GIT histology

• Digestive system


  Includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body

  
• Food processing


  Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated
• Digestive tract


  Also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus

  
• Digestive system components


  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine

  
• Digestive system accessory organs

  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas

  
• Accessory structures in the mouth

  • Tongue
  • Teeth
• Salivary glands


  Produce saliva and pour their secretions into the oral cavity through the ducts
• Salivary glands


  • Covered by capsule (connective tissue)
  • Divided into lobules
  • Lobules are separated from one another by the connective tissue septa

  
• Lobules


  Contain secreting units called acini, ducts which drain the acini
• Acinus


  An oval or spherical structure which has a lumen in the center, enclosed by a basement membrane, with a layer of epithelial cells (acinar cells) which are usually columnar or pyramidal cells, and myoepithelial cells lie between the epithelial cells and the basement membrane

  
• Types of acini


  • Mucous acinus
  • Serous acinus
  • Mixed acinus

  
• Mucous acinus


  Oval shaped structure, has a large lumen, lined by columnar cells (acinar cells) with flat nucleus situated on the basement membrane, contains large secretory granules of mucinogen near the apex, cytoplasm has a distinct "spongy" appearance

  
• Serous acinus


  Spherical structure and smaller than mucous acinus, has a small lumen, lined by columnar or pyramidal cells with a round nucleus placed near basal part, cytoplasm at the apical part appears pinkish and contains secretory granules, secretes the digestive enzyme α-amylase

  
• Mixed acinus


  Mucous acini with a half moon (crescent) shaped cap of serous cells on them, the serous cells attach to the mucus producing acini and empty their secretory product into interstices between the mucus-producing cells

  
• Types of salivary gland ducts

  • Intercalated ducts
  • Striated ducts
  • Interlobular ducts

  
• Intercalated ducts


  Drain the acini and lined by simple cuboidal epithelium
• Striated ducts


  Formed by the intercalated ducts and lined by simple columnar epithelium
• Interlobular ducts


  Embedded in the connective tissue surrounding the lobules of the glands, lined by stratified cuboidal or columnar epithelium

  
• Serous salivary gland


  • Parotid salivary gland

  
• Mucous salivary gland


  • Sublingual salivary gland

  
• Mixed salivary gland


  • Submandibular salivary gland
• Tongue


  A muscular organ with a dorsal surface (dorsum) and ventral surface, covered by a mucous membrane (mucosa) made up of an outer epithelium and inner lamina propria (corium), deeper part contains bundles of skeletal muscle fibers
• Types of papillae on the tongue

  • Filiform papillae
  • Fungiform papillae
  • Circumvallate papillae
• Filiform papillae


  Small conical projections covered by horny keratinized cap, present all over the dorsum of anterior 2/3 of tongue, do not contain taste buds

  
• Fungiform papillae


  Have rounded top and narrow base, contain taste buds, present at the apex and along the lateral margins of anterior 2/3 of dorsum of tongue

  
• Circumvallate papillae


  Largest papilla, situated in a depression surrounded by a circular (vallate) sulcus, have rounded top and narrow base, contain taste buds on their lateral wall, the sulcus receives the opening ducts of serous glands (Von Ebner's gland) which distribute the substance to be tasted and wash out the debris

  
• Layers of the alimentary canal

  Mucosa, submucosa, muscularis externa, serosa
• Mucosa


  Consists of epithelium, lamina propria and muscularis mucosa
• Submucosa


  Composed of irregular fibroelastic connective tissue with elastic and collagen fibres, contains blood and lymph vessels and Meissner's plexus of nerves

  
• Muscularis externa


  Contains inner circular and outer longitudinal layers of smooth muscle fibres, with Auerbach's plexus of nerves between the layers
• Serosa


  The outermost layer formed by visceral peritoneum, or replaced by connective tissue coat called adventitia if the organ is retroperitoneal
• Esophagus


  Wall of esophagus presents 4 layers - mucosa, submucosa, muscularis externa, and adventitia/serosa
• Esophageal mucosa


  Innermost layer, contains 3 parts - epithelium, lamina propria and muscularis mucosa, epithelium is mostly stratified squamous non-keratinized type

  
• Esophageal submucosa


  Contains dense irregular connective tissue, blood vessels and mucous glands (esophageal glands) which secrete mucus to aid in food passage
• Esophageal muscularis externa


  Thick layer of muscle fibres arranged in inner circular and outer longitudinal layers, upper part is skeletal muscle, lower part is smooth muscle, middle part is both
• Esophageal adventitia/serosa


  Outermost layer, made of loosely arranged connective tissue with blood vessels, replaced by serosa (visceral peritoneum) in the abdominal part
• Stomach


  Most dilated part of the GI tract, has 4 parts - cardia, fundus, body, and pyloric part
• Esophageal glands


  • Extend to the lumen
  • Lined by stratified columnar epithelium

  
• Esophageal glands


  Secrete mucus, which aids in the passage of food
• Layers of muscularis externa

  • Inner layer of circularly arranged muscle fibres
  • Outer layer of longitudinally arranged muscle fibres