The Gastrointestinal System II: Small & Large Intestines Histology SSN March 7, 2005 Presented by: Anne O’Donnell & Nadia Goodwin

Small & Large Intestines Large Intestine: reabsorption of water and elimination of undigested food and waste Small Intestine: principal site for digestion of food and absorption of the products of digestion

4 Layers of Digestive System Mucosa - Epithelium - Lamina Propria - Muscularis Mucosae Submucosa - Connective tissue, blood vessels, submucosal plexus Muscularis Externa - 2 muscle layers, myenteric plexus Adventitia / Serosa - Loose connective tissue

Layer 1: Mucosal Layer Epithelium Lamina Propria Muscularis Mucosae - enterocytes (with microvilli), - goblet cells, - paneth cells (SI only), - entero-endocrine cells, - M cells Lamina Propria - loose connective tissue - Peyer’s Patches - lacteals Muscularis Mucosae - Boundary b/t mucosa and submucosa - 2 layers of smooth muscle

Layer 2: Submucosa & Layer 3: Muscularis Externae Connective Tissue Blood vessels Submucosal (Meissner’s) Plexus (Brunner’s glands in duodenum) Muscularis Externae: 2 smooth muscle layers Myenteric (Auerbach’s) Plexus b/t muscle layers Submucosal Plexus Myenteric Plexus

Layer 4: Adventitia/Serosa Loose connective tissue

Plicae Circularis vs Villi Both are present in the small intestine as specializations to maximize absorptive surface area Plicae Circularis: circular folds with a core of submucosa. Contains many villi. Villus: finger-like mucosal projections with a core of lamina propria

Cells of Mucosal Epithelium: Enterocytes Absorptive Cells Columnar cells with basal nuclei Microvilli on apical surface Digestive enzymes in microvilli glycocalyx (breakdown sugars & proteins) Junctional complexes and terminal web

Cells of Mucosal Epithelium: Enterocytes Terminal bar Microvilli

Cells of Mucosal Epithelium: Enterocytes Junctional complexes and terminal web Terminal Bar

Cells of Mucosal Epithelium: Goblet Cells Produce mucus Interspersed among enterocytes Appear empty because mucus is washed out in slide preparation (more goblet cells in colon than SI)

Cells of Mucosal Epithelium in SI: Paneth Cells Located in Crypts of Lieberkuhn (bases of glands of SI b/t villi) Secrete lysozymes Basophilic base, eosinophilic granules at apex Regulate bacterial flora of gut

Cells of Mucosal Epithelium: Enteroendocrine Cells Secrete Hormones GIP, CCK, Secretin Seen with silver stain (Also in stomach)

Features of Mucosal Lamina Propria in SI Lamina Propria: loose cellular connective tissue Contains: Peyer’s Patches: lymph nodules and immune cells Lacteals: endothelial-lined lymphatic channels w/in villi that uptake lipid droplets

Lipid Absorption Triglycerides and other lipids are packaged into droplets Droplets are absorbed from lumen, then reach basolateral surface where they are released by excocytosis Transported into lacteal

Features of Muscularis Mucosae in SI Boundary b/t mucosa and submucosa Composed of two thin layers of smooth muscle

Duodenum Characterized by submucosal Brunner’s Glands (protect duodenum by neutralizing stomach acid)

Duodenum vs Jejunum and Ileum Submucosal Brunner’s Glands No Brunner’s Glands

Large Intestine Function: Reabsorbs water and electrolytes and eliminates undigested food and waste

Large vs Small Intestine Similarities Same Layers Mucosa Submucosa Muscularis Externa Adventitia/Serosa Same Epithelial Cells Enterocytes Goblet cells Enteroendocrine cells M cells BUT Differences Large Intestine has NO plicae circularis and NO villi Large Intestine has MORE GOBLET cells NO PANETH cells in LI

Large vs Small Intestine

Junction: Rectum and Anal Canal simple columnar epithelium with goblet cells glands Recto-Anal Junction -stratified columnar/ cuboidal epithelium No glands

Recto-Anal Junction -stratified columnar/cuboidal epithelium - as move toward anus, more stratified squamous - at perineum, stratified squamous epithelia is keratinized

Stratified Columnar/Cuboidal Stratified Squamous (non-keratinized)

Hemorrhagic Vessels These vessels are in the submucosa of the anal canal. When dilated, they are internal hemorrhoids. Hemorrhagic vessel

Internal Anal Sphincter Inner circular layer of muscularis externa thickens to form internal anal sphincter

Question 1 The organ shown : Digests carbohydrates and proteins Contains paneth cells Reabsorbs water Conjugates bile

Question 2 Villi are present in both images: True False

Question 3 Which sequence is the correct oral to anal order? B A D C