Digestive System - Small and Large Intestine PDF

4/13/21 Small Intestine - ~7 m long; 3 regions: the duodenum (proximal), jejunum (middle), and ileum (distal) - Function: (i) several hormones secretion; (ii) it continues and largely completes the digestion of foodstuffs and absorbs the resulting metabolites 111 The Small Intestine 112 56 4/13/21 M...

4/13/21 Small Intestine - ~7 m long; 3 regions: the duodenum (proximal), jejunum (middle), and ileum (distal) - Function: (i) several hormones secretion; (ii) it continues and largely completes the digestion of foodstuffs and absorbs the resulting metabolites 111 The Small Intestine 112 56 4/13/21 Mucous Membrane – Structures 1. Circular or semilunar folds (plicae circulares): consisting of mucosa and submucosa (best developed in the jejunum) – to increase the absorptive area 115 Mucous Membrane – Structures 2. Villi: finger-like projections consisting of lamina propria connective tissue with microvasculature and lymphatics 116 58 4/13/21 Mucous Membrane – Structures 2. Villi: covered with a simple columnar epithelium composed of absorptive enterocytes and goblet cells 117 Mucous Membrane – Structures 3. Short tubular intestinal glands or crypts (intestinal crypts or crypts of Lieberkühn): between the villi the covering epithelium 118 59 4/13/21 Mucous Membrane – Structures 3. Short tubular intestinal glands or crypts (intestinal crypts or crypts of Lieberkühn): stem cells for the epithelium and Paneth cells (prevent intestinal flora from becoming concentrated in these glands where damage to the stem cells could occur) 119 Mucous Membrane – Structures mitotic figure Stem cells differentiation: cells continue to move up each villus and within a week are shed at the tip, with billions shed throughout the small intestine each day 120 60 4/13/21 Luminal Surface Modifications Plicae circulares, intestinal villi, and microvilli: collectively increase the luminal surface area by a factor of 400 to 600 1.Plicae circulares (valves of Kerckring): permanent spiral folds of the mucosa and submucosa. Present in the distal half of the duodenum, the entire jejunum, and the proximal half of the ileum. They increase the surface area twofold to threefold 121 Luminal Surface Modifications 2. Intestinal villi: permanent evaginations: connective tissue core (lamina propria) + numerous plasma cells and lymphocytes, fibroblasts, mast cells, smooth muscle cells, capillary loops, and a single lacteal (blindended lymphatic capillary). Villi increase the surface area 10-fold 3. Microvilli: actin filaments that interact with myosin filaments in the terminal web. Microvilli increase the surface area about 20fold 122 61 4/13/21 Goblet Cells ileum, H&E 125 Mucosa of the Small Intestine The Epithelium Simple columnar – composed of goblet cells, surface absorptive cells, and some DNES cells (both types, open and closed) 1. Goblet cells: - unicellular glands – produce mucinogen, which accumulates in membranebounded granules, distending the apical region (theca) of the cell 126 63 4/13/21 Mucosa of the Small Intestine The Epithelium 1. Goblet cells: - After being released: mucinogen becomes hydrated and is thus converted to mucin (a protective coating of the epithelial lining of the lumen; mucin, when mixed with luminal contents is known as mucus). - Nucleus and other organelles in the basal region (stem) of the cell. - Increase in number from the duodenum to the ileum 127 Surface Absorptive Cells Enterocytes: the absorptive cells 128 64 4/13/21 Mucosa of the Small Intestine The Epithelium 2. Surface absorptive cells: - tall columnar cells with numerous mitochondria, SER and RER, and Golgi C - A layer of closely packed microvilli (striated border) on their free apical surface - Glycocalyx: overlies the microvilli and binds various enzymes, including disaccharidases and dipeptidases - Well-developed tight junctions and zonula adherens 131 Small Intestine Enterocytes à absorption: Disaccharidases and peptidases à bound to the microvilli à hydrolyze the disaccharides and dipeptides into monosaccharides and amino acids à absorbed through active transport Action of pancreatic lipase and bile à digestion of fats à in humans, most of the lipid absorption takes place in the duodenum and upper jejunum 132 66 4/13/21 Digestion of Fats Enterocytes involved in lipid absorption accumulate many small lipid droplets in vesicles of the smooth ER These vesicles fuse near the nucleus, forming larger globules that are moved lacteals laterally and cross the cell membrane to the extracellular space (arrows) for eventual uptake by lymphatic capillaries (lacteals) in the lamina propria 133 lacteals 134 67 4/13/21 Mucosa of the Small Intestine The Epithelium 3. DNES cells: - Produce and secrete gastrin, cholecystokinin, gastric inhibitory peptide, and several other hormones - 3 genes for G protein-linked taste receptors: (similar to those of the taste buds located on the tongue) The taste cells of the gastrointestinal tract communicate with the islets of Langerhans (signaling the beta cells to release insulin once sweet taste is detected in the lumen) 137 DNES Cells Enteroendocrine cells: Cells with spherical nuclei and clear cytoplasm – the secretory granules are oriented basally E.g., “argentaffin” cells - these cells reacts with the silver causing a black precipitate to form à “S” or serotonin-secreting enteroendocrine cells 138 69 4/13/21 Lamina Propria through Serosa 139 Mucosa of the Small Intestine Lamina Propria The cores of the villi + the interstices between the numerous glands (crypts) of Lieberkühn - Loose connective tissue with lymphoid cells, fibroblasts, mast cells, smooth muscle cells, nerve endings, and lymphoid nodules - Lacteals (blind-ended lymphatic vessels) + capillary loops 140 70 4/13/21 Mucosa of the Small Intestine Lamina Propria 1. Crypts of Lieberkühn: simple tubular glands, from the intervillous spaces to the muscularis mucosae of the intestine - Composed of goblet and oligomucous cells, columnar cells (similar to surface absorptive cells), DNES cells, regenerative cells, Paneth cells, intermediate cells, and M cells 143 Paneth Cells Brightly eosinophilic (almost orange) secretory granules in the apical cytoplasm 144 72 4/13/21 Mucosa of the Small Intestine Lamina Propria 1a. Paneth cells: - Located at the base of the crypts of Lieberkühn - Pyramidal cells: secrete the antibacterial enzyme lysozyme stored in large, apical, membrane-bounded secretory granules - Also release other antibacterial agents, defensins and tumor necrosis factor 145 Mucosa of the Small Intestine Lamina Propria 1a. Paneth cells: - Extensive RER (basally), a large supranuclear Golgi complex, and many mitochondria. - These agents: the capability of killing bacteria as well as certain protozoa 146 73 4/13/21 Mucous Membrane – Structures 7. Paneth Cells: Basal nucleus (N) Abundant rough ER Large secretory granules surrounded by halos of polysaccharide-rich material Key components of the gut's innate immunity, secreting into the crypt lumens enzymes and peptides called defensins à prevent microorganisms from permanently lodging in the crypts and affecting stem cell and differentiative activities 147 Mucosa of the Small Intestine Lamina Propria 1b. Regenerative cells: - Located in the basal half of the crypts of Lieberkühn - Columnar stem cells: to replace themselves and the other types of epithelial cells 1c. Intermediate cells: - Make up the bulk of the epithelial lining of the crypts of Lieberkühn. Regenerative properties (they are derived from regenerative cells, but have not as yet committed themselves to a certain cell line) 148 74 4/13/21 Lamina Propria through Serosa 151 Mucosa of the Small Intestine Muscularis Mucosae - An inner circular and an outer longitudinal layer of smooth muscle 152 76 4/13/21 Muscularis Mucosae 153 Submucosa - Fibroelastic connective tissue + blood and lymphatic vessels, nerve fibers, and Meissner plexus - Brunner glands: only in the duodenum - an alkaline fluid (protects the duodenal epithelium from the acidic chyme) and urogastrone (a polypeptide hormone - human epidermal growth factor - that enhances epithelial cell division and inhibits gastric HCl production) 154 77 4/13/21 Muscularis Externa - Two layers of smooth muscle: an inner circular and an outer longitudinal layer - The inner layer: participates in the formation of the ileocecal sphincter - Auerbach (myenteric) plexus is housed between the two layers 155 Innervation of the Intestine 156 78 4/13/21 The Myenteric Plexus 159 External Layer - Serosa covers all of the jejunum and ileum and part of the duodenum - Adventitia covers the remainder of the duodenum 160 80 4/13/21 Cecum & Colon Mucosa No villi & no specialized folds 1. The epithelium: simple columnar with numerous goblet cells, surface absorptive cells, and occasional DNES cells 2. The lamina propria: similar to that of the small intestine: lymphoid nodules, blood and lymph vessels, and closely packed crypts of Lieberkiihn, which lack Paneth cells 165 Mucosa 166 83 4/13/21 Mucosa TEM micrograph of colonocytes: - Short microvilli at their apical ends - Prominent Golgi complexes above the nuclei - Dilated intercellular spaces with interdigitating leaflets of cell membrane (L) à a sign of active water transport The absorption of water is passive, following the active transport of sodium from the basolateral surfaces 169 Cecum & Colon Mucosa: Lamina Propria 170 85 4/13/21 Lamina Propria through Serosa 171 Cecum & Colon Mucosa 3. Muscularis mucosae: an inner circular and outer longitudinal layer of smooth muscle cells 172 86 4/13/21 Lamina Propria through Serosa 173 Cecum & Colon - - Submucosa Fibroelastic connective tissue + blood and lymphatic vessels, nerves, and Meissner (submucosal) plexus Muscularis Externa An inner circular and a modified outer longitudinal layer of smooth muscle The outer layer: three flat, longitudinal ribbons of smooth muscle - the teniae coli When continuously contracted: the teniae coli form sacculations of the wall: the haustra coli Auerbach (myenteric) plexus 174 87 4/13/21 Cecum & Colon The External layer - Adventitia: the ascending and descending portions of the colon - Serosa: the cecum and the remainder of the colon. - Fat-filled outpocketings of the serosa (appendices epiploicae) are characteristic of the transverse and sigmoid colon 175 Cecum & Colon The Lumen of the Large Intestine - Trillions of microbes: ~10 times the number of cells in the entire human body - These microorganisms: body's microbiome – physiological balance has shown to reflect the health of the individual harboring them - 3 groups of intestinal flora: enterotypes – each individual possesses a predominance of one of these enterotypes How these enterotypes develop and what their significance is are not as yet understood 176 88 4/13/21 Rectum Similar to the colon but contains fewer and deeper crypts of Lieberkühn 177 The Anal Canal - Constricted continuation of the rectum - Anal mucosa: longitudinal folds called anal columns (or rectal columns of Morgagni), which join each other to form anal valves. The regions between adjacent valves: anal sinuses - Epithelium: simple columnar changing to: - Simple cuboidal proximal to the anal valves Stratified squamous nonkeratinized distal to the anal valves Stratified squamous keratinized (epidermis) at the anus 178 89 4/13/21 The Anal Canal - The lamina propria: fibroelastic connective tissue + sebaceous glands, circumanal glands, hair follicles, and large veins - The muscularis mucosae: an inner circular and an outer longitudinal layer of smooth muscle, both of which terminate at the anal valves 179 Recto-Anal Junction Simple columnar epithelium of the intestine Keratinized stratified squamous epithelium of skin 180 90 4/13/21 The Anal Canal - Submucosa: dense, irregular fibroelastic connective tissue that houses large veins - Muscularis externa: an inner circular and an outer longitudinal layer of smooth muscle. The inner circular layer – the internal anal sphincter - Adventitia attaches the anus to surrounding structures 181 182 91