DIGESTIVE SYSTEM AND ASSOCIATED GLANDS

• Digestive Tract


  Oral Cavity, Esophagus, Stomach, Small and Large Intestines, and Anus
• Associated Glands of the Digestive Tract

  Salivary Glands, Liver, Gallbladder and Pancreas
• Other Names for Digestive Tract

  • Gastrointestinal Tract
  • Alimentary Canal
• Function of Digestive System

  Obtain molecules from the ingested food
• Specific Functions of Gastrointestinal Tract

  • Ingestion
  • Mastication
  • Motility
  • Secretion
  • Hormone Release
  • Chemical Digestion
  • Absorption
  • Elimination
• General Structure of GIT

  • Hollow tube with lumen of variable diameter and has a wall made up of four main layers: Mucosa, Submucosa, Muscularis, Serosa

  
• Mucosa


  Epithelial Lining, Lamina Propria, Muscularis mucosae

  
• Submucosa


  Denser CT with larger blood and lymph vessels, Submucosal (Meissner) Plexus

  
• Muscularis


  Thick layer of smooth muscle cells organized as two or more sublayers, Myenteric (Auerbach) Nerve Plexus

  
• Serosa


  Thin sheet of loose CT covered with a simple squamous epithelium or mesothelium
• Oral Cavity


  • Lined by Stratified Squamous Epithelium, Keratinized, Partially Keratinized, Non-keratinized

  
• Lips/Labia


  • Well-developed core of striated muscles, Highly mobile structure for ingestion and speech, Internal mucous surface, Vermillion Zone, Outer surface

  
• Tongue


  • A mass of striated skeletal muscle covered by a thick keratinized stratified squamous epithelium, Manipulates ingested material during mastication and swallowing, Lower surface is smooth with typical lining mucosa, Dorsal surface is irregular with hundreds of papillae

  
• Lingual Papillae

  • Filiform Papillae
  • Fungiform Papillae
  • Foliate Papillae
  • Vallate Papillae
• Taste Buds


  Ovoid structures within the stratified epithelium on the tongue's surface, Contains Gustatory Cells, Supportive Cells, Immature Cells, Basal Stem Cells, Detect 5 broad categories of tastants

  
• Teeth


  • Adult: 32 permanent teeth, Primary teeth (Deciduous/Milk Teeth), Dentin, Pulp, Enamel, Periodontium (Cementum, Periodontal Ligament, Alveolar Bone)
• Esophagus


  • A muscular tube which transports swallowed material from the pharynx to the stomach, Mucosa, Submucosa, Muscularis Propria, Serosa
• Stomach


  • A greatly dilated segment of the digestive tract, Main functions: Continue digestion of carbohydrates, Add acidic fluid and mix contents into chyme, Begin digestion of triglycerides and proteins, Four major regions: Cardia, Pylorus, Fundus, Body, Four layers: Mucosa, Submucosa, Muscularis Propria, Serosa
• Mucosa of the Stomach


  • Changes abruptly at the Esophagogastric Junction, Stratified Squamous Epithelium to Simple Columnar Epithelium, Epithelium invaginates deeply into the lamina propria to form gastric pits, Macroscopically: series of infoldings called "Rugae", Bonded by a thin layer of smooth muscles called "Muscularis Mucosae"

  
• Major regions of the stomach

  • Cardia
  • Pylorus
  • Fundus
  • Body
• Cardia


  • Narrow transitional zone between the stomach and the esophagus
  • Involved in mucus production

  
• Pylorus


  • Funnel-shaped region that opens into the smaller intestine
  • Involved in mucus production

  
• Layers of the stomach wall

  • Mucosa
  • Submucosa
  • Muscularis Propria
  • Serosa

  
• Muscularis Propria


  • Outer Longitudinal Layer
  • Middle Circular Layer
  • Innermost Oblique Layer

  
• Mucosa of the stomach

  • Changes abruptly at the Esophagogastric Junction (EGJ)
  • Stratified Squamous Epithelium (SSE) of the esophagus to Simple Columnar Epithelium (SCE) of the stomach
  • Epithelium invaginates deeply into the lamina propria (LP) that forms millions of gastric pits, each with an opening to the stomach lumen
  • Macroscopically: series of infoldings called "Rugae"
  • Bonded by a thin layer of smooth muscles called "Muscularis Mucosae (MM)

  
• Gastric Pits


  • Secrete a thick adherent and highly viscous mucous layer, rich in bicarbonate ions
  • Protects the mucosa from the abrasive effects of foods and corrosive effects of stomach acid
• Lamina propria


  • Highly vascularized
  • Surrounds and support the gastric pits and gastric glands
  • Also contains smooth muscle fibers, lymphoid cells, capillaries and lymphatics

  
• Submucosa


  • Composed of connective tissue with large blood and lymph vessels and many lymphoid cells, macrophages, and mast cells

  
• Muscularis propria


  • Has three poorly defined layers of smooth muscle
  • Rhythmic contractions thoroughly mix ingested food and chyme, mucus, HCl, and digestive enzymes from the gastric mucosa
  • At the pylorus, the middle layer is greatly thickened to form the Pyloric Sphincter

  
• Segments of the small intestine

  • Duodenum
  • Jejunum
  • Ileum
• Mucosa of the small intestine

  • Macroscopically: series of permanent circular or semilunar folds called "Plicae Circulares", consisting of mucosa and submucosa
  • Villi are short mucosal outgrowths that project into the lumen and densely cover the entire mucosa
  • Intestinal Glands or Crypts (Crypts of Lierberkuhn) are openings of short tubular glands in between the villi

  
• Enterocytes


  • Absorptive cells
  • Tall columnar cells with an oval nucleus located basally
  • Apical end displays a prominent ordered region called "Striated (or Brush) Border" which are densely packed microvilli
  • Together with the plicae and villus, they increases the absorptive capacity of the organ

  
• Goblet cells


  • Are cells that are interspersed among the absorptive enterocytes
  • Secrete glycoprotein mucins which are then hydrated to form mucins
  • Main function: To protect and lubricate the lining of the intestine

  
• Paneth cells


  • Located in the basal portion of the intestinal crypts below the stem cells
  • Exocrine cells with large, eosinophilic secretory granules in their apical cytoplasm
  • Granules release lysozyme, phospholipase A2 and hydrophobic peptides (defensins) to breakdown membranes of microorganisms and bacterial cell walls
• Enteroendocrine cells


  • Present in varying numbers throughout the length of the small intestine
  • Secrete various peptide hormones
  • Has chemoreceptors similar to taste buds

  
• M cells (microfold cells)


  • Unique epithelial cells specialized for transepithelial transport of particles and microorganisms
  • Located mainly in the ileum's mucosa overlying the lymphoid follicles of Peyer patches
  • Has a pocket, formed by deeply invaginated basolateral membrane containing lymphocytes and macrophages

  
• Submucosa of the small intestine

  • Larger blood vessels than those in the lamina propria
  • Submucosal (Meissner) Nerve Plexus
  • Duodenal Glands / Brunner Glands
  • MALT (Peyer Patches)

  
• Muscularis of the small intestine

  • Well-developed with 2 distinct layers: Inner Circular Layer and Outer Longitudinal Layer
  • Myenteric (Auerbach) Nerve Plexus

  
• Regions of the large intestine

  • Cecum with the Ileocecal valve and Appendix
  • Ascending Colon
  • Transverse Colon
  • Descending Colon
  • Sigmoid Colon
  • Rectum

  
• Mucosa of the large intestine

  • Lacks villi and has no major folds except in the rectum
  • Penetrated throughout by tubular intestinal glands
  • Cells: Absorptive cells "Colonocytes", Goblet cells, Epithelial stem cells
  • Lamina propria is rich in lymphoid cells and often extend into the submucosa
  • Appendix has little absorptive function but is a significant component of MALT (Mucosa-Associated Lymphoid Tissue)