histology

• Alimentary canal


  Part of the digestive system
• Accessory digestive structures


  • Tongue
  • Teeth
  • Salivary glands
  • Pancreas
  • Liver
  • Gall bladder
• Tongue


  • Composed of striated muscle
  • Lingual muscles are both extrinsic and intrinsic
  • Striated muscle arranged in bundles in three planes at right angles to each other
• Some fat is found between muscle bundles in the tongue
• Sulcus terminalis


  Divides the tongue into anterior 2/3 and posterior 1/3
• The apex of the V-shaped sulcus terminalis points posteriorly and is the location of the foramen cecum, the remnant of the site from which the thyroid gland formed
• Lingual papillae


  • Numerous mucosal irregularities and elevations covering the dorsal surface of the tongue anterior to the sulcus terminalis
  • Four types: filiform, fungiform, circumvallate, and foliate
• Filiform papillae


  • Most numerous and smallest
  • Conical
  • Elongate projections of connective tissue covered with highly keratinized stratified squamous epithelium
  • No taste buds present
  • Found on entire anterior dorsal surface of tongue
• Fungiform papillae


  • Mushroom-shaped
  • Located on dorsal surface
  • More numerous near the tip
  • Taste buds present in epithelium
• Circumvallate papillae


  • Large, dome-shaped
  • Located just anterior to sulcus terminalis
  • Humans have 8-12
  • Each papilla surrounded by a moat-like space lined with stratified squamous epithelium
• Circumvallate papillae


  • Lateral surface epithelium contains many taste buds
  • Ducts of lingual salivary glands (von Ebner's glands) deposit a serous secretion into the moat to clean debris
• Foliate papillae


  • Aligned at right angles to the long axis of the tongue
  • Located on the posterior lateral surface
  • Contain many taste buds in the epithelium of the facing walls of neighboring papillae
  • Small serous glands empty into the clefts
• Taste buds


  Detect five basic stimuli: sweet, salty, bitter, sour, and umami
• Type I familial dysautonomia (Riley-Day syndrome) causes severe hypogeusia (decreased ability to detect taste) due to the developmental absence of taste buds and fungiform papillae
• Type I familial dysautonomia is an autosomal recessive disorder caused by a mutation in the DYS gene, and in addition to hypogeusia, individuals experience other symptoms related to developmental defects in the peripheral and autonomic nervous systems
• Alimentary canal


  • Same basic construction end to end
  • Four distinct layers: mucosa, submucosa, muscularis, serosa (adventitia)
• Mucosa - Epithelium


  • Functions to facilitate passage of the bolus and provide a selectively permeable barrier between body and external environment
  • Epithelium absorbs products of digestion and transports them to the vascular system
• Mucosa - Lamina Propria


  • Contains glands
  • Components of the immune system
  • Vessels to receive absorbed substances (fenestrated type)
  • Absorption occurs in small and large intestine
  • Numerous lymphatic capillaries receive lipids and some proteins
• Mucosa - Muscularis Mucosae


  • Forms boundary between mucosa and submucosa
  • Consists of inner circular and outer longitudinal layers
  • Can produce movement of mucosa independent of movement of entire gut wall
• Submucosa


  • Consists of moderately dense irregular connective tissue
  • Contains larger blood vessels that send branches to mucosa, muscularis externa, and serosa
  • Contains lymphatics
  • Contains glands in esophagus and initial part of duodenum
  • Contains nerve plexuses (Meissner's plexus) that innervate smooth muscle
• Muscularis Externa


  • Usually consists of two concentric thick layers of smooth muscle
  • Inner layer forms tight spiral circular layer
  • Outer layer forms loose spiral longitudinal layer
  • Contains the myenteric plexus (Auerbach's plexus)
  • Part of the enteric division of the autonomic nervous system
• Hirschsprung disease (congenital megacolon) is caused by mutations that result in the absence of the enteric nervous system in one segment of the distal colon
• Serosa


  • A membrane containing simple squamous epithelium (mesothelium)
  • Absent in extraperitoneal organs, which are covered by loose connective tissue (adventitia)
• Esophagus


  • 25 cm long
  • Narrowest part of alimentary path
  • Most muscular segment of GI tract
  • Lumen normally collapsed by tonus of muscularis externa, throwing mucosa into longitudinal folds
  • Conforms to the typical 4-layered alimentary tube structure
• Structural mechanisms for protection of tissue and facilitation of passage in the esophagus

  • Lined with non-keratinized stratified squamous epithelium
  • Esophageal glands in submucosa and esophageal cardiac glands in lamina propria
  • Muscularis externa is thick with striated muscle in upper portion to propel food rapidly (peristalsis)
  • Diffuse lymphatics in lamina propria
  • Nodules
• Esophageal glands


  • Two types: esophageal glands proper in submucosa and esophageal cardiac glands in lamina propria
  • Esophageal glands proper produce slightly acidic mucous to lubricate the lumen
  • Esophageal cardiac glands produce neutral mucous to protect against regurgitated material
• Barrett's esophagus is an abnormal change (metaplasia) in the cells of the lower end of the esophagus caused by damage from chronic acid exposure, where the normal squamous epithelium is replaced by an intestinal-type columnar epithelium
• The stomach is the expanded part of the alimentary canal and has the same basic organization as the rest of the gut: mucosa, submucosa, muscularis, and serosa
• Stomach


  • Inner surface has longitudinal folds called rugae
  • Lining absorbs some water, salts, lipid-soluble drugs, alcohol, and certain drugs like aspirin which can damage the surface epithelium
• Gastric pits or foveolae


  Numerous openings seen in the mucosal surface of the stomach
• Histological regions of the stomach

  • Cardia
  • Pylorus
  • Fundus
• Cardiac glands


  • Limited to the narrow region of the stomach near the esophageal orifice
  • Tubular, tortuous, sometimes branched glands composed of mucous secreting cells
  • Secretion contributes to gastric juice along with that of esophageal cardiac glands
• Fundic glands


  • Also called gastric glands
  • Present throughout the gastric mucosa except where cardiac and pyloric glands occur
  • Simple branched tubular glands that extend from the bottom of the gastric pits to the muscularis mucosae
  • Several glands open into one gastric pit
• Cell types in fundic glands

  • Mucous neck cells
  • Chief cells
  • Parietal cells (oxyntic cells)
  • Enteroendocrine cells
  • Undifferentiated cells