Histology

• Digestive system


  Consists of digestive (GI) tract and accessory organs

  
• Digestive (GI) tract


  • Oral cavity
  • Esophagus
  • Stomach
  • Small and large intestine
  • Anus

  
• Accessory organs


  • Salivary glands
  • Liver
  • Pancreas

  
• Main function of digestive system


  To extract nutrients by digesting ingested food
• Digestion


  Hydrolysis of complex macromolecules to their simplest constituents that are more readily absorbed
• GI tract

  • Hollow tube with a lumen of variable diameter and a wall made up of four main layers: Mucosa, Submucosa, Muscularis, Serosa/Adventitia
• Layers of the GI tract wall

  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa/Adventitia

  
• Mucosa


  Consists of an epithelial lining, underlying lamina propria, and muscularis mucosae

  
• Submucosa


  Contains larger blood vessels and lymph vessels, submucosal (Meissner) plexus of autonomic nerves, some glands and lymphoid tissue

  
• Muscularis


  Two or more sublayers of smooth muscles and myenteric (Auerbach) plexus
• Serosa/Adventitia


  Mesothelium or a thick adventitial layer
• Structures of the oral cavity


  • Lips
  • Tongue
  • Teeth
• Oral cavity lining


  Stratified squamous epithelium, which can be keratinized, partially keratinized, or nonkeratinized depending on the location

  
• Lips

  • Guard the entrance to the digestive tract, have three surfaces: outer cutaneous part, red vermilion zone, internal mucous surface (oral mucosa)

  
• Tongue


  • A mass of striated muscles covered by mucosa, manipulates ingested material during mastication and swallowing, has an irregular dorsal surface with various types of papillae

  
• Types of tongue papillae

  • Filiform papillae
  • Fungiform papillae
  • Foliate papillae
  • Vallate (or circumvallate) papillae

  
• Taste buds


  Ovoid structures that sample the general chemical composition of ingested materials, contain gustatory cells that respond to five categories of taste chemicals

  
• Categories of taste chemicals


  • H+ ions for acids (sour)
  • Sugars and related compounds (sweet)
  • Alkaloids and certain toxins (bitter)
  • Amino acids (umami)
• Teeth


  • Thirty-two permanent teeth in adults, twenty of which are preceded by primary (deciduous or milk teeth), each jaw has 4 incisors, 2 canines, 10 molars/premolars

  
• Parts of a tooth

  • Crown
  • Neck
  • Roots
  • Pulp cavity
  • Dental pulp

  
• Mineralized tissues of the teeth

  • Dentin
  • Enamel
  • Cementum

  
• Dentin


  Surrounds the pulp cavity and is the bulk of the tooth, hard and resilient structure, comprised of 70% hydroxyapatite, 18% type I collagen fibers, and 12% water

  
• Enamel

  Hardest substance in the body but very brittle, 96% hydroxyapatite, 4% matrix comprising of glycoproteins amelogenins and enamelins, produced by ameloblasts
• Cementum

  Similar to bone but is avascular and has no osteons, maintained by cementocytes
• Esophagus


  • A 25-cm (in adults) long muscular tube that transports swallowed material from the pharynx to the stomach, divided into upper, middle, and lower segments, has four layers: mucosa, submucosa, muscularis, adventitia

  
• Esophageal mucosa

  Features a nonkeratinized stratified squamous epithelium, with underlying lamina propria and prominent muscularis mucosae

  
• Esophageal submucosa


  Contains small mucus-secreting glands - esophageal glands

  
• Esophageal muscularis


  Consists of inner circular and outer longitudinal layers of muscle, with skeletal muscles at the upper one-third segment, combination of skeletal and smooth muscle at middle third segment, and smooth muscles at the lower third segment, ganglia of the myenteric (Auerbach) plexus are found between the muscle layers

  
• Esophageal adventitia


  Loose connective tissue that supports and protects the esophagus, anchoring it to nearby structures in the mediastinum, distal 1-2cm is covered by serosa in the peritoneal cavity

  
• Enteric nervous system


  Two distinct components: myenteric (Auerbach) plexus regulates smooth muscle contraction, peristalsis and gastrointestinal motility, submucosal (Meissner) plexus regulates glandular secretion and local blood flow

  
• Gastroesophageal junction


  Border between the epithelial lining of the esophagus and stomach, stratified squamous epithelium changes to simple columnar epithelium, referred to as the Z line, common site of esophageal carcinoma

  
• Stomach


  • Dilated segment of the digestive tract, functions: add acidic fluid, mix ingested contents to form chyme, promote initial digestion of protein with pepsin, has four regions: cardia, pylorus, fundus, body

  
• Stomach mucosa

  Lined by a simple columnar epithelium invaginating deeply into the lamina propria to form gastric pits and long, branched, tubular gastric glands

  
• Cell types in gastric glands


  • Surface mucus cells
  • Stem cells
  • Parietal (oxyntic) cells
  • Chief (zymogenic) cells
  • Enteroendocrine (G) cells

  
• Surface mucus cells


  Secrete a highly viscous mucous layer rich in bicarbonate to protect the mucosa
• Parietal cells


  Produce HCl and intrinsic factor (for uptake of vitamin B12 in the small intestine)
• Chief cells

  Release pepsinogen (inactive proenzyme which is converted to pepsin by HCl; pepsin is the principal enzyme for protein digestion) and gastric lipase
• G cells

  Scattered epithelial cells with endocrine or paracrine functions, produce peptide gastrin
• Other layers of the stomach


  • Submucosa: connective tissue with large blood and lymph vessels and many lymphoid cells, macrophages, and mast cells

  Muscularis: three poorly defined layers of smooth muscle: outer longitudinal, middle circular, and innermost oblique

  Serosa: a thin layer covering the external surface

  
• Small intestines


  • Site of completion of digestion and absorption of nutrients