Four Layers of Digestive System

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 - 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

Submucosa: • 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 Myenteric Plexus

Submucosal 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 w/in a core of submucosa. Contains many villi.

Villus: finger-like mucosal projections w/in 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 in SI: Paneth Cells • Located in Crypts of Lieberkuhn (base of glands of SI b/t villi) • Basophilic base, 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: 1. 2.

Peyer’s Patches: lymph nodules and immune cells Lacteals: endotheliallined lymphatic channels w/in villi that uptake lipid droplets

Features of Mucosal 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

Rectum -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

Anal Transition Zone Stratified Columnar/Cuboidal

Anal Stratified Squamous Epithelium

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

Question 1 The organ shown : a. Digests carbohydrates and proteins b. Contains paneth cells c. Reabsorbs water d. Conjugates bile

Question 2 Villi are present in both images: True or False

Question 3 Which sequence is in the correct oral to rectum/anal order?

B

A D

C