General Structure of Digestive Tract

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1 Dr. Nabil Khouri

2 General Structure of Digestive Tract Common Characteristics: Hollow tube composed of a lumen whose diameter varies. Surrounded by a wall made up of 4 principal layers: Mucosa Epithelial lining; A lamina propria of loose connective tissues rich in blood, lymph vessels and smooth muscle cells; Muscularis mucosae. Submucosa Dense connective tissues with many blood and lymph vessels. Muscularis Contains smooth muscle cells, divide into 2 layers; internal (circular); external (longitudinal) Serosa Thin layer of loose connective tissue rich in blood and lymph vessels and adipose and single squamous covering epithelium (mesothelium)

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4 Basic Mucosal Forms in GI. Protective mucosa Found in oral cavity, pharynx, esophagus, anal canal Type stratified sqaumous epithelium.. Secretory mucosa Found only in stomach Type glandular epithelium (packed tubular), Simple branched.. Absorptive mucosa Found in the Entire small intestine Arranged in a fingerlike projection manner villi Duodenum only sub-mucosal mucus Brunner s glands crypts - extend into the muscularis mucosa. Absorptive/protective mucosa Found in large intestine Type glandular epithelium (packed tubular), Strait, cells specialized for water and mucus absorption - Goblet cells

5 Esophagus

6 The Stomach Wall

7 Histology of the Stomach Mucosa Epithelial lining is composed of Simple columnar, cells with round or oval nucleus at the base with granules Goblet cells that produce a coat of alkaline mucus Mucosa is folded to form gastric pits Gastric pits (GP) containing gastric glands (Open into the bottom of GP) that secrete: Gastric glands secrete gastric juice Mucus protects stomach wall from pepsin Pepsin splits protein Hydrochloric acid needed for pepsin to work Intrinsic factor aids in absorption of vitamin B 12

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10 Small Intestine Duodenum Jejunum Ileum

11 Microscopic Anatomy of the Small Intestine Structural modifications of the small intestine wall increase surface area Plicae circulares: Characteristics of the jejunum Transverse folds of the intestinal lining (deep circular folds of the mucosa and submucosa) semi lunar, Circular or spiral form Villi: fingerlike extensions of the mucosa (epithelium and lamina propria) Microvilli: tiny projections of absorptive mucosal cells plasma membranes Lacteals Terminal lymphatic in villus Intestinal glands Lined by entero-endocrine, goblet and stem cells

12 Small intestine

13 The Intestinal Wall

14 The Intestinal Villi Simple columnar epithelium Produces Mucin Enterocytes Tall columnar cells W microvilli form the brush border - Paneth cells found at the base of the crypt Lymphatic vessels Central lacteal (lymphatic vessel) Core capillaries Core of connective tissue Epithelial cells Tall columnar with striated border

15 Small Intestine: Histology of the Wall The epithelium of the mucosa is made up of: Absorptive cells and goblet cells Interspersed T cells (intraepithelial lymphocytes), and Entero-endocrine cells Intestinal crypts cells secrete intestinal juice Peyer s patches are found in the submucosa Whereas, Brunner s glands are found only in the duodenum and secrete alkaline mucus

16 Duodenum V Mucosa (M)-Villous form Glands- Coiled and branched Crypts of LieberKuhn. Brunner glands Tall columnar cells. Basally located nucleus Secret alkaline mucus Lysozyme and EG Factor Ducts pass through the MM and openinto the crypts Muscularis mucosa (MM) Extends between the glands M ucosa G

17 Mucosa of the Small Intestine The mucosal surface is folded into villi Increases surface area for absorption Interior contains blood vessels and a lacteal The mucosa and submucosa are thrown into circular folds Plicae Circulares PC Microvilli are present on the luminal surface of the enterocytes. Prominent future Peyer s Patches lymphoid tissue aggregates Lamina propria Intestinal Glands Secrete intestinal juice Contains enzymes that break down proteins, carbohydrates, fats

18 Differences between duodenum, jejunum & ileum Duodenum Jejunum Ileum Epithelium Columnar Striated border Few goblet cells Columnar Striated border goblet cells+ Columnar goblet cells++ Villi Broad Spatula Shaped Closely packed Tongue-shaped Different heights Few thin fingershaped Lamina Propria Crypts+ No Peyer s patches Crypts+ Diffuse infiltration of lymphocytes No Peyer s patches Crypts+ Peyer s patches extend into submucosa Submucosa Mucus secreting Brunner s glands Only connective tissues and blood vessels Peyer s patches

19 Small Vs Large Intestine Villi Small intestine 1. Crypts shallow 2. Goblet cells less Large intestine 1. Absence of villi 2. Crypts deeper, More Goblet cells Longitudinal muscle coat of muscularis externa 1. Uniformly thick 1. Three bands of Taenia coli