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

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

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

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

If stain with PAS, will see the Glycocalyx Macrophages Mast cells

Mucosal Epithelium: Enterocytes Terminal bar Microvilli The terminal web contains actin filaments in a dense network of spectrin molecules overlying a layer of intermediate filaments. Cell organelles are excluded from this terminal web and in this perparation is it relatively unstained. Terminal web

Cells of Mucosal Epithelium: Enterocytes 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) Antibiotic: Secrete lysozymes Regulate bacterial flora of gut

Cells of Mucosal Epithelium in SI: Paneth Cells Located in Crypts of Lieberkuhn (base of glands of SI b/t villi) Basophilic base (cytoplasm), eosinophilic granules at apex

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 M Cells: more squamous epithelial cell (overlies the Peyer’s Patch). No microvilli.

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

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 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 Inner - segementation Outer - peristalsis Myenteric (Auerbach’s) Plexus b/t muscle layers Submucosal Plexus Myenteric Plexus

Layer 4: Adventitia/Serosa Loose connective tissue