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GI tract Function: Obtain food

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1 GI tract Function: Obtain food
Digest food- pharynx, stomach, small intestine Absorb nutrients (monosaccharides,amino acids, fatty acids, monoglycerides)-small intestine, large intestine Protection- large immune defense organ

2 From Gray’s anatomy: “Treves states that, in one hundred cases, the average length of the small intestine in the adult male was 22 feet 6 inches, and in the adult female 23 feet 4 inches: but that it varies very much, the extremes in the male being 31 feet 10 inches, and 15 feet 6 inches. He states that in the adult the length of the bowel is independent of age, height, and weight. “



5 © 2000 by W. H. Freeman and Company.
Sodium deoxcholate- natural detergent © 2000 by W. H. Freeman and Company.

6 © 2000 by W. H. Freeman and Company.
Detergents cause lipids to form micelles © 2000 by W. H. Freeman and Company.


8 Organization of the GI tract
Mucosa Surface epithelium, columnar cells Loose areolar connective tissue Thin layer of smooth muscle, muscularis mucosae Specializations of mucosa Villi M (microfold) cells- antigen presentation

9 © 2001 by Garland Science


11 Figure 15—23. Electron micrograph of epithelium of the small intestine
Figure 15—23. Electron micrograph of epithelium of the small intestine. Abundant microvilli at the cell apex can be seen to form the brush border. At the left are 2 lymphocytes migrating in the epithelium. In the center is an enteroendocrine cell (E) with its basal secretory granules. x1850. cap

12 Antibody production in GI tract
IgA- produced by plasma cells in secretions including saliva, tears, milk, and intestinal secretions Secreted with “secretory component”, which protects the antibody from digestion




16 Figure 15—33. Some aspects of immunologic protection of the intestine
Figure 15—33. Some aspects of immunologic protection of the intestine. A: A condition that is more frequent in the upper tract, such as in the jejunum. There are many IgA-secreting plasma cells, scattered lymphocytes, and some macrophages. Note that the lymphocytes in the epithelial lining are located outside the epithelial cells, and below the tight junctions. B: A condition that is more frequent in the ileum, where aggregates of lymphocytes are located under M cells. The M cells transfer foreign material (microorganisms and macromolecules) to lymphocytes located deep in the cavities of the M cells. Lymphocytes spread the information received from this foreign material to other regions of the digestive tract, and probably to other organs, through blood and lymph.

17 Submucosa Moderately dense connective tissue
Plicae circulares, circular folds, valvulae conniventes; valves of Kerkring Outfoldings of submucosa Begin in duodenum, 5 cm after pylorus in human Submucosal plexus Innervates mucosa Innervates muscularis mucosa

18 Enteric nervous system
© 2001 by Sinauer Associates, Inc. Enteric nervous system


20 Muscularis externa taenia coli Inner circular smooth muscle
Outer longitudinal smooth muscle Exception: upper esophagus, anal canal-this is skeletal muscle Auerbach’s plexus Parasympathetic ganglion cells

21 Autonomic Reflex 101 Afferent sensory into nucleus of the solitary tract (IX, X) Motor outputs from IX (glossopharyngeal, gag reflex) X (DMV, to everywhere) XII (hypoglossal, to tongue, motor only) To intermediolateral cell column and SNS


23 Stomach 4 regions Cardiac Fundus Body Pylorus
Digestion and mixing- food enters as a bolus (ball), leaves 3-4 hr later as a semi-fluid called chyme Mechanical churning Enzymes (pepsin, renin, lipase) HCl Mucous Lined with folds called ‘rugae’

24 Gastric glands Surface covered with gastric pits
Receive contents of glands Surface epithelium and cells lining pit produce mucous


26 Cardiac glands 2-4 cm from opening of stomach
Wide lumen, often coiled, see glands in X-section Gastric pits extend ¼ to 1/3 of mucosa surface Mostly mucous cells, long coiled glands

27 Fundic glands Occupy largest area
Produce most of the acid and mucous of stomach Short pits, long simple branched tubular glands Many cell types Surface mucous Mucous neck cells Chief cells- produce pepsinogen Parietal cells- produce HCL, intrinsic factor

28 Pyloric glands Long pits, short glands Many enteroendocrine glands


30 Fundic gland

31 Pyloric gland Figure 15—19. Photomicrograph of a section of the pyloric region of the stomach. Note the deep gastric pits with short pyloric glands in the lamina propria. H&E stain. Low magnification. (Courtesy of MF Santos.)

32 Figure 15—17. Electron micrograph of a section of gastric gland in the fundus of the stomach. Note the lumen and the parietal cells, containing abundant mitochondria; chief cells, with extensive rough endoplasmic reticulum; and enteroendocrine cells (closed type), with basal secretory granules. x5300.

33 Figure 15—14. Electron micrograph of an active parietal cell
Figure 15—14. Electron micrograph of an active parietal cell. Note the microvilli (MV) protruding into the intracellular canaliculi and the abundant mitochondria (M). x10,200. (Courtesy of S Ito.)

34 Figure 15—15. Composite diagram of a parietal cell, showing the ultrastructural differences between a resting cell (left) and an active cell (right). Note that the tubulovesicles (TV) in the cytoplasm of the resting cell fuse to form microvilli (MV) that fill up the intracellular canaliculi (IC).


36 © 2000 by W. H. Freeman and Company.
100 mM HCl Ion transport in a parietal cell © 2000 by W. H. Freeman and Company.

37 Clinical correlations
Gastric atrophy- loss of mucosa, antibodies produced against parietal cells, can lead to anemia (pernicious) Peptic ulcer- too much acid production relative to mucous protection Normal stomach HCl- 18 mEq per night Ulcer – 300 mEq per night Gastric ulcer May result not from too much acid, but from loss of mucous lining- alcohol, aspirin- NSAIDS!

38 NSAIDS and ulcers NSAIDs inhibit the COX-1 enzyme, ultimately reducing prostaglandin synthesis which is required for mucous and bicarbonate production. Inhibition of stomach acid secretion.

39 Role of Bacteria- 2005 Nobel in Physiology or Medicine (announced October 3, 2005) Barry Marshall, Robin Warren Helicobacter pylori Old thinking- treat ulcers with antihistamines (Zantac, ranitidineH2 antagonists, etc) to reduce acid secretion 50% relapse rate/6 months, 95% relapse rate 2 yrs Antihistamines and antibiotics, 12% relapse for duodenal ulcer, 13% for gastric ulcer Now also use proton pump blockers (Nexium)

40 Incidence of H. pylori infection
Helicobacter pylori may be transmitted orally by means of fecal contaminated food or water IFHII: About Helicobacter pylori. International Foundation for Helicobacter and Intestinal Immunology

41 Table 4: Estimated Ulceration Prevalence Per 1,000,000 People Per Year  Source: Non-linear optimization by author, based on various studies subject to the constraints listed in Table 3  Scenario  Duodenal Ulcer  Gastric Ulcer  Both Types of Ulcers  No Ulcer  TOTALS (people)  Neither NSAID user nor H. pylori infection  651,288  NSAID user only  671  51,035  51,706  Both NSAID user and H. pylori infection  7,092  822  7,377  15,294  H. pylori infection only  2,862  3,484  67  275,300  281,712  9,953  4,977  70  985,000  1,000,000 

42 Small intestine General organization
Duodenum Jejunum Ileum Villi- finger like projections of the mucosa


44 Villi Single epithelial cell layer Lamina propria
Capillaries and central lacteal, a blind ended lymphatic capillary, drains into lymph nodes


46 Small intestine


48 Intestinal epithelium
Absorptive cells Enteroendocrine cells (15 different subtypes producing different hormones) Mucous (goblet) cells Paneth cell

49 Four cell types of the intestinal epithelium

50 Intestinal epithelial cells are short lived
Cells differentiate in crypts or glands from stem cells Cells go through 5-6 divisions before reaching terminal cell stage Cells (except Paneth cells) translocate to villus and then tip of villus After 2-5 days cells reach villus tip and apoptosis begins



53 Figure 15—22. Electron micrograph of an absorptive epithelial cell of the small intestine. Note the accumulation of mitochondria in its apex. The luminal surface is covered with microvilli (shown in transverse section in the inset). Actin filaments, sectioned transversely, constitute the principal structural feature in the core of the microvilli. x6300. (Courtesy of KR Porter.)

54 Microvilli 3000 per cell 0.5-0.1 um diameter
Extensive actin microfilament network Found only in absorptive cells of SI, thyroid, proximal convoluted tubule


56 Figure 15—24. Structure of a microvillus
Figure 15—24. Structure of a microvillus. A cytoskeleton of actin filaments, associated with other proteins, keeps the shape of the microvillus. The actin filaments are continuous with the microfilaments of the terminal web (see Chapter 4), which also contains intermediate filaments. Note that in this location actin filaments have a structural role and are not related to movement, as is usually the case when these microfilaments are present. To fulfill its supportive role, actin is associated with other proteins that link the microfilaments to one another to fibrin, and to the cell membrane and a specific protein–villin–in its tip.

57 © 2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

58 Figure 15—40. Electron micrograph of epithelial cells of the large intestine. Note the microvilli at the luminal surface, the well-developed Golgi complex, and dilated intercellular spaces filled by interdigitating membrane leaflets, a sign of active water transport. x3900.

59 Intestinal surface area

60 Villin 95-kd protein Found only in cells with microvilli (kidney, proximal convoluted tubules; small intestine) Stabilizes actin microfilaments in microvilli

61 Glycocalyx Protein rich coat that covers microvilli. Contains dipeptidases, enzymes that breakdown disaccharides, mucous.

62 Bush border

63 Carbohydrate chemistry
anphys-fig jpg


65 Fat absorption Emulsification- stomach
Enzymatic breakdown of triglycerides to monoglycerides and fatty acids Micelle formation requiring bile salts from liver, stored in gall bladder


67 Triglycerides are composed of a 3 carbon (glycerol) backbone and three fatty acids


69 Micelles are formed by bile salts, fatty acids, and monoglycerides
© 2000 by W. H. Freeman and Company.

70 Figure 15—25. Lipid absorption in the small intestine
Figure 15—25. Lipid absorption in the small intestine. Lipase promotes the hydrolysis of lipids to monoglycerides and fatty acids in the intestinal lumen. These compounds are stabilized in an emulsion by the action of bile acids. The products of hydrolysis cross the microvilli membranes passively and are collected in the cisternae of the smooth endoplasmic reticulum (SER), where they are resynthesized to triglycerides. These triglycerides are surrounded by a thin layer of proteins that form particles called chylomicrons (0.2—1 micrometers in diameter). Chylomicrons are transferred to the Golgi complex and then migrate to the lateral membrane, cross it by a process of membrane fusion (exocytosis), and flow into the extracellular space in the direction of the blood and lymphatic vessels. Most chylomicrons go to the lymph; a few go to the blood vessels. The long-chain lipids (>C12) go mainly to the lymphatic vessels. Fatty acids of fewer than 10—12 carbon atoms are not reesterified to triglycerides but leave the cell directly and enter the blood vessels. RER, rough endoplasmic reticulum. (Based on results of Friedman HI, Cardell RR Jr: Anat Rec 1977;188:77.)

71 Figure 15—26. Electron micrograph of intestinal epithelium in the lipid-absorption phase. Note the accumulation of lipid droplets in vesicles of the smooth endoplasmic reticulum. These vesicles fuse near the nucleus, forming larger lipid droplets that migrate laterally and cross the cell membrane to the extracellular space (arrows). x5000. (Courtesy of HI Friedman.)

72 © 2002 by W. H. Freeman and Company.

73 Properties of lipoproteins in blood circulation
Source: After M. S. Brown and J. L. Goldstein, The Pharmacological Basis of Therapeutics. 7th ed., A. G. Gilman, L. S. Goodman, T. W. Rall, and F. Murad, Eds. (Macmillan, 1985), p Biochemistry, Berg Lipoproteins Major core lipids Apoproteins Mechanism of lipid delivery Chylomicron Dietary triacylglycerols B-48, C, E Hydrolysis by lipoprotein lipase Very low density lipoprotein (VLDL) Endogenous triacylglycerols B-100, C, E Intermediate-density lipoprotein (IDL) Endogenous cholesterol esters B-100, E Receptor-mediated endocytosis by liver and conversion into LDL Low-density lipoprotein (LDL) B-100 Receptor-mediated endocytosis by liver and other tissues High-density lipoprotein (HDL) A Transfer of cholesterol esters to IDL and LDL

74 Intestinal crypts Paneth cells- lie at the base of the crypts
Produce antibacterial and antifungal peptides called cryptidins or a-defensins Paneth cell dysfunction may contribute to Crohn’s disease

75 HNP, human neutrophil defensin


77 Western blot for cryptidin
Paneth cells secrete crytidin in response to bacteria (LPS, lipopolysaccharide) Ayabe T, Satchell DP, Wilson CL, Parks WC, Selsted ME, Ouellette AJ. Secretion of microbicidal alpha-defensins by intestinal Paneth cells in response to bacteria. Nat Immunol (2):113-8

78 Paneth cells

79 Figure 15—28. Section treated immunohistochemically to demonstrate the presence of lysozyme in the Paneth’s cells of the small intestine (arrowheads) and the macrophages (M) of the connective tissue. Medium magnification.

80 Figure 15—29. Electron micrograph of a Paneth’s cell
Figure 15—29. Electron micrograph of a Paneth’s cell. Note the basal nucleus with prominent nucleolus, abundant rough endoplasmic reticulum, and large secretory granules with a protein core surrounded by a halo of polysaccharide-rich material. These granules contain lysozyme, a lytic enzyme involved in the regulation of intestinal bacteria. x3000.

81 Factors involved in barrier functions for gastrointestinal tract
Low pH of gastric juice Mucus Intestinal motility Tight junctions Regeneration of epithelial cells Antimicrobial peptides (e.g. defensins) Antimicrobial proteins (e.g. lysozyme and lactoferrin) Paneth cells Phagocytic cells Lymphocytes Antibodies Gut-associated lymphoid tissues M cells Normal flora Ayabe T, Ashida T, Kohgo Y, Kono T. The role of Paneth cells and their antimicrobial peptides in innate host defense. Trends Microbiol :394-8.


83 Enteroendocrine cells
Located in stomach and small intestine Produce several different types of peptide hormones

84 Figure 15—18. Electron micrograph of an enteroendocrine cell (open type) of the human duodenum. Note the microvilli in its apex. x6900. (Courtesy of AGE Pearse.)

85 Figure 15—23. Electron micrograph of epithelium of the small intestine
Figure 15—23. Electron micrograph of epithelium of the small intestine. Abundant microvilli at the cell apex can be seen to form the brush border. At the left are 2 lymphocytes migrating in the epithelium. In the center is an enteroendocrine cell (E) with its basal secretory granules. x1850.

86 Enteroendocrine hormones
Cell type stimulus Target/ action gastrin G- pylorus Vagus nerve Inc HCL prod. Inc contraction secretin S- Small Intestine Low pH Inc HCO3- from pancreas cholecystokinin I- Small Intestine Fatty acids Contraction of gall bladder somatostatin D- pylorus, duodenum High gastrin Reduce gastrin secretion Reduce HCL secretion


88 Goblet cells Secrete a mixture of glycoproteins and proteoglycans
Also found in epithelial lining of trachea © 2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

89 Apomucin, mucous associated polypeptide- 8 genes encoding apomucins in humans
O-glycan chains are attached throughout the polypeptide called apomucin and encoded by the MUC-2 gene. The extensive glycosylation allows mucin to hydrate and gelatinize upon secretion.

90 Figure 15—21. Photomicrograph of the epithelium covering the small intestine. A: Columnar epithelial cells with the brush border (arrowhead) interspersed with mucus-secreting goblet cells. The PAS-hematoxylin staining gives a positive reaction for the glycoproteins present in mucus and the brush border. Medium magnification. B: Numerous absorptive cells with their brush borders and the clearly visible intercellular limits. PT stain. High magnification.

91 Figure 15—39. Section of a large intestinal gland showing its absorptive and mucous goblet cells. Observe that the goblet cells are secreting and beginning to fill the lumen of the gland with its secretions. The microvilli in the absorptive cells participate in the process of water absorption. PT stain. High magnification.

92 Other specializations
Brunner’s glands Only in duodenum, secrete alkaline mucous Only found in eutherian mammals Microfold (M) cells Microfolds rather than microvilli No glycocalyx Form epithelium of lymphatic nodules Lymphoid nodules Peyer’s patches- each patch, nodules Bursa of Fabricius- in birds, site of B lymphocytes

93 Large intestine No villi No digestive enzymes secreted
Lots of lymphoid tissue in submucosa Primary function is water reabsorption

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