monoglycerides of any hydrocarbon chain length, as well as short chain di- and triglycerides, and to a lesser extent medium-chain triglycerides and cholesterol esters. (Potyrev S.S.)
A number of food products contain nucleoproteins. Their initial hydrolysis is carried out by proteases, then RNA and DNA cleaved from the protein part, respectively, are hydrolyzed by RNA and DNases to oligonucleotides, which, with the participation of nucleases and esterases, are degraded to nucleotides. The latter are attacked by alkaline phosphatases and more specific nucleotidases, releasing nucleosides that are then absorbed. Phosphatase activity of intestinal juice is very high.
The enzyme spectrum of the mucous membrane of the small intestine and its juice changes under the influence of certain long-term diets.
regulation of intestinal secretion. Eating, local mechanical and chemical irritation of the intestine enhance the secretion of its glands with the help of cholinergic and peptidergic mechanisms.
In the regulation of intestinal secretion, local mechanisms play a leading role. Mechanical irritation of the mucous membrane of the small intestine causes an increase in the release of the liquid part of the juice. Chemical stimulants of the secretion of the small intestine are the products of digestion of proteins, fats, pancreatic juice, hydrochloric and other acids. The local action of the products of digestion of nutrients causes the separation of intestinal juice rich in enzymes. (Briefly T.F.)
The act of eating does not significantly affect intestinal secretion, at the same time, there are data on the inhibitory effects on it of irritation of the antrum of the stomach, modulating effects of the central nervous system, on the stimulating effect on the secretion of cholinomimetic substances and the inhibitory effect of anticholinergic and sympathomimetic substances. Stimulate intestinal secretion of GIP, VIP, motilin, inhibits somatostatin. The hormones enterocrinin and duocrinin, produced in the mucous membrane of the small intestine, stimulate the secretion of intestinal crypts (Lieberkün's glands) and duodenal (Brunner's) glands, respectively. These hormones have not been isolated in purified form.
The small intestine consists of 3 parts: 1) duodenum (intestinum duodenum), 2) jejunum (Intestinum jejunum) and 3) ileum (intestinum lleum). The wall of the small intestine consists of 4 membranes: 1) mucous membrane, including a layer of the epithelium, its own plate and muscular plate; 2) submucosa; 3) muscular membrane, consisting of the inner circular and outer longitudinal layers of smooth myocytes. and 4) sevbnoi. SOURCES OF DEVELOPMENT of the epithelium - intestinal endoderm, loose connective and smooth muscle tissue - mesenchyme, mesothelium of the serous membrane - visceral sheet of splanchnotome.
RELIEF (SURFACE) of the mucous membrane is represented by folds, villi and crypts (simple tubular glands). The folds of the mucous membrane are formed by the mucous membrane and the submucosa, have a circular direction and are called semilunar (plica semilunalls), or circular (plica circularls). VILLI (Villl Intestinalls) are protrusions of the mucous membrane, which include loose connective tissue of the lamina propria, smooth myocytes of the muscular lamina and a single-layer prismatic (intestinal) epithelium covering the villi. The composition of the villi also includes an arteriole branching into capillaries, a venule and a lymphatic capillary. The height of the villi in the duodenum is 0.3-0.5 mm; jejunum and ileum - up to 1.5 mm. The thickness of the villi in the duodenum is greater than that of the jejunum or ileum. There are up to 40 villi per 1 sq. mm in the duodenum, and no more than 30 in the jejunum and ileum.
The epithelium covering the villi is called columnar (eptheli-um colmnarae). It consists of 4 types of cells: 1) columnar epitheliocytes with a striated border (epitheliocytus columnar is cum lim-bus striatus); 2) M-cells (cells with microfolds): 3) goblet exocrinocytes (exocrinocyts caliciformis) and 4) endocrine, or basal-granular cells (endocrinocytus). Striated-rimmed columnar epitheliocytes are so named because they have microvilli on their apical surface. The average height of the microvilli is about 1 µm, the diameter is 0.01 µm, the distance between the microvilli is from 0.01 to 0.02 µm. Between the microvilli contains a highly active alkaline phosphatase, nucleoside diphosphatases, L-glycosidase, O-glycosidase, aminopeptidases. Microvilli contain microtubules and actin filaments. Thanks to these ultrastructures, microvilli carry out movement and absorption. The surface of microvilli is covered with glycocalyx. Digestion in a striated border is called parietal. In the cytoplasm of columnar epitheliocytes, EPS, the Golgi complex, mitochondria are well developed, there are lysosomes and contain multivesicular bodies (a vesicle or vesicle, inside which there are smaller vesicles) and microfilaments, which form a cortical layer in the apical part. The nucleus is oval, active, located closer to the basal part. On the lateral surface of columnar epitheliocytes in the apical part of the cells there are intercellular connections: 1) tight insulating contacts (zonula occludens) and 2) adhesive belts (zonula adherens), which close the intercellular gaps. Closer to the basal part of the cells, there are desmosomes and interdigitations between them. The lateral surface of the cell cytolemma contains Na-ATPase and K-ATPase. which are involved in the transport of Na and K through the cytolemma. Functions of columnar epithelial cells with a striated border: 1) produce digestive enzymes, involved in parietal digestion 2) participation in parietal digestion and 3) absorption of cleavage products. M-CELLS are located in those places of the intestine where there are lymph nodes in the lamina propria of the mucous membrane. These cells belong to a variety of columnar epithelial cells, have a flattened shape. There are few microvilli on the apical surface of these cells, but the cytolemma here forms microfolds. With the help of these microfolds, M-cells capture macromolecules (antigens) from the intestinal lumen, endocytic vesicles are formed here, which then enter the lamina propria of the mucous membrane through the basal and lateral plasmolemma, come into contact with lymphocytes and stimulate them to differentiate. Goblet exocrinodites are mucous cells (mucocytes), have a synthetic apparatus (smooth EPS, Golgi complex, mitochondria), a flattened inactive nucleus is located closer to the basal part. A mucous secretion is synthesized on the smooth EPS, the granules of which accumulate in the apical part of the cell. As a result of the accumulation of secretion granules, the apical part expands and the cell acquires the shape of a glass. After secretion from the apical part, the cell again acquires a prismatic shape.
ENDOCRINE (ENTEROCHROSHRPHILIC) CELLS are represented by 7 varieties. These cells are contained not only on the surface of the villi, but also in the crypts. Crypts are tubular depressions located in the lamina propria. In fact, these are simple tubular glands. Their length does not exceed 0.5 mm. The composition of the crypts includes 5 types of epithelial cells; 1) columnar epitheliocytes (enterocytes), differ from the same cells of the villi by a thinner striated border: 2) goblet ekeocrinocytes are the same as in the villi:
3.) epitheliocytes without a striated border are undifferentiated cells, due to which the epithelium of crypts and villi is renewed every 5-6 days; 4) cells with acidophilic granularity (Paneth cells) and 5) endocrine cells. CELLS WITH ACIDOPHILIAN GRAIN are located one by one or in groups in the area of the body and the bottom of the crypts. In these cells, the Golgi complex, granular ER, and mitochondria are well developed. located around the round core. In the apical part of the cells there are acidophilic granules containing a protein-carbohydrate complex. The acidophilia of the granules is explained by the presence of the alkaline protein arginine in them. The cytoplasm of cells with acidophilic granularity (Paneth cells) contains zinc and enzymes: acid phosphate, dehydrogenase and dipephydases, which break down dipeptides to amino acids, in addition, there is lysozyme, which kills bacteria. Functions of Paneth cells; cleavage of dipetidases to amino acids. antibacterial and neutralization of HC1. Crypts and villi of the small intestine represent a single complex due to: 1) anatomical proximity (the crypts open between the villi); 2) crypt cells produce enzymes involved in parietal digestion, and 3) crypt and villus cells are renewed every 5-6 days due to undifferentiated crypt cells. ENDOCRINE CELLS of the villi and creep of the small intestine are represented by 1) Ec cells that produce serotonin, motilin and substance P; 2) A-cells that secrete enteroglucagon, which breaks down glycogen into simple sugars; 3) S-cells that produce secretin, which stimulates the secretion of pancreatic juice; 4) 1-cells that secrete cholecystokinin. stimulating liver function, and pancreozymin. activating the function of the pancreas; 5) G-cells. producing gastrin; 0) D-cells secreting somatostatin; 7) D1 cells that produce VIL (vasoactive intestinal peptide). The lamina propria of the mucous membrane is represented by loose connective tissue, which contains many reticular fibers and reticulo-like cells. In addition, in its own plate there are single lymphatic nodules (nodull lymphatlcl solita-rl), the diameter of which reaches 3 mm. and grouped lymph nodules (nodull lyinphatlcl aggregati), whose width is 1 cm and length up to 12 cm. starts to decrease. Functions of lymph nodes: hematopoietic and protective.
The muscular plate of the mucous membrane of the small intestine consists of 2 layers of smooth myocytes: internal circular and external longitudinal. Between these layers there is a layer of loose connective tissue. The submucosal basis consists of loose connective tissue, in which there are all plexuses: nervous, arterial, venous and lymphatic. In the submucosa of the duodenum there are complex branched tubular glands (giandulae submucosae). The terminal sections of these glands are lined mainly with mucocytes with a light cytoplasm, a flattened inactive nucleus. The cytoplasm contains the Golgi complex, smooth ER, and mitochondria; in the apical part, there are granules of mucous secretion. In addition, apical-granular, goblet, undifferentiated, and sometimes parietal cells are found in the terminal sections. The small ducts of the glands of the duodenum are lined with cuboidal epithelium, the larger ducts opening into the intestinal lumen are lined with columnar limbic. The secret of submucosal_zhedez has an alkaline reaction, contains di-peptidases. Meaning of the secret: it breaks down dipeptides to amino acids and alkalizes the acidic contents that came from the stomach into the duodenum. The MUSCLE COATING of the wall of the small intestine consists of 2 layers of smooth myocytes: the inner circular and the outer longitudinal. Between these layers there is a layer of loose connective tissue, in which 2 nerve plexuses are located: 1) the muscular-intestinal nerve plexus and 2) the muscular-intestinal sensitive nerve plexus. Due to the local contraction of the myocytes of the inner layer, the contents of the intestine are mixed, due to the friendly contraction of the inner and outer layers, peristaltic waves arise, which contribute to pushing food in the caudal direction. The serous membrane of the small intestine consists of a connective tissue base covered with mesothelium. Duplication of the serous membrane forms the mesentery of the intestine, which attaches to the dorsal wall abdominal cavity. In animals whose body occupies a horizontal position, the intestines are suspended on the mesentery. Therefore, the intestines of animals always occupy the correct position, i.e. it does not rotate around the mesentery. In humans, the body is in a vertical position, therefore, conditions are created for the intestine to rotate around the mesentery. With a significant turn of the intestine around the mesentery, partial or complete obstruction occurs, which is accompanied by pain. In addition, the blood supply to the intestinal wall is disrupted and its necrosis occurs. At the first signs of intestinal obstruction, a person needs to give the body a horizontal position so that the intestines are suspended on the mesentery. This is sometimes enough for the intestines to take the correct position and restore its patency without surgical intervention. BLOOD SUPPLY OF THE SMALL INTESTINE is carried out at the expense of those arterial plexuses: 1) submucosal, located in the submucosal base; 2) intermuscular, located in a layer of connective tissue between the outer and inner muscle layers of the muscular membrane and 3) mucous, located in the lamina propria of the mucous membrane. Arterioles depart from these plexuses, branching into cacillaries in all membranes and layers of the intestinal wall. Atrerioles extending from the mucous plexus penetrate into each villus of the intestine and branch into capillaries that flow into the venule of the villus. Venules carry blood to the venous plexus of the mucosa, and from there to the plexus of the submucosa. LYMPH OUTFLOW from the intestine begins with lymphatic capillaries located in the villi of the intestine and in all its layers and membranes. Lymphatic capillaries empty into larger lymphatic vessels. through which lymph enters a well-developed plexus of lymphatic vessels located in the submucosa. The innervation of the SMALL INTESTINE is carried out by two intermuscular plexuses: 1) the muscular-intestinal plexus and 2) the sensitive muscular-intestinal plexus. The SENSITIVE MUSCULAR-INTESTINAL nerve plexus is represented by afferent nerve fibers, which are dendrites of neurons coming from 3 sources: a) neurons of the spinal ganglia, b) sensory neurons of the intramural ganglia (Dogel cells type II) and c) sensory neurons of the node vagus nerve. The musculo-intestinal nerve plexus is represented by various nerve fibers, including axons of neurons of sympathetic ganglions (sympathetic nerve fibers) and ascons of efferent neurons (Type II Dogel cells) embedded in the intramural ganglia. Efferent (sympathetic and parasympathetic) nerve fibers end with motor effectors on smooth muscle tissue and secretory ones on crypts. Thus, there are sympathetic and parasympathetic reflex arcs in the gut, which are already well known. In the gut there are not only three-membered, but also four-membered reflex sympathetic arcs. The first neuron of the four-membered reflex arc is a neuron of the spinal ganglion, the second is a neuron of the lateral intermediate nucleus spinal cord, the third neuron is in the sympathetic nerve ganglion and the fourth - in the intramural ganglion. There are local reflex arcs in the small intestine. They are located in the intramural ganglia and consist of Type II Dogel cells, whose depdrites end in receptors, and whose axons end in synapses on Type I Dogel cells, which are the second neurons of the reflex arc. Their axons terminate in effector nerve endings. FUNCTIONS OF THE SMALL INTESTINE: 1) chemical treatment food; 2) suction; 3) mechanical (motor); 4) endocrine. CHEMICAL PROCESSING OF FOOD is carried out due to 1) intracavitary digestion; 2) parietal digestion and 3) parietal digestion. Intracavitary digestion is carried out due to the enzymes of pancreatic juice entering the duodenum. Intracavitary digestion provides the breakdown of complex proteins to simpler ones. Parietal digestion is carried out on the surface of the villi due to enzymes produced in the crypts. These enzymes break down simple proteins into amino acids. Membrane digestion occurs on the surface of epithelial mucosal overlays due to intracavitary enzymes and enzymes produced in crypts. What are epithelial mucous overlays 7 The epithelium of the villi and crypts of the small intestine is updated every 5-G days. Rejected epithelial cells of crypts and villi are mucous epithelial overlays.
Cleavage of proteins in the small intestine is carried out with the help of trypsin, kinazogen, erypsin. Cleavage of nucleic acids occurs under the influence of nuclease. Cleavage of carbohydrates is carried out with the help of amylase, maltava, saccharase, lactase, glucosidases. Cleavage of lipids occurs due to lipases. The absorption function of the small intestine is carried out through a striated border of columnar epitheliocytes covering the villi. These villi are constantly contracting and relaxing. At the height of digestion, these contractions are repeated 4-6 times per minute. The contractions of the villi are carried out by smooth myocytes located in the stroma of the villus. Myocytes are located radially and obliquely with respect to the longitudinal axis of the villi. The ends of these myocytes are braided with reticular fibers. The peripheral ends of the reticular fibers are woven into the basement membrane of the epithelium of the villi, the central ends into the stroma surrounding the vessels inside the villi. With the contraction of smooth myocytes, there is a decrease in the volume of the stroma located between the vessels and the epithelium of the villi, and a decrease in the volume of the villi themselves. The diameter of the vessels, around which the stroma layer becomes thinner, does not decrease. Changes in the villi during their contraction create conditions for the entry of cleavage products into the blood and lymphatic capillaries of the villi. At the moment when smooth myocytes relax, the volume of the villi increases, intravillous pressure decreases, which favorably affects the absorption of cleavage products into the stroma of the villi. Thus, it seems that the villi are then increasing. then decreasing, they act like an eyedropper; when the rubber cap of the pipette is squeezed, its contents are released, when relaxed, the next portion of the substance is sucked. In 1 minute, about 40 ml of nutrients are absorbed in the intestines. PROTEIN ABSORPTION is carried out through the brush border after their splitting to amino acids. LIPID ABSORPTION IS CARRIED OUT IN 2 WAYS. 1. On the surface of the striated border, with the help of lipase, lipids are broken down to glycerol and fatty acids. Glycerin is absorbed into the cytoplasm of epithelial cells. Fatty acids undergo esterification, i.e. with the help of cholinesterol and cholinesterase, they are converted into fatty acid esters, which are absorbed through the striated border into the cytoplasm of columnar epithelial cells. In the cytoplasm, esters decompose with the release of fatty acids, which, with the help of kinasogen, combine with glycerol. As a result, lipid droplets with a diameter of up to 1 micron are formed, called chylomicrons. Chylomicrons then enter the stroma of the villi, then into the lymphatic capillaries. The 2nd WAY of lipid absorption is carried out as follows. On the surface of the striated border, lipids are emulsified and combined with protein, resulting in the formation of droplets (chylomicrons) that enter the cytoplasm of cells and intercellular spaces, then into the stroma of the villi and the lymphatic capillary. The MECHANICAL FUNCTION of the small intestine is to stir and push the chyme in a caudal direction. The endocrine function of the small intestine is carried out due to the secretory activity of endocrine cells located in the epithelium of the villi and crypts.
Small intestine
The small intestine provides the final digestion of food, the absorption of all nutrients, as well as the mechanical movement of food towards the large intestine and some evacuation function. The small intestine is divided into several sections. The plan of the structure of these departments is the same, but there are some differences. The relief of the mucous membrane forms circular folds, intestinal villi and intestinal crypts. Folds are formed by the mucous membrane and submucosa. Villi are finger-like outgrowths of the lamina propria, covered with epithelium on top. Crypts are deepenings of the epithelium in the lamina propria. The epithelium lining the small intestine is single-layer prismatic. This epithelium is divided into:
- Columnar enterocytes
- goblet cells
- M cells
- Paneth cells (with acidophobic granularity)
- endocrine cells
- undifferentiated cells
- EC cells, produce serotonin
- ECL cells, produce histamine
- P cells, produce bambasin
- Cells that synthesize enteroglucagon
- K cells that produce pancreosinin
- Inner circular layer
- Outer longitudinal layer
After the products of fat hydrolysis have entered the enterocytes, fats begin to be synthesized in the intestinal wall, specific to given organism , which by their structure different from dietary fat. The mechanism of fat resynthesis in the intestinal wall is as follows: first happens glycerol activation And IVH then sequentially will occur acylation of alpha-glycerophosphate with education mono- And diglycerides. active form diglyceride - phosphatidic acid occupies a central place in the synthesis of fat to the intestinal wall. From it after activation in the presence CTF formed CDP-diacylglyceride which gives rise to complex fats.
IVH activation.
RCOOH + HSKoA + ATP → RCO~SCoA + AMP + H 4 P 2 O 7 The reaction is catalyzed acyl-CoA synthetase.
Glycerol activation.
Glycerol + ATP → α-glycerophosphate + ADP Enzyme – glycerate kinase.
In the reactions of resynthesis of fats, as a rule, only long chain fatty acids. These are not only fatty acids absorbed from the intestines, but also fatty acids synthesized in the body, therefore, the composition of resynthesized fats differs from fats obtained from food.
In the cells of the mucous membrane of the small intestine, the absorbed cholesterol molecules are also converted into esters by interacting with acyl-CoA. This reaction is catalyzed acetlcholesterolacyltransferase (AHAT). The activity of this enzyme depends the rate at which exogenous cholesterol enters the body. In the epithelial cells of the small intestine, lipoprotein complexes are formed from fats formed as a result of resynthesis, as well as from cholesterol esters, fat-soluble vitamins that come with food. chylomicrons (HM). XM further deliver fats to peripheral tissues.
42. Human blood lipoproteins, their formation and functions.
Lipids are insoluble compounds in water, therefore, for their transfer by blood, special carriers are needed that are soluble in water. These forms of transport are lipoproteins. Synthesized fat in the intestinal wall, or fat synthesized in other tissues, organs, can be transported by the blood only after inclusion in the composition of lipoproteins, where proteins play the role of a stabilizer (various apoproteins). According to its structure lipoprotein micelles have outer layer And core. outer layer It is formed from proteins, phospholipids and cholesterol, which have hydrophilic polar groups and show an affinity for water. Core consists of triglycerides, cholesterol esters, fatty acids, vitamins A, D, E, K. Thus, insoluble fats are transported throughout the body after synthesis in the intestinal wall, as well as synthesis in other tissues.
Allocate 4 classes of blood lipoproteins, which differ from each other in their chemical structure, micelle size and transportable fats. Because they have different settling rates in solution table salt , they are divided into: 1.) Chylomicrons. Formed in the intestinal wall and have the largest particle size. 2.) Very low density lipoproteins - VLDL. Synthesized in the intestinal wall and liver. 3.) Low density lipoproteins - LDL. Formed in the endothelium of capillaries from VLDL. 4.) Lipoproteins high density - HDL. Formed in the intestinal wall and liver.
Chylomicrons (HM) the largest particles. Their maximum concentration is reached by 4 - 6 hours after a meal. They are broken down by the action of an enzyme. lipoprotein lipase, which is formed in the liver, lungs, adipose tissue, vascular endothelium. It is generally accepted that chylomicrons (ChM) are absent in fasting blood and appear only after eating. XM is predominantly transported triacylglycerides(up to 83%) and exogenous IVH.
The largest number of lipoproteins are involved in transport of dietary fat, which includes over 100g triglycerides And about 1g cholesterol per day. IN epithelial cells intestines, dietary triglycerides and cholesterol are included in large lipoprotein particles - chylomicrons. They are secreted into the lymph, then through the general bloodstream they enter into the capillaries of adipose tissue And skeletal muscle.
Chylomicrons are targeted by the enzyme lipoprotein lipase. Chylomicrons contain a special apoprotein CII activating lipase releasing free fatty acids and monoglycerides. Fatty acids pass through the endothelial cell and enter adjacent adipocytes or muscle cells, in which either reesterified to triglycerides, or are oxidized.
After removal of triglycerides from the core chylomicron residue separated from the epithelium of the capillaries and again enters the blood. Now it has turned into a particle containing a relatively small amount of triglycerides, but a large number of cholesterol esters. There is also an exchange apoproteins between it and other plasma lipoproteins. Final result - transformation of a chylomicron into a particle of its residue rich cholesterol esters, as well as apoprotein B-48 And E. These residues are carried to the liver, which absorbs them very intensively. This uptake is mediated by the binding of apoprotein E to a specific receptor called chylomicron residue receptor on the surface of the hepatocyte.
Bound residues are taken up by the cell and degraded in lysosomes in the process - receptor-mediated endocytosis. The overall result of transport performed by chylomicrons is delivery of dietary triglycerides to adipose tissue, and cholesterol to the liver.
VLDL particles enter the tissue capillaries, where they interact with the same enzyme - lipoprotein lipase, which the destroys chylomicrons. Triglyceride core VLDL hydrolyzed and the fatty acids are used to synthesize triglycerides in adipose tissue. Remaining particles resulting from the action of lipoprotein lipase on VLDL are called intermediate density lipoproteins(LPPP). Part of the LPP particles is degraded in the liver by binding to receptors, named low density lipoprotein receptors (LDL receptors), which are distinct from receptors chylomicron residues.
The rest of the LPPP remains in plasma, in which it is exposed subsequent transformation, during which almost all remaining triglycerides are removed. In this transformation, the particle loses all of its apoproteins except for apoprotein B-100. As a result, a cholesterol-rich particle is formed from the LPPP particle. LDL. Core LDL almost entirely composed of cholesterol esters, but surface sheath contains only one apoprotein B-100. Humans have a large proportion of LDL not absorbed by the liver, and therefore their level in human blood high. Normally approx. 3/4 total cholesterol blood plasma is in LDL.
One of the functions of LDL found in the supply of cholesterol to various extrahepatic parenchymal cells, such as adrenal cortex cells, lymphocytes, muscle cells, and kidney cells. They all bear on their surface LDL receptors. LDL bound to these receptors are taken up by receptor-mediated endocytosis and inside cells destroyed by lysosomes.
Cholesterol esters from LDL are hydrolyzed lysosomal cholesterylesterase (acid lipase), and free cholesterol is used for membrane synthesis and as precursor of steroid hormones. Like extrahepatic tissues, the liver has an abundance of LDL receptors; It uses LDL cholesterol to bile acid synthesis And to form free cholesterol secreted into bile.
A person daily receptor-mediated pathway removed from plasma 70-80% LDL. The rest is destroyed by the cellular system "cleaners" - phagocytic RES cells. In contrast to the receptor-mediated pathway for the destruction of LDL, the pathway for their destruction in "cleansing" cells serves for the destruction of LDL with an increase in their level in plasma rather than to supply cells with cholesterol.
Since the membranes of parenchymal cells and "cleaner" cells are subject to circulation, and since cells die and are renewed, unesterified cholesterol enters the plasma, where it usually binds high density lipoproteins (HDL). This unesterified cholesterol then forms esters with fatty acids under the action of an enzyme present in the plasma - lecithincholesterol acyltransferase (LHAT).
Cholesterol esters formed on the surface of HDL are transferred to VLDL and eventually included in LDL. Thus, a cycle is formed in which LDL deliver cholesterol to extrahepatic cells and again receive it from them through HDL. A significant portion of the cholesterol released by the extrahepatic tissues is transported to the liver, where it is excreted into the bile.
VLDL and LDL mainly transport cholesterol and its esters into organ cells And fabrics. These fractions are atherogenic. HDL is commonly referred to as antiatherogenic drugs who carry out cholesterol transport(excess cholesterol, cholesterol released as a result of the breakdown of cell membranes) to the liver for subsequent oxidation with the participation cytochrome P450 with education bile acids which are excreted from the body as coprosterols.
The breakdown of blood lipoproteins after endocytosis in lysosomes And microsomes: Under the influence lipoprotein lipase in the cells of the liver, kidneys, adrenal glands, intestines, adipose tissue, capillary endothelium. Products of LP hydrolysis are involved in cellular metabolism.
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