DIGESTIVE SYSTEM

• Digestion

  • (breaking down of ingested food into absorbable substances), absorption of digested substances
  • excretion of undigested materials
  • involves the mechanical action of the digestive tract
  • chemical action of the substances and enzymes that are secreted by the digestive glands
  • empty their secretions into the lumen of the tract
• Digestive system

  consist of:

  • Digestive tract (alimentary canal)
  • Digestive glands
• Digestive tract

  • a long tube that consists of several segments that differ morphologically and functionally
  • proximal to distal:
  • oral cavity
  • pharynx
  • esophagus
  • stomach
  • small intestine
  • large intestine
• Digestive glands

  • glands that are embedded in the walls of the digestive tract and the accessory glands of the digestive system
  • located outside the digestive tract but are connected to the tract by ducts
  • include
  • major salivary glands
  • exocrine portion of the pancreas
  • liver gallbladder
• 4 histologic layers of the walls of digestive tract:

  • mucosa (tunica mucosa; mucous membrane)
  • submucosa (tunica submucosa)
  • muscularis externa (tunica muscularis)
  • adventitia/serosa (tunica adventitial serosa)
• Mucosa (Tunica Mucosa; Mucous Membrane)

  • three components:
  • epithelium
  • lines the luminal surface
  • lamina propia
  • loose connective tissue layer that underlies the epithelium
  • contains fine blood, lymphatic vessels, and mucosa-associated lymphoid tissue (MALT)
  • numerous digestive glands are embedded
  • muscularis mucosae
  • refer to the thin sheet of smooth muscle fibers
  • outermost layer of the mucosa
  • inner layer: circularly-oriented
  • outer layer: longitudinally-oriented.
• Submucosa (Tunica Submucosa)

  • lies external to the muscularis mucosae
  • loose connective tissue
  • connective tissue
  • denser
  • abundant
  • vascular
  • is also well supplied with GALT
  • submucous plexus (of Meissner)
• Submucous plexus (of Meissner)

  • submucosa of the esophagus, stomach, and intestines contains autonomic neurons that form a ganglionated network
• Muscularis Externa (Tunica Muscularis)

  • has 2 relatively thick smooth muscle coats or layers
  • inner layer - circularly-oriented
  • outer layer - longitudinally-oriented
  • responsible for the mechanical digestion of food
  • contraction
  • interstitial cells of Cajal (ICC)
  • myenteric plexus (of Auerbach)
• Interstitial cells of Cajal (ICC)

  • a population of cells that act as pacemakers for the contraction of the smooth muscles
  • located between the nerves and the smooth muscle cells
  • methylene blue staining and zinc-iodide-osmium impregnation
  • contain actin and myosin filaments, dense bodies, and numerous mitochondria
  • intermediate filament is vimentin
  • sER is flattened
  • mitochondria are smaller
• Myenteric plexus (of Auerbach)

  • between the inner and outer layers of the muscularis extern a of the esophagus, stomach, and intestines
  • composed of ganglionated autonomic neurons
  • widely interconnected with submucous plexus
  • comprise the enteric division (enteric nervous system)
• Enteric division (Enteric Nervous System)

  • responsible for regulating the contractions of the smooth muscles in the muscularis mucosae and muscularis externa
  • secretions of the mucosal and submucosal glands
  • caliber of the blood vessels
  • water absorption and electrolyte exchange
• Adventitia/Serosa (Tunica Adventitia/Serosa)

  • outermost histologic layer of the digestive tract
  • loose connective tissue
  • serosa
  • adventitia
• serosa

  • areas of the digestive tract covered by peritoneum, and connective tissue is lined externally by mesothelium
• Adventitia

  • areas where the tract has no peritoneal covering
• General Histological Organization of the Digestive Tract

  
• Lip (Mucocutaneous Junction)

  
• Lip (Cutaneous Surface)

  
• Oral cavity (Mouth)

  • 2 regions:
  • vestibule
  • oral cavity proper
  • continuous with the pharynx at the faucial isthmus
  • mechanical and chemical digestion of food starts here
• Vestibule

  • region of the mouth that is anterior to the teeth and gums (gingivae)
• Oral cavity proper

  • formed by the hard and the soft palates, the floor mainly by the tongue and the lateral walls by the cheeks
• Submucosa (Oral cavity)

  • submucosa is not well-delineated from the lamina propria because there is no muscularis mucosae
  • contains some small, mucus-secreting, and compound tubuloalveolar glands
  • labial glands (in the upper and lower lips)
  • buccal glands (in the cheek)
• Palatine glands

  • mucus-secreting glands of the hard palate
• hard palate and gums

  • keratinized stratified squamous
  • no submucosa
  • lamina propria merges with the periosteum of the underlying bones
• Lip (Mucosal Surface)

  
• Muscularis Externa (Oral Cavity)

  • present only in the lips and cheeks
  • skeletal muscles - responsible for mastication and certain facial expression
• Adventitia (Oral Cavity)

  • External to the muscularis externa in the lips and cheeks
  • binds the muscularis externa to the overlying skin
• Tongue

  • responsible for mastication, deglutition, and speech
  • contains most of the sense organs for taste
  • anterior tongue - 2/3 of tongue
  • posterior tongue - 1/3 of tongue
  • foramen cecum - "V" marks the site of ingrowth of the thyroglossal duct from which the thyroid gland originated
  • skeletal muscle

  
• Anterior tongue

  • forms much of the floor of the oral cavity
  • forms small protrusions called lingual papillae

  
• Mucosa (Tongue)

  • ventral - smooth
  • dorsal - rough
  • made up of dense connective tissue
• Posterior tongue

  • forms part of the anterior wall of the oropharynx
  • attached portion of the tongue is called its root
  • forms lumps due to the presence of the lingual tonsils

  
• Lingual Papillae

  • projections of the lingual mucosa and are confined to the dorsal surface of the anterior tongue
  • stratified squamous epithelium
  • tip - keratnized
  • 3 types
  • filiform
  • fungiform
  • circumvallate

  
• Filiform papillae

  • most numerous of the lingual papillae
  • slender and tapering
  • found all over the dorsal surface of the anterior tongue
• Fungiform papillae

  • have flattened surfaces
  • broader and scattered
  • connective tissue core contains numerous capillaries such that the papillae appear as pinhead-sized red dots to the naked eye
  • Taste bud - occasionally present
• Circumvallate papillae

  • largest (1 to 3 mm in diameter) but fewest (only 6 to 14) of the lingual papillae
  • arranged in a single file along the sulcus terminalis
  • cylindrical structures
  • 200 to 300 taste buds

  
• Muscles (Tongue)

  • skeletal muscle fibers
  • three planes: horizontal, vertical, and longitudinal
  • attached to the hyoid bone, mandible, styloid processes of the temporal bones, soft palate, and pharyngeal wall
• Glands (Tongue)

  • glands of von Ebner
  • anterior lingual glands (of Nuhn and Blandin)
• Glands of von Ebner

  • Embedded in the lamina propria and muscles in the region of the circumvallate papillae are serous glands
  • compound tubuloalveolar glands whose ducts open into the moat that surrounds the circumvallate papillae
  • its secretions flush the moat and serve as a medium where substances are dissolved in order to be tasted
• Anterior lingual glands (of Nuhn and Blandin)

  • submucosa in the ventral surface of the apex of the tongue
  • structurally similar to the glands of von Ebner
• Taste Buds

  • responsible for the sense of taste
  • ovoid structures that are embedded in the epithelium of the circumvallate papillae
  • 50 to 80 um tall and 30 to 50 um wide
  • consists of 50 - 150 densely-packed fusiform cells whose apices converge on a small opening on the surface of the epithelium called taste pore