Digestive System

Digestive System

1. Mouth:

• Outer surface: Skin, numerous hairs, sebaceous glands, sweat glands.

• Lip margin: No sweat glands, hairs. Contains sebaceous glands. Abundant capillaries and nerve endings.

• Inner surface: Stratified squamous non-keratinized epithelium, lamina propria.

2. Oral mucosa:

• Inner lining of the oral cavity.

• Stratified squamous non-keratinized epithelium.

• Cells are replaced by proliferation from the basal layer.

3. Tongue:

• Covered by oral mucosa.

• Underneath the tongue: Thin, smooth.

• On top of the tongue: Numerous papillae.

4. Lingual papillae:

• Three types of papillae: Filiform, Fungiform, Circumvallate.

5. Salivary glands:

• Exocrine glands, saccular, compound alveolar type.

• Parotid gland: Serous, secretes IgA.

• Submandibular gland: Serous + mixed.

• Sublingual gland: Mostly mixed, abundant mucous cells.

6. Esophagus:

• Mucosa: 500-800 micrometers thick.

• Epithelium: Stratified squamous non-keratinized.

• Lamina propria: Loose connective tissue, with collagen fibers, elastic fibers, fibroblasts, esophageal-gastric glands. Contains lymphocytes, small lymphatic nodules.

• Muscularis mucosa: Very thick, 200-400 micrometers near the stomach. Consists of an inner circular and outer longitudinal layer.

• Submucosa: Loose connective tissue, contains esophageal proper glands.

• Esophageal proper glands: Compound alveolar type, secrete mucus.

• Muscular layer: 0.5-2.2 mm thick. Upper 1/4: Striated muscle; lower 3/4: Smooth muscle.

7. Stomach:

• Mucosa: Numerous small furrows (gastric pits), 2-4mm in diameter, with funnels on the surface.

• Epithelium: Simple columnar mucous-secreting.

• Lamina propria: Loose connective tissue with numerous glands (secreting HCl, mucus, pepsin, lipase).

• Cardiac glands: Straight tubular glands, branched, consisting of neck, body, and base segments. Contains mucous cells, parietal cells, chief cells, and argyrophil cells.

• Pyloric + Fundic glands: Curved tubular glands, branched, containing mucous cells and argyrophil cells.

• Parietal cells: Produce H+ and Cl-.

• Mucous cells: Secrete mucus.

• Argyrophil cells: Secrete serotonin and gastrin.

• Submucosa: Loose connective tissue, numerous adipose cells, mast cells.

• Muscular layer: Oblique-circular-longitudinal.

• Pyloric sphincter: Thickened circular muscle.

8. Small intestine:

• Mucosa: Three ways to increase absorption: Circular folds, villi, microvilli.

• Circular folds: 8-10mm high, 3-4mm wide, 2-5cm long.

• Epithelium: Simple columnar, containing 3 types of cells: Absorptive, goblet, argyrophil.

• Lamina propria: Loose connective tissue, numerous components. Large lymphatic nodules. Blood capillaries closely attached to the epithelial basement membrane. Smooth muscle fibers from the muscularis mucosa attach to the basement membrane (Brucke’s muscle). Nerve fibers from Meissner’s plexus ascend.

• Lieberkuhn’s glands: Deep epithelial invaginations, formed by 4 types of cells: Absorptive, goblet, argyrophil, Paneth.

• Brunner’s glands: In the lamina propria and submucosa of the duodenum. Simple cuboidal or columnar epithelium. Product: Protects the duodenal mucosa.

• Muscularis mucosa: Branched, interspersed between Brunner’s glands.

• Submucosa: Loose connective tissue, forming the core of the circular folds, numerous elastic fibers. May contain: Large lymphatic nodules, nerve plexuses.

• Muscular layer: Two layers of smooth muscle, with a nerve plexus between them.

9. Large intestine:

• Mucosa: Smooth, without villi or folds.

• Epithelium: Simple columnar, containing 3 types of cells: Absorptive, goblet (abundant), argyrophil.

• Lamina propria: Loose connective tissue, abundant plasma cells and lymphocytes. Lymphatic nodules. Lieberkuhn’s glands are long and straight. Abundant goblet cells, few argyrophil cells.

• Muscularis mucosa: Two thin layers of smooth muscle.

• Muscular layer: Longitudinal muscle layer —> 3 prominent muscle bands.

• Serosa: Loose connective tissue, continuous with the visceral peritoneum.

10. Liver:

• Division of liver units (3):

• Classical

• Portal

• Hepatic lobule

• Radial capillaries: Receive blood from the portal vein + hepatic artery, deliver it to the central vein.

• Space of Disse: Exchange of substances between liver cells –> blood. Fat storage.

• Portal area: Portal vein, hepatic artery. Bile duct:

• Canal of Hering

• Pericentral duct

• Interlobular duct

• Canal of Hering: Simple cuboidal epithelium.

• Pericentral duct: Simple cuboidal epithelium.

• Interlobular duct (liver): Simple columnar epithelium.

• Hepatic duct, common bile duct, cystic duct, gallbladder (3 layers):

• Mucosa

• Fibromuscular layer

• Serosa

• Function:

• Exocrine: Bile secretion.

• Endocrine: Glucagon metabolism, urea, triglyceride, cholesterol, iron storage.

• Means of liver fixation (3):

• Inferior vena cava + hepatic artery

• Falciform ligament

• Round ligament

11. Pancreas:

• Exocrine part:

• Lobules.

• Compound alveolar glands.

• Pancreatic duct: Drains the head, body, and tail of the pancreas.

• Accessory pancreatic duct: Drains the upper part of the head of the pancreas.

• Intercalated duct: Small, short. Cuboidal epithelium.

• Intralobular duct: Regular lumen. Cuboidal (columnar) epithelium.

• Interlobular duct (pancreas): Wide lumen. Cuboidal (columnar) epithelium.

• Main excretory duct: Wide lumen. Simple columnar epithelium.

• Endocrine part:

• Islets of Langerhans – tail of the pancreas contains the most.

• Cells within islets: A, B, D, PP.

• A cells:

• Contain alpha granules.

• Secrete glucagon.

• Largest.

• Peripheral region of the islet.

• B cells:

• Contain beta granules.

• Secrete insulin.

• Central region of the islet.

• D cells:

• Contain sigma granules.

• Secrete somatostatin – inhibits A and B cell secretion.

• Peripheral region.

• PP cells:

• Secrete pancreatic polypeptide – inhibits exocrine pancreatic secretion.