Histology of Digestive System

• Digestive system


  Made of two major parts: Digestive tract and Accessory (supporting) glands of digestion
• Digestive tract


  • Mouth
  • Esophagus
  • Forestomach (rumen, reticulum, omasum in ruminants)
  • Stomach (abomasum in ruminants)
  • Small and large intestines and rectum
• Accessory (supporting) glands of digestion

  • Salivary gland
  • Liver
  • Pancreas
• General plan of the wall of the digestive tract

  Made up of 4 layers: Tunica mucosa, Tunica submucosa, Tunica muscularis, Tunica serosa
• Tunica mucosa


  • Epithelium
  • Lamina propria: loose areolar or reticular connective tissue
  • Muscularis mucosa: smooth muscle
• Tunica submucosa


  • Loose connective tissue
  • Nerves
  • Blood vessels
  • Glands in some organs
• Tunica muscularis


  • Inner circular layer
  • Outer longitudinal layer
  • Parasympathetic ganglia located between the layers

  
• Tunica serosa


  • Connective tissue
  • If surrounded by other tissues = tunica adventitia
  • If suspended in the body cavity = tunica serosa covered by simple squamous epithelium (mesothelium)

  
• Tongue


  • Muscular organ, covered by mucosa
  • Spatula shape
  • Functions: Prehension, mastication & deglutition
• Tongue epithelium


  • Stratified squamous
  • Keratinized: ruminants & horses
  • Non-keratinized: cats & dogs
• Tongue muscles


  • Arranged cross-sectionally, longitudinally, and obliquely
• Tongue papillae


  • Foliate (leaf shaped)
  • Filiform
  • Circumvallate
• Esophagus


  • Mucosa: Stratified squamous epithelium
  • Submucosa with submucosal glands
  • Tunica muscularis with inner circular and outer longitudinal layers
  • Auerbach's plexus located between muscle layers
• Stomach


  • Cardia: Contains many mucus-secreting glands
  • Fundus: Mostly glands secreting acid-peptic gastric juices, some mucus-secreting glands
  • Pylorus: Contains two different types of mucus-secreting glands, also contains endocrine cells secreting gastrin
• Fundic stomach mucosa


  • Simple columnar epithelium with surface-lining cells, mucus-secreting glands (fundic/oxyntic glands)
  • Fundic glands contain 6 cell types: surface lining cells, parietal cells, regenerative cells, mucous neck cells, chief cells, diffuse neuroendocrine system cells
• Stomach wall


  • Tunica mucosa: Thrown into deep longitudinal folds (rugae) when empty, reduced when full
  • Tunica submucosa: Loose connective tissue with parasympathetic ganglia (Meissner's plexus)
  • Tunica muscularis: Inner circular, outer longitudinal smooth muscle layers with parasympathetic ganglia (Auerbach's plexus)
  • Tunica serosa: Connective tissue, covered by simple squamous epithelium (mesothelium)
• Fundic stomach


  Fundic glands open into the bottom of the gastric pits, and each gland is subdivided into an isthmus, a neck, and a base
• Tunica mucosa


  • If empty stomach, this layer is thrown into deep longitudinal folds called rugae that extend from lamina muscularis mucosae to the lumen
  • If full stomach, the rugae are much reduced in size (distension of the tunica mucosa) to accommodate the presence of a large amount of food material
• Tunica submucosa


  • Typical loose connective tissue contains parasympathetic ganglia located in submucosal plexuses also known as Meissner's plexuses
• Tunica muscularis


  • Typical smooth muscle consisting of at least two layers, an inner circular layer and an outer longitudinal layer
  • Parasympathetic ganglia located between the two muscle layers in the myenteric or Auerbach's plexus

  
• Tunica serosa


  • Typical small amount of loose connective tissue
  • Overlying simple squamous epithelium or mesothelium
• Four cell types in the gastric gland

  • Mucous Neck cells
  • Parietal (oxyntic) cells
  • Chief (zymogenic) cells
  • Neuroendocrine cells
• Mucous Neck cells


  • Found dispersed between the parietal cells
  • Secrete a mucus that is thinner than that secreted by the surface mucous cells
  • Mucus protects other glandular cells from action of proteases and HCl
• Parietal (oxyntic) cells


  • Found throughout the gastric gland
  • Round cells that contain distinct eosinophilic (pink) cytoplasm
  • Round, prominent nucleus
  • Secrete HCl and intrinsic factor, needed for absorption of vitamin B12
• Chief (zymogenic) cells


  • Found mostly near the base of the gastric glands
  • Very basophilic (purple) containing basally positioned nucleus
  • Prominent basophilic apical cytoplasm filled with many ribosomes
  • Secrete pepsinogen, which is converted to pepsin in the acidic milieu of the stomach
• Neuroendocrine cells


  • Difficult to distinguish by conventional light microscopy
  • Several types are present
  • Some secrete serotonin, gastrin, glucagon & somatostatin, among other hormones
• Submucosal plexus (Meissner's plexus)

  • Ganglionic complex in the Submucosal tunic
  • Controls contraction of smooth muscle in lamina propria
  • Found throughout entire length of tract
• Myenteric plexus (Aeurbach's plexus)


  • Located between layers of tunica muscularis
  • Coordinates contractions of muscularis
  • Controls stomach contraction & peristalsis
  • Responsive to ANS input
• Parts of the compound stomach in ruminants

  • Rumen
  • Reticulum
  • Omasum
  • Abomasum
• Rumen


  • Tunica mucosa: long (1.0-1.5 cm long) conical projections called papillae that extend in to the lumen
  • Lamina epithelialis mucosae - keratinized stratified squamous
  • Lamina propria - typical; no glands
  • Lamina muscularis- absent
  • Tunica submucosa: merges with LP = Propria-submucosa; no glands or lymphoid aggregates
  • Tunica muscularis: typical
  • Tunica serosa: typical
• Reticulum


  • Tunica mucosa: When viewed from the lumen, looks like a "honeycomb" or reticulum
  • Honeycomb is a series of connected vertical primary folds that give rise to secondary and tertiary papillae which project into the lumen
  • Lamina muscularis mucosa: a layer of smooth muscle extends from the tips of the papillae down to the position of the lamina muscularis mucosa although in the reticulum this layer is not quite typical
  • Other tunics are typical

  
• Omasum


  • Tunica mucosa: Highly keratinized stratified squamous epithelium
  • Lamina propria: Sparse loose connective tissue
  • Laminae muscularis mucosae extends into the primary laminae usually in two layers
  • Between the two layers of the laminae muscularis mucosae there is a layer of smooth muscle belonging to the tunica muscularis
  • These three layers of smooth muscle intertwine as they extend toward the tip of the laminae and eventually fuse to form one large mass of muscle at the tip

  
• Abomasum


  • Histologically it is essentially the same as a simple stomach
  • Only 3 layers seen: Mucosa, Muscularis, Serosa
• Villi


  Finger-like projections that increase the surface area to enhance the process of absorption
• Duodenum


  • Presence of Brunner's glands in the submucosa
  • Plicae circulares present
  • Longest villi of all three regions
  • Highest number of goblet cells

  
• Jejunum


  • No glands in submucosa
  • Lymphoid nodules present but not as prominent as Peyer's patches in ileum
  • Finger-like villi

  
• Ileum


  • No glands in submucosa
  • Peyer's patches
  • More goblet cells
  • Shorter villi

  
• Duodenum, jejunum and ileum are regions of the small intestine

  
• Large Intestine


  • Lack/Absence of villi
  • Less-coiled
  • Simple tubular intestinal glands with lots of Goblet cells
  • Absence of Panneth cells
  • Increase number of lymphatic nodules
  • No plicae circulares

  
• Taenia coli


  Three broad longitudinal bands of smooth muscle in the colon