digestive tract

• Mucosa


  Consists of epithelium, lamina propria, muscularis mucosae
• Submucosa


  Dense connective tissue and submucosal (Meissner) plexus
• Muscularis (propria/externa)


  Smooth muscle organized in 2 layers: inner circular, outer longitudinal interspersed by myenteric (Auerbach) plexus
• Serosa


  Loose connective tissue lined by simple squamous epithelium (mesothelium)
• Lip


  • Inner: thick, nonkeratinized epithelium and many minor labial salivary glands
  • Red vermilion zone: very thin keratinized stratified squamous epithelium, transitional between oral mucosa and skin
  • Outer: epidermal and dermal layers, sweat glands, and many hair follicles with sebaceous glands
• Tongue


  • Mass of striated muscle covered by mucosa
  • Muscle fibers oriented in all directions, allowing high mobility
  • Lower surface smooth, with typical lining mucosa
  • Dorsal surface irregular, with papillae and lingual tonsils
  • Papillary and tonsillar areas separated by sulcus terminalis
• Types of Lingual Papillae

  • Filiform
  • Fungiform
  • Foliate
  • Vallate/circumvallate
• Taste Buds


  • Ovoid structures within the stratified epithelium on the tongue's surface, which sample the chemical composition of ingested material
  • Widely scattered, continuously flushed by minor salivary glands
  • 50-100 cells (supportive, stem, gustatory), about half of which are elongated gustatory cells with 7-10 day life span
  • Microvilli at ends of gustatory cells project through taste pore
• 5 Broad Categories of Tastants

  • Salty (sodium ions)
  • Sour (hydrogen ions from acids)
  • Sweet (sugars and related compounds)
  • Bitter (alkaloids and certain toxins)
  • Savory/Umami (glutamate and aspartate)
• Teeth


  • 8 teeth per quadrant: 2 incisors, 1 canine, 2 premolars, 3 permanent molars
  • Parts: crown, neck, root fitting into dental alveoli
  • Dentin: calcified tissue harder than bone, 70% hydroxyapatite, organic matrix of type 1 collagen and proteoglycans
  • Enamel: hardest component, 96% calcium hydroxyapatite, 2-3% organic material, uniform interlocking enamel rods
  • Ameloblast process contains secretory granules with enamel matrix proteins
  • Periodontium: cementum, periodontal ligament, alveolar bone, gingiva
• Esophagus


  • Mucosa: nonkeratinized stratified squamous epithelium, cardiac glands at lower end
  • Submucosa: small esophageal glands (mainly mucous)
  • Muscularis: striated muscle in upper region, smooth muscle in lower region, mixed in middle
  • Adventitia/Serosa: adventitia except lower end with serosa
  • Esophagogastric junction: abrupt change from stratified squamous to simple columnar epithelium
• Stomach


  • Mucosa of fundus and body penetrated by gastric pits leading to gastric glands
  • Surface mucous cells secrete thick mucus layer with bicarbonate
  • Gastric glands have 4 major cell types: mucous neck, parietal, chief, enteroendocrine
  • Cardiac and pyloric regions have branching glands of mostly mucous cells
• 4 Main Regions of the Stomach

  • Cardia
  • Fundus & Body
  • Pylorus
• Major Cell Types of Gastric Glands

  • Mucous neck cells
  • Parietal cells
  • Chief (zymogenic) cells
  • Enteroendocrine cells
• Gastric glands


  Sites of releasing acidic gastric juice
• Stomach mucosa and submucosa

  • Have large, longitudinally directed folds called rugae, which flatten when the stomach fills with food
• The wall in all regions of the stomach is made up of all four major layers
• Pylorus


  Funnel-shaped region that opens into the small intestine
• Major Cell Types of Gastric Glands

  • Mucous neck cells
  • Parietal cells
  • Chief (zymogenic) cells
  • Enteroendocrine cells
• Mucous neck cells


  Include immature precursors of the surface mucous cells but produce less alkaline mucus while migrating up into the gastric pits
• Parietal cells


  Large cells with many mitochondria and large intracellular canaliculi for production of HCl in the gastric secretion; they also secrete intrinsic factors for vitamin B12 uptake
• Chief (zymogenic) cells


  Clustered mainly in the lower half of the gastric glands, secrete the protein pepsinogen that is activated by the low pH in the lumen to form the major protease pepsin
• Enteroendocrine cells


  Scattered epithelial cells of the diffuse neuroendocrine system, which release peptide hormones to regulate activities of neighboring tissues during food digestion
• Layers of the Stomach

  • Mucosa (Epithelium, LP, MM)
  • Submucosa (with submucosal plexus)
  • Muscularis (Inner Circular and Outer Longitudinal Layers, with Myenteric Plexuses Between Them)
  • Adventitia/Serosa
• The small intestine has three regions: the duodenum, the jejunum, and the ileum with the large mucosal and submucosal Peyer patches
• Small Intestine Mucosa


  • Has millions of projecting villi, with simple columnar epithelium over cores of lamina propria, and intervening simple tubular intestinal glands (or crypts)
• Major Cell Types of the Small Intestines

  • Enterocytes
  • Goblet cells
  • Paneth cells
  • Enteroendocrine cells
  • M (microfold) cells
• Enterocytes


  Absorptive cells that are tall columnar cells, each with an oval nucleus located basally, and a prominent ordered region called the striated (or brush) border with densely packed microvilli covered by glycocalyx through which nutrients are taken into the cells
• Goblet cells


  Interspersed among the absorptive enterocytes, secrete mucins that are then hydrated to form mucus
• Paneth cells


  Located in the basal portion of the intestinal crypts below the stem cells, are exocrine cells with large, eosinophilic secretory granules in their apical cytoplasm that release lysozyme, phospholipase A2, and hydrophobic peptides called defensins, all of which bind and break down membranes of microorganisms and bacterial cell walls
• Enteroendocrine cells


  Present in varying numbers throughout the length of the small intestine, secreting various peptide hormones
• M (microfold) cells


  Specialized epithelial cells in the mucosa of the ileum overlying the lymphoid follicles of Peyer patches that selectively endocytose antigens and transport them to the underlying lymphocytes and dendritic cells, which then migrate to lymph nodes for an appropriate immune response
• Layers of the Small Intestine

  • Mucosa (Epithelium, LP, MM)
  • Submucosa (with submucosal plexus)
  • Muscularis (Inner Circular and Outer Longitudinal Layers, with Myenteric Plexuses Between Them)
  • Adventitia/Serosa
• The large intestine has three major regions: the short cecum, with the appendix; the long colon, with its ascending, transverse, descending, and sigmoid portions; and the rectum
• Large Intestine Mucosa


  • Has millions of short simple tubular intestinal glands, lined by lubricant goblet cells and absorptive cells for the uptake of water and electrolytes
• Teniae coli


  Outer longitudinal layer of the muscularis of the colon, subdivided into three bands of smooth muscle that act in the peristaltic movement of feces to the rectum
• Layers of the Large Intestine


  • Mucosa (Epithelium, LP, MM)
  • Submucosa (with submucosal plexus)
  • Muscularis (Inner Circular and Outer Longitudinal Layers, with Myenteric Plexuses Between Them)
  • Adventitia/Serosa
• At the anal canal the simple columnar epithelium lining the rectum shifts abruptly to stratified squamous epithelium of the skin at the anus
• Internal anal sphincter


  Formed by the thickened circular layer of the rectum's muscularis, with further control exerted by striated muscle of the external anal sphincter
• Layers of the Anal Canal

  • Mucosa (Epithelium, LP, MM)
  • Submucosa (with submucosal plexus)
  • Muscularis (Inner Circular and Outer Longitudinal Layers, with Myenteric Plexuses Between Them)
  • Adventitia/Serosa