Histology of the Lower Digestive Tract

Digestive Tract
- Upper: Mouth, esophagus, stomach (Gaster)
- Lower: Small intestine (intestine), large intestine (colon), rectum, and anus
Digestive Glands
- Small (mouth): Parotid, submandibular, sublingual
- Large: Liver and pancreas
- Gallbladder

Functions
- Transports digested food from the stomach
- Mixes food with secretions from the intestines, pancreas, and liver (digestion process)
- Absorption of digestive products
- Hormone production: Serotonin, secretin, cholecystokinin
Movements
- Segmental contractions
- Peristalsis
Sections and Lengths
- Duodenum: $+ 25$ cm
- Jejunum: $+ 240$ cm
- Ileum: $+ 360$ cm

Tunica Mucosa
- Mucosa has villi with simple columnar epithelium
- Villi become less numerous and shorter distally
- Crypts of Lieberkühn: Invaginations of small glands between villi, extending to the muscularis mucosae
- Plica circularis (Kerckring folds): Circular folds involving the submucosa, visible in the duodenum and disappearing towards the end of the ileum (characteristic of the jejunum)
  - Function: Increases absorption surface area and slows down intestinal movement for better mixing
- Lamina propria: Loose connective tissue
  - In the ileum, contains lymphatic nodules forming Peyer's patches
Submucosa
- Meissner's plexus: Nerve plexus in the submucosa
- Brunner's glands: Complex tubular glands only in the duodenum
  - Produce mucus containing bicarbonate to neutralize stomach acid from the pylorus
Tunica Muscularis
- Two layers of muscle: Outer longitudinal and inner circular
- Auerbach's plexus: Nerve plexus between the muscle layers
Tunica Serosa
- Loose connective tissue covered by mesothelium

Mucosa
- Epithelium: Cells for nutrient absorption and mucus secretion
- Lamina Propria: Connective tissue with blood vessels, lymph vessels, and immune cells
- Muscularis Mucosae: Thin layer of smooth muscle for movement and contraction of the mucosa
Submucosa
- Connective tissue containing blood vessels, lymph vessels, and nerve fibers
Muscularis
- Two layers of smooth muscle: Longitudinal (fibers run parallel to the intestine) and circular (fibers encircle the intestine)
- Contractions aid in the movement and mixing of food
Serosa
- Outermost layer of the duodenum
- Anterior portion covered by mesothelium that produces serous fluid
- Posterior portion replaced by adventitia (connective tissue) that protects and connects the organ to surrounding structures

Mucosa
- Epithelium: Cells for nutrient absorption; tall, dense villi
- Lamina Propria: Contains blood vessels, lymph vessels, and immune cells
- Muscularis Mucosae: Thin layer of smooth muscle for movement and contraction
Submucosa
- Connective tissue with blood vessels, lymph vessels, and nerve fibers
- Provides blood and nutrients to the mucosa
Muscularis
- Two layers of smooth muscle: Longitudinal and circular
- Contractions aid in movement and mixing
Serosa
- Outermost layer covered by mesothelium that produces serous fluid
- Protects and holds the small intestine in the abdominal cavity

Mucosa
- Epithelium: High, dense villi, but shorter and less frequent than in the jejunum
- Peyer's patches: Lymphoid tissue aggregates for the immune system
- Lamina Propria: Connective tissue with blood vessels, lymph vessels, and immune cells; involved in substance exchange
- Muscularis Mucosae: Thin layer of smooth muscle for movement and contraction
Submucosa
- Connective tissue with blood vessels, lymph vessels, and nerve fibers
- Provides blood and nutrients to the mucosa
Muscularis
- Two layers of smooth muscle: Longitudinal and circular
- Contractions aid in movement and mixing
Serosa
- Outermost layer covered by mesothelium that produces serous fluid
- Protects and holds the small intestine in the abdominal cavity

Absorptive cells (columnar): Contain glycocalyx to protect from digestion
Goblet cells: Produce mucus to protect the mucosa from the lytic contents of the lumen; more abundant in the jejunum and ileum
Argentaffin cells: Produce hormones (serotonin, secretin, and cholecystokinin)
Paneth cells: Located at the base of intestinal crypts; contain lysosomes for bacterial hydrolysis
Primitive columnar cells: Divide in the crypts and differentiate into absorptive, goblet, argentaffin, and Paneth cells

Tunica Mucosa
- No plica or villi
- Thicker mucosa with long crypts of Lieberkühn
- More goblet cells; more lymphatic nodules
Tunica Muscularis
- Three bands of smooth muscle (taenia coli)
- Shorter muscularis
Tunica Serosa
- Fat deposits (appendices epiploicae)

Diverticulum of the cecum
Microscopic Structure
- Tunica Mucosa: Similar to the colon; lamina propria with abundant lymphatic tissue
- Submucosa: Dense connective tissue network
- Tunica Muscularis: Circular and longitudinal muscle layers; no taenia coli
- Tunica Serosa: Covered by mesothelium

Mucosa
- Similar to the colon
- Distal portion: Stratified squamous keratinized epithelium
- Lamina propria: Large venous plexus (hemorrhoids)
Submucosa
- Connective tissue with many blood capillaries
Tunica Muscularis
- Proximal 1/3: Two layers of circular and longitudinal muscle (internal anal sphincter)
- Distal portion: Skeletal muscle (external anal sphincter)

Ulcerative Colitis
- Chronic inflammatory disease affecting the mucosa of the large intestine and rectum. Histology shows inflammation, ulceration, and immune cell infiltration.
Crohn's Disease
- Chronic inflammatory disease affecting the entire digestive tract. Histology shows inflammation involving all layers of the intestinal wall, immune cell infiltration, ulceration, and granuloma formation.
Colorectal Cancer
- Cancer in the large intestine or rectum. Histology shows abnormalities in the epithelial cells, dysplasia (precancerous changes), and invasion into deeper layers.
Inflammatory Bowel Syndrome (IBS)
- Mild inflammation, immune cell infiltration, and mucosal damage may be observed histologically.