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    Related terms:

    • Bile
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    • Secretin
    • Pylorus
    • Gastrin
    • Gastric Acid
    • Brush Border
    • Liver
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    Learn more about Chyme

    Improving in vitro simulation of the stomach and intestines

    Venema K. , … Minekus M. , in Designing Functional Foods , 2009

    Composition of the chyme

    The composition of the chyme is influenced by the transit of the meal, secretion of digestive fluids, and absorption of nutrients and water. None of the models described above simulates these combined aspects and obtains a physiological chyme composition for the different digestive stages in time.

    Enzyme, bile and electrolyte concentrations are not physiological (Boisen and Eggum, 1991). Especially the single enzyme methods are limited in their use since enzymes usually work together to digest a meal (Savoie, 1994). Pancreatin or intestinal fluid is relatively cheap and contains a mixture of relevant enzymes. A disadvantage of these preparations is that their composition is not well defined, with a batch-to-batch variation of enzyme activities. Also, pancreatin contains a considerable amount of nonenzyme material. Generally, the gastric pH profile after ingestion of the meal is not simulated, which may result in an unrealistic exposure to peptic and acidic conditions.

    None of the colonic models described so far includes removal of metabolites and water (Rumney and Rowland, 1992). This results in lower microbial density than found in the colonic content in vivo.

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    Chemical Regulation of Feeding, Digestion and Metabolism

    David O. Norris Ph.D. , James A. Carr Ph.D. , in Vertebrate Endocrinology (Fifth Edition) , 2013

    1 Secretin

    The presence of acidic chyme (pH less than 4) from the stomach directly stimulates the S cell in the duodenal mucosa to release the peptide secretin into the blood. Although H+ are the primary stimulus, secretin release also is stimulated by bile salts, fatty acids, sodium oleate, and several herbal extracts. Secretin stimulates the pancreas to secrete basic juice (rich in HCO3) and helps to neutralize the acidity of the chyme that has entered the small intestine. Although secretin levels in the blood do not increase following ingestion of a meal, the action of secretin on the exocrine pancreas is potentiated by another intestinal peptide hormone, CCK, that also increases in the blood following ingestion of a meal (see ahead).

    Originally it was believed that secretin also was responsible for stimulating secretion of the digestive enzymes normally present in the pancreatic juice, including the proteases chymotrypsin and trypsin, pancreatic lipase, pancreatic amylase, and nucleases for DNA and RNA. After 40 years of controversy following the demonstration of secretin, it was confirmed finally by Harper and Raper (1943) that purified secretin stimulates secretion of pancreatic fluid that is rich in sodium bicarbonate but poor in digestive enzymes. Secretion of the digestive enzymes was attributable to a second duodenal peptide that was found to contaminate some secretin preparations. Because zymogen granules represent vesicles of stored enzyme within the acinar (exocrine) pancreatic cell, the peptide that caused extrusion of zymogen granules from pancreatic acinar cells initially was called pancreozymin (pancreas–zymogen). It was postulated that the release of pancreozymin into the blood in response to the presence of peptides and amino acids in the chyme is due to direct actions of these molecules on pancreozymin-producing cells, similar to the action of H+ on the S cell. Sometimes the term secretagogue is applied to substances present in food, substances secreted from the mucosa into the gut lumen, or products of digestion that induce gastric or intestinal secretions. Pancreozymin later turned out to be another intestinal peptide hormone previously named for a different function (see ahead).

    Secretin consists of 27 amino acids and chemically is related to several other peptides of the PACAP (pituitary adenylate cyclase-activating polypeptide) family (see Chapter 4), several of which are involved in digestion. Secretin has been isolated from several mammalian species and the primary sequence appears to be conserved (Figure 12-14), although mammalian secretins differ markedly from avian secretin. Receptors for secretin are G-protein linked, and secretin apparently operates through production of a cAMP second messenger to stimulate pancreatic HCO3 secretion (see Chapter 3).

    FIGURE 12-14. Comparison of secretins from mammals and the chicken.

    Mammalian secretins are very conservative whereas more than half of the amino acids are different in the bird. See Appendix C for an explanation of the letters coding for individual amino acids.

    (Adapted with permission from Leiter, A.B. et al., in “Gut Peptides” (J.H. Walsh and G.J. Dockray, Eds.), Raven, New York, 1994, pp. 147–173.)

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    Integrative Functions of the Enteric Nervous System

    Jackie D. Wood , in Physiology of the Gastrointestinal Tract (Fifth Edition) , 2012

    22.2.7 Gastric Emptying

    The orderly delivery of gastric chyme to the duodenum at a rate that does not overload the digestive and absorptive functions of the small intestine is another function that requires neural integration of gastric motility. Integrative neural control compensates for minute-to-minute variations in the volume, composition, and physical state of the duodenal contents by adjusting the rate of delivery into the duodenum. This is necessary because the intraluminal milieu of the small intestine is different from that of the stomach and undiluted gastric contents have a composition that is poorly tolerated by the duodenum. Neural control of gastric emptying automatically adjusts the delivery of gastric chyme to an optimal rate for the small intestine and guards against overloading the small intestinal mechanisms for the neutralization of acid, dilution to iso-osmolality, and enzymatic digestion of the foodstuff.

    Some of the moment-to-moment neural control of the rate of gastric emptying involves feedback regulation of the gastric reservoir. An example is the powerful actions of lipids in the duodenum to slow gastric emptying. In this case CCK released from enteroendocrine cells in the intestinal mucosa act to stimulate CCK receptors on vagal gastric afferents, thereby initiating vagovagal reflex relaxation of the reservoir.

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    Carbohydrate Digestion

    Larry R. Engelking , in Textbook of Veterinary Physiological Chemistry (Third Edition) , 2015

    Luminal Phase (Pancreatic α-Amylase)

    The entry of partially digested acidic chyme into the duodenum stimulates specialized mucosal cells to release two important polypeptide hormones into blood; secretin (from duodenal S cells), and cholecystokinin (CCK, from duodenal I cells). These hormones then stimulate exocrine pancreatic secretions into the duodenal lumen containing NaHCO3 (needed to neutralize acidic chyme), and digestive enzymes (including α-amylase). Both salivary and pancreatic α-amylase (which are similar enzymes), continue internal starch, glycogen, and dextrin digestion in a favorable neutral duodenal pH environment (i.e., pH 7). Polysaccharides are digested to a mixture of dextrins and isomaltose (which contain all of the α-1,6 branch-point linkages), as well as maltose and maltotriose (Fig. 38-1). Most salivary and pancreatic α-amylase is destroyed by trypsin activity in lower portions of the intestinal tract, although some amylase activity may be present in feces.

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    Gastrointestinal Toxicology

    J.-M. Sauer , in Comprehensive Toxicology , 2010 Colonic motility

    Whereas small bowel mechanical activities propel and mix the chyme, the major colonic mechanical function is storage of chyme, usually for several days with the chyme held mainly in the cecum (Johnson 1994, 1997). The benefit of this prolonged storage is absorption of salt and water, amounting to 5–10% of that which occurred in the small bowel during several hours. As chyme is propelled to the ileocecal junction by peristaltic waves, the sphincter relaxes transiently, thereby allowing the chyme to enter the cecum. Then the sphincter closes abruptly and tightly to prevent retropulsion of cecal contents back into the ileum.

    An important structural feature of the colon contributes to its storage function. The walls of the cecal, ascending, transverse, and descending portions of the colon lack a continuous layer of longitudinal smooth muscle. Therefore, peristalsis cannot take place in the proximal 85% of the colon. Since there is a continuous layer of circular muscle, rhythmic segmentation is unaffected and is the prevalent mechanical event in the colon. Because rhythmic segmentation prompts retropulsion and does not cause aboral propulsion of the chyme from the cecum, the colonic storage function depends upon this mechanical activity.

    Nevertheless, sooner or later, absorption of salt and water from cecal chyme is complete and converts the liquid into semisolid feces. This waste material has to be propelled to the rectosigmoid portion of the colon in advance of defecation. The motor event responsible for this transit is termed ‘mass movement.’ Mass movement occurs once or twice daily and usually following a meal. The most distal portion of the cecal fecal mass is sequestered, is squeezed up the ascending colon, and then passes along the hepatic flexure, the transverse colon, the splenic flexure, and the descending colon. This colonic fecal transit covers 75  cm of large bowel in about 15   min.

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    The multifunctional gut of fish

    Anne Marie Bakke , … Åshild Krogdahl , in Fish Physiology , 2010

    3.3 Intestine and Accessory Organs

    The food entering the intestine, subsequently known as chyme, is mixed with secretions from the intestine as well as the accessory digestive organs—the pancreas and liver/gall bladder—the latter via ducts, the pancreatic and bile ducts, respectively. The secretions include components involved in digestion (see Sections 3.3.1 and 3.3.2) as well as electrolytes, most notably bicarbonate, which neutralizes the acidic pH of the chyme coming from the stomach so that the digestive enzymes in the intestine can perform at a pH close to their optimums. The pH in the intestinal content of fishes appears to be more alkaline than in mammals, ranging from 7 to 9 (Deguara et al. 2003; Fard et al. 2007), suggesting bicarbonate secretion may be higher in fishes than in mammals. One reason may be that bicarbonate is also important in osmoregulation by mediating ion and water absorption in the intestine, at least in marine species (see Chapters 4 and 5Chapter 4Chapter 5). The relative contribution of bicarbonate secretion from the pancreas, pancreatic ducts, bile, and/or intestine is not known (Wilson and Grosell 2003; Grosell and Genz 2006). In marine fishes, bicarbonate from the intestinal epithelial cells is secreted by an apically located Cl/ HCO3 exchanger (Grosell et al. 2001), rather than the basolaterally located exchanger in mammals. The alkaline pH may increase along the intestinal tract with a slight drop towards the anus, possibly due to an increase in content of short-chain fatty acids produced by microbial fermentation as shown for both young and adult Senegal sole (Solea senegalensis; Yufera and Darias 2007). Table 2.2 shows pH along the intestinal tract of Atlantic cod (Gadus morhua; Albrektsen et al. 2009).

    Table 2.2. pH of gut contents along the digestive tract of Atlantic cod (Gadus morhua)*

    Standard deviation0.920.350.
    The results are averages of measurements on 160 cod of about 300   g (Albrektsen et al. 2009). PR: Pyloric region; MI1: first 1/3 of the mid intestine; MI2: mid 1/3 of the mid intestine; MI3: distal 1/3 of the mid intestine; DI: distal intestine.

    3.3.1 Pancreas

    Although not well characterized functionally, the exocrine pancreas of various fish species, whether discrete or diffuse in its anatomical structure and location, contains acinar cells with zymogen granules which produce and store digestive enzymes (Kurokawa and Suzuki 1995; for review see Krogdahl and Sundby 1999). The zymogen granules and/or intestinal content of at least some fish species have been shown to contain the pancreatic enzymes or enzymatic activities corresponding to lipase, co-lipase, phospholipase, α-amylase, the proteolytic enzymes trypsin, chymotrypsin, elastase, carboxypeptidases A and B, as well as DNAase and RNAase (Kurokawa and Suzuki 1995; Pivnenko et al. 1997; Krogdahl and Sundby 1999; Kurtovic et al. 2009). Some important characteristics of some enzymes studied so far are given in Table 2.1.

    From the pancreatic tissue, these enzymes are released into ductules which converge into pancreatic ducts, finely structured and numerous in fish with diffusely located pancreatic tissue. The ducts release the enzymes into the lumen of the pyloric ceca and/or proximal intestine, or into the bile duct(s) (Kurokawa and Suzuki 1995; Krogdahl and Sundby 1999; Morrison et al. 2004). The proteolytic enzymes and co-lipase are secreted as pro-enzymes that are activated in the intestinal lumen, whereas the lipases and α-amylase are released in active forms. The cascade of events that lead to activation of the pro-enzymes is initiated by enterokinase, which is secreted from intestinal cells. Enterokinase activates trypsinogen to form trypsin (Ogiwara and Takahashi 2007), which in turn activates the other pro-proteases (Fig. 2.1; see review by Krogdahl and Sundby 1999).

    Fig. 2.1. Illustration of the activation of proenzymes secreted from the pancreas. Enterokinase from the intestinal mucosa activates trypsinogen to trypsin, which in turn activates the other proenzymes. Trypsin also shows autoactivation. Design: F. Venold.

    There is some uncertainty as to which lipolytic enzymes predominate in fish. The various pancreatic lipases found in higher vertebrates may be more limited in number and/or their functions modified in fish. Colipase activity has been found in rainbow trout (Oncorhynchus mykiss; Leger et al. 1979) and the elasmobranch dogfish (Squalus acanthius; Sternby et al. 1984), whereas in other fish species such as Atlantic cod it has not been isolated despite efforts to do so (Gjellesvik et al. 1989, 1992). Several studies have attempted to verify the existence of an sn-1,3-specific pancreatic lipase, but this has been elusive (Olsen and Ringø 1997; Tocher 2003; Gottsche et al. 2005). Lipases purified or identified from red sea bream (Pagrus major) hepatopancreas and winter flounder (Pseudopleuronectes americanus) pancreatic tissue exhibit a bile-salt dependency (Iijima et al. 1998; Murray et al. 2003), indicating that bile salt-dependent carboxyl ester lipase (CEL) is secreted from pancreatic tissue of at least some species. In their review, Kurtovic et al. (2009) conclude that fish digestive lipases are either of the co-lipase-dependent pancreatic lipase (PL) or CEL type, and that PL may be present mainly in freshwater fish and CEL in marine species. However, some research indicates that in some fish species the wall of the digestive tract from the foregut to the distal-most regions may also be a source of lipases (Tocher 2003), since highest lipase activity was observed in the proximal intestinal region of most fish and in the distal region of others.

    The amounts and activities of various secreted pancreatic enzymes appear to differ with species and/or their natural dietary preferences, although the diffusely located pancreatic tissue of many species makes such studies challenging. However, nutrient delivery into the intestinal lumen is the most important stimulus of exocrine pancreatic secretion in fish as in mammals (see review by Krogdahl and Sundby 1999). The nutrient composition and digestibility of the diet also differentially influences secretion of specific enzymes and other factors. For example, a diet containing a high level of protein, protein with low digestibility, and/or components that inhibit proteases, i.e. the antinutritional factors known as trypsin inhibitors in plant feedstuffs, has been shown to stimulate the pancreas to deliver a secretion with higher levels of trypsin in Atlantic salmon (Salmo salar L.; Olli et al. 1994; Krogdahl et al. 1999, 2003) and European sea bass (Dicentrarchus labrax; Péres et al. 1996, 1998). The relationship between dietary nutrient levels and corresponding enzyme secretions regarding lipids and carbohydrates appear to be more complicated (see reviews by Krogdahl and Sundby 1999; Morais et al. 2007; Zambonino Infante and Cahu 2007; Bogevik et al. 2009). Piscivorous species, such as Atlantic salmon, rainbow trout and sea bream (Pagrus pagrus) appear to have the capacity to secrete only low levels of amylase. In Atlantic salmon, amylase secretion is apparently not significantly up-regulated by carbohydrate content in the diet (Krogdahl et al. 2005; Frøystad et al. 2006). This is similar to the situation in some strictly carnivorous mammals.

    3.3.2 Liver

    Bile acids, which are acidic steroids with powerful detergent properties, are produced in the hepatocytes and secreted from the liver/gall bladder via the bile duct(s). They aid digestion by emulsifying dietary lipids and fat-soluble vitamins and thereby allow for efficient action of lipases (see Section 3.3.1) and formation of micelles (see Section 4.3). The primary bile acids are cholic and chenodeoxycholic acids, which are formed from cholesterol. A large proportion of these are conjugated to taurine to form taurocholic acid, taurolithocholic acid and taurochenodeoxycholic acid, and some to glycine to form glycocholate in the few fish species studied to date (Haslewood 1967; Une et al. 1991; Velez et al. 2009). These primary bile acids may be further modified in the intestinal lumen by bacterial enzymes to form secondary bile acids, although the extent and significance of this in fish is largely unknown. Numerous studies have demonstrated that bile is also a medium for the excretion of many metabolites of endogenous and exogenous substances from the blood and liver in fishes.

    The gall bladder is emptied by signals indicating that chyme is entering the intestine. Cholecytokinin (CCK) secreted via neural signals from endocrine cells lining the intestine may play a part in signaling both gall bladder and pancreatic secretion (see Chapters 7 and 8Chapter 7Chapter 8; Holmgren and Olsson 2009). During the course of fasting, the gall bladder becomes fuller and the bile more concentrated and darker in color. After a meal, it is usually more or less empty and light in color.

    In many fishes, activities and amounts of the pancreatic enzymes and bile acids in the chyme decrease as the intestinal content moves distally toward the anus (see review by Krogdahl and Sundby 1999), presumably due to their recycling as in higher vertebrates. However, the extent of enterohepatic and enteropancreatic recycling of intact bile acids and pancreatic enzymes in fish is not known. At least some may be broken down into constituent parts, which are subsequently absorbed by the intestine.

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    Alimentary Tract

    James S. Lowe BMedSci, BMBS, DM, FRCPath , Peter G. Anderson DVM, PhD , in Stevens & Lowe's Human Histology (Fourth Edition) , 2015


    When the pyloric sphincter opens, partially digested food (chyme) empties from the stomach into the small intestine, which is the main site for the absorption of amino acids, sugars, fats, and some larger molecules produced by digestion. The small intestine also secretes enzymes to complete the digestive processes begun in the stomach. It begins at the pylorus, the distal limit of the stomach, and ends at the ileocaecal valve, the proximal limit of the large intestine. At autopsy, when the longitudinal muscle is relaxed, the small intestine usually measures about 6 m, but in life it measures only about 3 m long. It is divided into three sections (duodenum, jejunum and ileum), although the transitions from one to the other are not precise.

    The duodenum is the proximal 20–25 cm of small intestine and is entirely retroperitoneal. It has the shape of a letter C, with the head of the pancreas fitting into its concave edge. The bile and pancreatic ducts open into the duodenum in this concavity.

    The jejunum begins where the duodenum emerges from behind the peritoneum and extends to an ill-defined junction with the ileum.

    The ileum extends from the jejunum to the ileocaecal valve.

    The small bowel has several modifications to increase its surface area

    As the major absorptive site, the small intestine shows architectural modifications to its mucosa and submucosa to increase its surface area.

    The mucosa and submucosa are thrown up into a large number of folds or plicae arranged circularly around the lumen. These are most prominent in the jejunum (Fig. 11.36a) and are absent from the distal end of the small intestine.

    FIGURE 11.36. Small intestine – general architecture.

    (a) Macroscopic view of small intestinal mucosal surface showing tightly packed circumferential mucosal folds or plicae. (b) Low-power micrograph of the plicae (P) showing their complex mucosal surface, which is composed of large villi (V). (c) Micrograph showing the villi (V) protruding into the small intestinal lumen; note the crypts (C) between their bases.

    The surface of the plicae is further arranged into villi, which protrude into the intestinal lumen (Figs 11.36b,c; 11.37). Tubular glands or crypts extend down from the base of the villi to the muscularis mucosae. The small intestine has the standard arrangement of musculature (i.e. an external layer of longitudinal muscle and an inner circular layer) and a substantial submucosa in which GALT (see p. 203) is particularly prominent.

    FIGURE 11.37. In two dimensions, the villi appear to have the same structure, but in three dimensions their structure can be seen to correspond to one of three main patterns: they are either finger-like (F), leaf-like (L) or ridge-like (R), as shown in this scanning electron micrograph. There are also occasional intermediate forms. The proportions of these patterns vary from site to site and also with age. In babies and young children, the villi of the duodenum and proximal jejunum are almost entirely leaf or ridge-like, finger-like villi appearing with age. The adult pattern is present by 10–15 years.

     Clinical Example

    Gluten Enteropathy (Coeliac Disease)

    The absorptive function of the jejunum depends on the integrity of the villi. If enough villi are damaged, food material cannot be absorbed, leading to weight loss, diarrhoea, etc.

    One important cause of extensive loss of villi is coeliac disease, due to allergy to the wheat protein, gluten. The resulting immune-mediated inflammation leads to flattening of the jejunal surface with extensive loss of villi (Fig. 11.38).

    FIGURE 11.38. Gluten enteropathy (coeliac disease).

    Micrograph of villous damage resulting from gluten sensitivity. The jejunal mucosal surface is flattened owing to extensive loss of villi. Compare with Figure 11.36c.

    Gluten enteropathy commonly presents in babies and young children, with failure to thrive and to gain normal height and weight for their age.

    The villi usually assume their normal structure when wheat and its products are excluded from the diet (i.e. a gluten-free diet), and the malabsorptive state subsequently improves.

    There are three functional zones of small intestinal epithelium

    The small intestinal epithelium can be divided into three functional zones. These are the villi, the crypts and the neck zone, where villi and crypts merge. (The features of the villi are discussed with Figure 11.39, and crypt structure is illustrated in Figures 11.40 and 11.41.)

    FIGURE 11.39. Small intestine villus.

    (a) Thin acrylic resin stained with H&E, showing a single villus, which is covered by tall enterocytes (E) bearing a prominent microvillus brush border (MV). Scattered among the enterocytes are occasional mucous cells (M) and intraepithelial lymphocytes (L). The stromal core contains small capillaries and lymphatics (not shown) and a number of lymphocytes, plasma cells and macrophages. (b) Electron micrograph of a row of enterocytes. Note the microvillus border (MV), part of a mucous cell (M) and the endocrine cell with basal granules (E). (c) Electron micrograph of microvillus brush border (MV) at high magnification. The glycocalyx (G) can be seen as a faint greyish haze on the surface of the microvilli. (d) Scanning electron micrograph of part of the villus surface. Note the tightly packed enterocytes, the microvilli of which are partly obscured by the layer of glycocalyx. Mucous cells discharging their mucus (M) are clearly seen. (e) Histochemical preparation of small intestine showing the distribution of the enzyme lactase (blue staining), which is localized to the luminal surface of the enterocytes. Like many other cell-bound enzymes responsible for food breakdown in the small intestine, the enzyme molecules reside in the glycocalyx.

    FIGURE 11.40. Paneth cells.

    Micrograph of the base of a small intestinal crypt from a paraffin section showing numerous Paneth cells (P) containing large numbers of bright red granules. A small endocrine cell (E) with ill-defined fine basal eosinophilic granules can also be seen.

    FIGURE 11.41. Endocrine cell.

    Micrograph of the base of a small intestinal crypt showing a typical pale-staining enteroendocrine cell (E). In this thin acrylic resin section, the Paneth cell (P) granules are difficult to see.

    The cells of the epithelium are enterocytes, mucous cells, Paneth cells, endocrine cells and stem cells, and their numbers and distribution vary in the different zones of the epithelium.

    Enterocytes are the main cell in the villi and are absorptive in function

    Enterocytes are tall columnar cells with round or oval nuclei in the lower third of the cell.

    The luminal surface of enterocytes is highly specialized; each cell bears 2000–3000 tightly packed, tall microvilli, which are coated by a glycoprotein, the glycocalyx (see Fig. 11.39c). This is composed of fine filamentous extensions of the microvillus cell membrane.

    The glycocalyx contains a number of enzymes (brush border enzymes, e.g. lactase, sucrase, peptidases, lipases and alkaline phosphatase), which are important in digestion and transport (see Fig. 11.39e).

    Beneath the microvillus surface, the enterocyte cytoplasm contains lysosomes and smooth endoplasmic reticulum and paired centrioles in the terminal web region (see Fig. 3.15). Nearer the nucleus, the cell is rich in rough endoplasmic reticulum and mitochondria and there is a prominent Golgi. Between the nucleus and the basement membrane are mitochondria and many ribosomes and polyribosomes. The lateral walls of enterocytes show complex interdigitations, and are the sites of Na+ and K+ ATPase activity. The lateral walls are separated from the microvillus surface by desmosomes and tight junctions (see Fig. 3.7).

    The ultrastructural features of the enterocyte are linked to its absorptive function, and therefore many of the features and mechanisms are common to other active absorptive cells, such as those of the proximal convoluted tubule cell of the kidney. These absorptive mechanisms are illustrated and discussed in Chapter 15.

    Mucous (goblet) cells are mainly found in the upper two-thirds of the crypts

    Occasional mucous cells are scattered among the enterocytes of the villi (see Figs 11.39a,b).

    Mucous cells contain globules of mucin in their luminal cytoplasm, the mucin being discharged on to the surface when the cytoplasm is fully expanded by mucin granules. The scanty basal cytoplasm is rich in rough endoplasmic reticulum.

    Mucous cells are least frequent in the duodenum, and increase in number in the jejunum and ileum, being most numerous in the terminal ileum close to the caecum.

    Paneth cells are found in the lower third of the crypts

    Paneth cells have basal nuclei and prominent large eosinophilic granules in their luminal cytoplasm (Fig. 11.40). Ultrastructurally, these granules are spherical and electron dense, the remaining cytoplasm being rich in rough endoplasmic reticulum; these are features of a protein-secreting cell (see Fig. 3.20).

    Paneth cells contain substances called ‘defensins’, which are secreted and protect against infection.

    Endocrine cells are located mainly in the lower third of the crypts, but are also seen higher up in the villi

    Endocrine cells in the small bowel resemble those seen in the stomach (see Fig. 11.32), being roughly triangular in shape, the broad base being in contact with the basement membrane, the narrow apex reaching the lumen. Their nuclei are spherical and their cytoplasm pale staining (Fig. 11.41).

    Ultrastructurally, the cytoplasm contains neuroendocrine granules, and the luminal surface bears microvilli.

    Small intestinal endocrine cells secrete a number of hormones and peptides, including serotonin (5HT), enteroglucagon, somatostatin, secretin, gastrin, motilin and vasoactive intestinal peptide (VIP).

    Stem cells are found in the lower third of the crypts

    The replication of stem cells replenishes the stock of the other cells, including the Paneth and endocrine cells. Most replication is to replace the mucous cells and enterocytes of the villi, as these cells have a rapid turnover, being shed from the tips of the villi about 5 days after production.

    Before developing into the mature form of the two cell types, the stem cells differentiate into intermediate cells, which show some features of both mucous cells and enterocytes. This process appears to involve the Wnt signalling pathway. These cells occupy much of the upper two-thirds of the crypts.

    Stem cells and intermediate cells are particularly numerous when there is increased cell loss from the villi, which is a common feature of many diseases affecting the small intestine; the crypts increase in length and show increased numbers of cells in mitosis (i.e. crypt or gland hyperplasia).

    The lamina propria of the small intestine is most clearly seen in the core of the villi, but also surrounds and supports the gland crypts

    The lamina propria of the small bowel is composed of collagen, reticulin fibres, fibroblasts and glycosaminoglycan matrix, through which run blood capillaries, lymphatics and nerves. It contains some smooth muscle fibres.

    The blood vessels and lymphatics are particularly prominent in the villi, a central lymphatic (lacteal) running vertically down the centre of the core of each villus.

    The lamina propria also contains lymphocytes, plasma cells, eosinophils, macrophages and mast cells.

    The lymphocytes are largely T lymphocytes (approximately 70% T-helper and 30% T-suppressor; see Chapter 8); most of the plasma cells produce IgA. Lymphocytes are also present in the villous epithelium, usually in a basal position between the lateral intercellular spaces and, like those in the lamina propria, they are also T lymphocytes, but the subset pattern is different, about 80% being T-suppressor, the rest being T-helper types.

    Eosinophils are common in the lamina propria throughout the digestive tract.

    Macrophages are found mainly beneath the basement membrane in the upper reaches of the villi. They are thought to engulf particulate antigens and to ingest soluble antigens before presenting them to T lymphocytes.

    Mast cells are seen mainly in the crypt region.

    The submucosa of the small bowel contains vessels, lymphoid tissue and nerves

    The small intestinal submucosa contains lymphatics, blood vessels and the submucosal plexus of nerves and ganglion cells. In addition, it contains part of the larger lymphoid aggregates of the GALT, which cross the muscularis mucosae.

    In the first part of the duodenum, the submucosa contains mucus-secreting Brunner's glands.

     Key Facts

    Small Bowel

    Characterized by a mucosa raised into finger-like villi which contain abundant blood vessels and lymphatics

    Lined by columnar epithelium with goblet cells

    Glycocalyx of surface epithelium has enzymes, e.g. lactase and alkaline phosphatase

    Duodenum is associated with submucosal mucous glands (Brunner's glands)

    Contains mucosal endocrine cells which secrete gut hormones

    Has two layers of muscularis propria separated by a myenteric nerve plexus.

    The small intestine has regional specializations in duodenum, jejunum and ileum

    The duodenum differs from the rest of the small intestine as follows:

    It is entirely retroperitoneal

    Its villous pattern contains a high proportion of leaf and ridge forms (see Fig. 11.37)

    It contains prominent mucus-secreting submucosal glands (Brunner's glands, Fig. 11.42), which penetrate and split the muscularis mucosae so that some acini are located within the lamina propria of the mucosa

    FIGURE 11.42. Brunner's glands.

    The first part of the duodenum is characterized by the presence of large, fluid-secreting mucous glands called ‘Brunner’s glands’ (B), which empty into the neck of the crypts (C). The Brunner's glands are partly located in the lower mucosa, but pass through the muscularis mucosae (MM) into the submucosa.

    It receives secretions of glands located outside the digestive tract through long ducts; these glands are the liver (see Chapter 12) and the exocrine component of the pancreas.

    Brunner's glands are similar to the submucosal glands of the pyloric region of the stomach, being composed of mucous cells lining short ducts that open into the bases or sides of the crypts of the mucosa. Brunner's glands secrete an alkaline mucoid material (pH 8.0–9.5), which may protect the duodenal mucosa from the acid chyme, bringing its pH towards the level at which the pancreatic enzymes are most effective.

    Brunner's glands are also thought to secrete urogastrone, a peptide that inhibits acid secretion by the stomach. Endocrine cells can be demonstrated in these glands immunocytochemically.

    The jejunum is the main absorptive site of the digestive tract and shows not only the greatest development of plical folds (see Fig. 11.36a), but also the most complex villous system, with finger-like villi being predominant.

    The ileum is characterized by the greatest development of GALT. The lymphoid cells aggregate into large nodules (Peyer's patches), which expand the lamina propria of the mucosa, split the muscularis mucosae and extend into the submucosa.

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    Neurophysiologic Mechanisms of Human Large Intestinal Motility

    Adil E. Bharucha , Simon J.H. Brookes , in Physiology of the Gastrointestinal Tract (Fifth Edition) , 2012

    36.6.2 The Colon as a Storage Organ

    Under basal conditions, the healthy colon receives approximately 1500  ml of chyme over 24 hours, absorbing all but 100  ml of fluid and 1mEq of sodium and chloride, which are lost in the feces.348 Colonic absorptive capacity can increase to 5–6   L and 800–1000   mEq of sodium and chloride daily when challenged by larger fluid loads entering the cecum, as long as there is a slow infusion rate (i.e., 1–2   ml/minute). Since the work of Cannon (1902), the proximal colon has been recognized to be the primary site responsible for storage, mixing, and absorption of water and electrolytes.90 While the rectosigmoid colon functions primarily as a conduit, it can also participate in this compensatory absorptive response.201 For 25 years, secretory and absorptive processes were believed to be segregated to crypt and surface epithelial cells, respectively. It is now recognized that absorptive mechanisms are constitutively expressed in crypt epithelial cells; secretion is regulated by one or more neurohumoral agonists released from lamina propria cells, including myofibroblasts.407 When the colon is perfused with a plasma-like solution, water, sodium, and chloride are absorbed, while potassium and bicarbonate are secreted into the colon.395 Absorption of sodium and secretion of bicarbonate in the colon are active processes occurring against an electrochemical gradient. There are several different active (transcellular) processes for absorbing sodium, and these show considerable segmental heterogeneity in the human colon. The regional differentiation of colonic mucosal absorption is also demonstrated by regional effects of glucocorticoids and mineralocorticoids on sodium and water fluxes. For example, in the distal colon, epithelial Na+, K+, ATPase is activated by mineralocorticoids.52 On the other hand, the Na+/H+ exchange is activated in proximal colonic epithelium by the mineralocorticoid, aldosterone.100 Specific channels are involved in water transport across surfaces and epithelia. These water channels, or aquaporins (AQP), are a diverse family of proteins, of which AQP8 is expressed preferentially in colonic epithelium and small intestinal villous tip cells.

    Potassium is absorbed and secreted by active processes; it is unclear if chloride is absorbed by an active process. In contrast to the small intestine, glucose and amino acids are not absorbed in the colon. Colonic conservation of sodium is vital to fluid and electrolyte balance, particularly during dehydration, when it is enhanced by aldosterone.51 Patients with ileostomies are susceptible to dehydration, particularly when placed on a low-sodium diet or during an intercurrent illness. In addition to glucocorticoids and mineralocorticoids (aldosterone), other factors enhancing active sodium transport include somatostatin, α2-adrenergic agents, and short chain fatty acids. Clonidine mimics the effects of adrenergic innervation by stimulating α2-receptors on colonocytes. In contrast, stimulation of mucosal muscarinic cholinergic receptors inhibits active NaCl absorption and stimulates active chloride secretion.

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    Gastrointestinal System

    Robert G. Carroll PhD , in Elsevier's Integrated Physiology , 2007


    When the antrum contracts, the duodenum relaxes, allowing a small volume of gastric chyme to enter the duodenum. The pylorus functions as a sphincter, but anatomically it does not qualify, since the muscles across the pylorus are not electrically connected by gap junctions. The rate of gastric emptying is matched to duodenal buffering ability, preventing acid damage to the duodenal mucosa and subsequent duodenal ulcers. The pylorus also prevents regurgitation of duodenal contents back into the stomach, preventing bile damage to the gastric mucosa, which can lead to gastric ulcers.

    Negative feedback control balances the rate of chyme entry into the duodenum to the capability of the duodenum to digest and absorb the diet. Gastric chyme is acidic and often hyperosmolar. Within the duodenum, both acidity and hypertonicity initiate reflexes that inhibit gastric motility and therefore the further entry of gastric chyme. This allows the duodenum to process the luminal contents before any new gastric chyme is allowed to enter.

    Duodenal acidity (pH < 3.5) decreases the rate of gastric emptying through multiple routes. Acidity initiates a neural reflex that reduces gastric emptying. Duodenal acidity also stimulates secretin release, which increases HCO3 buffer secretion from ductal epithelia of the pancreatic and hepatic ducts. HCO3 secretion acts to neutralize the chyme present in the duodenum.

    Duodenal hypertonicity also decreases the rate of gastric emptying. Chyme becomes more hypertonic as digestion progresses. Duodenal hypertonicity decreases gastric emptying by a neural reflex and an unidentified hormonal component. The duodenal tonicity decreases with time as the digested components of the diet are absorbed across the duodenum. In addition, contraction of the pylorus is increased by unabsorbed products of digestion in the duodenum. Monoglycerides increase contraction of the pylorus and slow gastric emptying. Amino acids, especially tryptophan, and peptides in the duodenum also slow gastric emptying.

    Fat content and fatty acids (especially long chain, unsaturated, or both) decrease the rate of gastric emptying. Fats stimulate the release of cholecystokinin (CCK) from the duodenum and jejunum, which contracts the pylorus and relaxes the stomach fundus. Gastric inhibitory peptide (GIP) may also have a role in limiting gastric emptying.

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    Hormones of the Gastrointestinal Tract

    H. Maurice Goodman , in Basic Medical Endocrinology (Fourth Edition) , 2009

    Other Effects of Secretin

    The principal outcome of stimulating bicarbonate production by pancreatic and bile ducts is neutralization of chyme in the duodenum. This effect of secretin is complemented by actions on the stomach to decrease the delivery of hydrogen ions to the duodenum (Figure 6.17). Secretin indirectly inhibits gastric acid production and gastrin secretion by stimulating D cells in both the oxyntic and antral mucosae to secrete somatostatin. Somatostatin inhibits acid secretion by its effects on G cells, ECL cells, and parietal cells as already described. Some evidence indicates that secretin and CCK may have synergistic effects on somatostatin secretion. In addition, by activating receptors on vagal afferent neurons, secretin and CCK signal a reduction in the tonic inhibition of D cells, slow gastric emptying, and cause smooth muscle in the proximal stomach to relax. Other effects on the stomach include stimulation of mucus, pepsinogen, and gastric lipase secretion. Along with CCK, secretin is a trophic hormone that stimulates growth of the exocrine pancreas. Secretin receptors are found in a variety of cells in the brain and other organs, but its physiological role outside of the digestive remains to be established.

    Figure 6.17. Schematic representation of the actions of secretin and feedback regulation of its secretion. Solid arrows indicate stimulation; dashed arrows indicate inhibition. LSRF=luminal secretin releasing factors.

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