Small intestine. Small intestine

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1 General features Tubular organ longest part; 5-6 m most of chemical digestion absorption of nutrients reabsorption of H2O occurs. Two structural features; maximize the lumenal surface area villi microvilli striated brush border m 2 1 Brush border 2

2 Mucosa; Plicae circulares Lumenal surface folds; 3-10 mm Circular folds Has mucosa and submucosa. Simple columnar on villi with brush border (microvilli) scattered goblet cells 3 Simple columnar Capillary network Villus Lacteal

3 Mucosa; many villi (singular: villus). Villi long and narrow; circular cross-section; densely packed; several times lumenal thickness of epithelium Interior of villus lamina propria (LP) + network of blood capillaries; lacteal, SMF 5 6

4 Mucosal glands abundant simple tubular glands extend deep into mucosa under the lumenal epithelium at the base of the villi. Called intestinal glands (crypts of Leiberkuhn). Simple glandular epithelium cells produce mucus for protection resemble goblet cells have large mucus vacuole; most of cell's volume. 7 Glands Tubular part is mostly restricted to the mucosa Glands surrounded by LP glands very abundant (typical) LP between; very thin; not very obvious. Sometimes see extensions of gland into submucosa 8

5 Dynamic lumenal epithelium epithelial cells lost from ends of villi replaced by movement of epithelium from glands to base of villi to villi tips relatively high rate of cell division in deep parts of glands Muscularis mucosae relatively thin: sometimes difficult to detect just below the bottom of the glands 9 Some cells at the base of the glands produce enzyme lysozyme, destruction of bacteria in the lumen Paneth cells (AKA principal cells) cytoplasmic granules or vacuoles that are very strongly eosinophilic granules contain lysozyme, which may serve as an antibiotic. 10

6 Submucosa; Dense C.T Extends into plicae small muscular arteries and veins, nerves, ganglia small lymphatic vessels; adipocytes; areas of loose C.T. less cellular than LP and with larger vessels 11 Muscularis externa (ME) Two distinct layers of SMT inner, circular thicker than the outer outer, longitudinal Inner often misidentified as t. adventitia. Outer 12

7 Myenteric plexus visible masses of neural tissue between circular and longitudinal layers of ME some neuron bodies present Inner, circular layer scattered dense CT and capillaries 98% of volume is SMFs 13 14

8 Tunica adventitia, TA TA varies in thickness dense CT + a mesothelium In thick areas small to medium muscular arteries, veins and lymphatic vessels Veins; tributaries of the hepatic portal vein small unmyelinated nerves clumps or masses of adipocytes + areas of loose CT 15 Three regions of the small intestine differ in both structure and function; 1- duodenum 25 cm long associated with liver and pancreas 2- jejunum 2.5 m 3- ileum 3.5 m g. 16

9 Regions of the Small Intestine: Relative rates of several different functions 17 Special features of regions ` 1- DUODENUM First region very acidic material from the stomach (AKA acidic chyme) arrives receives pancreatic juice and bile highest intensity of chemical digestion 18

10 Duodenal Very thick layer of mucus protect the mucosa from acidic chyme from stomach digestive enzymes, especially proteases, from pancreas glands; AKA Brunner s glands additional mass of mucus- producing tissue not found in the other two regions Brunner s glands; in the submucosa branched tubular extensions of the mucosal glands through the MM stain light-basophilic do not look like goblet cells; more cuboidal shape; no mucus vacuoles. 19 g. 20

11 Continuations of the gland epithelium through MM very scarce. Brunner's glands are densely packed almost all of the submucosa's volume. 21 M. externa Submucosa Mucosa 22

12 JEJUNUM g. Middle region very active more active in immune defense, in the wall, than is the duodenum, some lymphatic nodules (small masses of lymphoid tissue; spherical in shape) in the wall scarce and you are not likely to see any in a cross-section of this region.- 23 Paneth cells may be more abundant Generally jejunum lacks special features It is boring je june/jiˈjo on/ Text Adjective; Dry and uninteresting 24

13 25 ILEUM Last region rates of digestion and mucus production; low nutrient absorption; low immune-defense activities, H 2 O reabsorption; high. Critical; all of the H 2 O in the food and the salivary, gastric and pancreatic juice, and bile must be reabsorbed Otherwise you would experience chronic diarrhea. 26

14 penetrate Ileum's special histological feature immune-defense abundant lymphatic nodules; AKA Peyer s patches. Part of GALT Gut Associated Lymphoid Tissue nodules form in the submucosa extend up into the mucosa muscularis mucosae. No distinct outer, dense CT capsule of nodules outer edge blends into the surrounding CT 27 28

15 Regions of the Small Intestine: Relative rates of several different functions 29