Histology of Alimentary canal in a Nutshell

Histology of Alimentary canal

GI tract functions:

• Sophagus- transport
• Stomach- mechanical disruption , absorption of water and alcohol
• Small intestine – chemical and mechanical digestion / absorption. It is here that majority of nutrients are absorbed.
• Large intestine – absorb electrolytes (salts and water) and vitamins
• Rectum and anus – defecation / makes poo go away

Layers of the GI tract:Starting with lumen

• Mucosa: 3 layers are always present in GI tube with varying thickness
  • Epithelium
  • Lamina propria- CT
  • Muscularis mucosae – thin layer of muscle
• Submucosa: contains vessels, some nerves. Contains glands in 2 regions of GI. Also contains neuronal plexus = submucosal plexus in most portions of GI
• Muscularis: Has 2 layers of smooth muscle
  • Inner circular
  • Outer longitudinal
  • There is another plexus of nerves between these two = myenteric plexus.
• Serosa: is connective tissue, has epithelium covering

Major changes from Proximal to Distal GI tube:

• Have glands in submucosa in duodenum / esophagus
• Have excretory ducts that come into proximal gut from pancreas (enzymes) and liver (bile salts).  There are no ducts in distal gut.
• Epithelium changes in distal gut.  Have lymphoid nodules in distal.
• Proximal: Esophagus / small intestine.  Distal gut = colon / rectum.

Details on layers:

• Mucosa:
  • Epithelium: Stratified squamous (esophagus and anus), is protective.  The remainder is simple columnar.  It secretes enzymes and absorbs nutrients.  Some specialized cells:

1. 1. • Goblet cells = secrete mucous onto cell surfaces. Not in esophagus or stomach. Increase in number as move through tube.
      • Enteroendocrine cells = “EC cells” secrete hormones. In the stomach and remainder of tract. Secrete 13+ types of hormones / neurotransmitters / enzymes.  Responsible for peristalsis.

1. • Lamina propria: Thin layer of loose connective tissue. Exception is in the esophagus where have more of a dense connective tissue (strength / protection)
   • Muscularis mucosae: Thin layer of smooth muscle.
   • Submucosa: Loose connective tissue.

1. 1. • Find blood vessels, lymphatic tissue.
      • Find glands only in esophagus and in duodenum.
      • Meissner’s plexus – enteric neurons

• Muscularis externa
  • Skeletal muscle – voluntary control in mouth, pharynx, upper esophagus and anus.  Control swallowing and defecation.
  • Smooth muscle – involuntary control

1. 1. • Inner circular fibers and outer longitudinal fibers (except in stomach where 3 layers exist)
      • Mixes, crushes, and propels food along by peristalsis

1. • Auerbach’s plexus (myenteric: innervation of circular and longitudinal smooth muscle layer.

So far, have large and complicated array of neurons that are intrinsic innervations of the GI tract.

Innervation of the Gut:

• all neurons and support cells are derived from the neural crest. These are intrinsic neurons.
• The GI does still receive signals from extrinsic sources, but are not required to drive muscles. They only modify the activity (sympathetic and parasympathetic sources).
• Scattered in epithelium are endocrine cells. They secrete mainly serotonin.  The stretch (presence of food) on these EC cells cause the release of seratonin.
• Myenteric plexus: mostly motor, regulates peristalsis. Located between muscle layers in muscularis externa.
• First neural input to submucosal ganglia (Meissner’s)à communicate with myenteric neurons (Auerbach’s)à drive peristalsis
• Submucosal plexus: Meissner’s plexus. Is secretory-motor. Release hormones and peptides.  Innervate the mucosa and are located in the submucosa.
• Autonomic inputs: extrinsic neurons.
  1. Parasympathetic – acetylcholine increases motility in GI
  2. Sympathetic – norepinephrine relaxes the gut

Esophagus:

• Moves food from oral cavity down to stomach
• Mucosa is stratified squamous epithelium (nonkeratinized)
  • Muscularis mucosae = single layer of smooth muscle
  • Lamina propria is dense irregular CT, has large amount of diffuse lymphoid tissue, arterioles and venules
  • Glands:

1. 1. • Esophageal-cardiac glands located at junction of esophagus and stomach and at junction of esophagus and pharynx
      • Sero-mucous glands: along entire length of esophagus

• Submucosa:
  • Dense fibroelastic CT, has esophageal glands
  • Serous glands: secrete pepsinogen and lysozyme
• Muscularis Extera
  1. Upper region: All skeletal
  2. Middle region: Skeletal and smooth
  3. Lower region: All smooth
• Adventitia / serosa: Has adventitia to diaphragm

Stomach:

• 4 Regions
  • Esophagus enters into cardia
  • Top = Fundus
  • Body = main portion. Has the same histology as the fundus.
  • Pylorus
• Chemical digestion
  • Hydrochloric acid – main chemical for digestion
  • Pepsin
  • Renin
  • Gastric lipase
• Muscularis: Where major difference is
  • Has 3 layers of smooth muscle instead of 2
  • Includes oblique layer of muscle underneath the longitudinal and circular layer.
• Epithelia has different structure
  • Invaginations (gastric pits) into lamina propria.
  • Gastric glands are present in the lamina propria. These glands open into the gastric pits.
  • These epithelial cells also have junctions at their top that prevents HCl from entering internal portion of lamina propria.
• No neurons in the submucosa layer
• Rugae: Folds of mucosa and submucosa.
• Muscularis externa:
  • Inner oblique layer (exception of GI tract)
  • Middle circular layerà forms the pyloric sphincter
  • outer longitudinal layer

Stomach:

1. 1. Gastric glands: 3 portions
   1. Base: blind end at the bottom
   2. Isthmus: junction to the gastric pit
   3. Neck: short connection between the base and the isthmus

Note that the gland is almost the same length as the gastric pit.

1. has 6 cell types
   1. surface lining cells and mucous neck cells produce mucous.  This is not the same as goblet cells.
   2. Regenerative cells are in the isthmus region at the top of the gland.  This changes when move to pyloric region in which they are found lower in the gland.
   3. Parietal cells: hydrochloric acid secretion.  Gastric intrinsic factor is synthesized and secreted along with the HCl.  This factor is required for absorption of B12.
   4. Chief cells: primarily in base of gland.  Synthesizes pepsinogen, gastric lipase, and rennin.  Pepsinogen / gastric lipase begin digestion of proteins / lipids.  HCl turns pepsinogen into pepsin.
   5. Enteroendocrine cells: secrete glucagons and gastric inhibitory peptideà influences parietal cells to regulate secretion of HCl.  These are concentrated at the base of the gastric pits.
2. gastrin: a hormone from G cells
   1. releases more gastric juice
   2. increases gastric motility
   3. relaxes the pyloric sphincter while constricting the esophageal sphincter.  Timing so that chyme is released in portions.

Stomach mucosa detail:

• Epithelium has 6 cell types, is simple columnar epithelium
• Fill entire lamina propria
• Cardiac mucosa:
  • Have primarily surface lining cells
  • Do not find chief cells at base of pit.
  • Gastric glands are highly coiled with shallow gastric pits
• Fundic mucosa:
  • Majority of cells are surface lining cells (mucosa)
  • Regenerative lining cells→ replace all of the other cell types except for EC cell.  They will migrate in both directions to replace cells.
  • The gastric glands in the fundus and body are long and straight while gastric pits are short.
• Pyloric Mucosa:
  • Mainly mucous neck cells
  • Have fewer chief cells.
  • Gastric glands here are branched and deep with deep gastric pits.

HCl secretion: By parietal cells

• Cells have receptors for
  • Gastrin – stretch on EC cells
  • Histamine – stretch on EC cells
  • Acetylcholine – vagus nerve, psychological
• Have structural differences depending on what state they are in (resting, stimulation, or active state).  Active state shows canaliculi whereas resting state shows tubulovesicles.

Small intestine

• 20 feet long, 1 in diameter
• 3 parts
  • Duodenum – 10 in.
  • Jejunum – 8 feet
• Ileum – 12 feet

Duodenum: Has specializations in submucosa.

Ileum: Has peyer’s patches – main immunological control mechanism, however there is still diffuse lymphatic tissue in entire GI tract.

Small intestine structure:

• Crypts of lieberkuhn = Occur between outpocketings (villi) in small intestine.  Villi are evaginations that stick out into lumen of gut whereas crypts of lieberkuhn open between villi, not at their base.
  • Inside the crypt: EC cells release digestive enzymes to break down lipids (turn them into chylomicrons), and proteins.  EC cells are also on villi.
  • Villi also contains goblet cells→ different kind of mucous than in stomach.  They increase in number as move down GI tract.
  • Vast numbers of surface absorptive cells all over villi
  • Regenerative cells are in the base of the crypt.
  • Paneth cells – secrete lysozyme, at base of pit.  Is diagnostic for small intestine.  Unknown function.
• Histological features
  • Plicae circularis = Folds of the small intestine. Includes the mucosa, muscularis mucosa, and submucosa.
  • Villi = Expand surface area.  These are large structures of mucosa and submucosa.
  • Microvilli = Even more surface area. These invest the villi.
• Epithelium: Surface absorption→ water, reesterify fatty acids, form chylomicrons, transport absorbed nutrients.

Regional differences:

• Duodenum:
  • Villi are broad, numerous, tall. Tight connections are present to stop materials from falling between them.
  • Few goblet cells.
  • Has specialization called Brunner’s glands.
• Jejunum:
  • Villi are narrow, shorter, less dens
  • Many goblet cells
• Ileum
  • Villi are the shortest, narrowest, smallest amount
  • Has peyer’s patches – with microfold cells which are considered MAC cells.
  • Has many goblet cells

Cells of intestinal gland:

• In the crypt- cells will secrete IgA.  In the ileum, if have Ag presentation→ plasma cells will reside in crypt to secrete antibodies.
• EC cells are on entire villus structure. Has a tear-drop shape
• paneth cells are filled with secretory granules, are at the base of crypts.
• Absorptive cells under constant renewal due to exfoliation in the lumen.

Crypts of Lieberkuhn:

• Simple or branched tubular glands
• Open between villi.
• Comprised of surface absorption cells, goblet cells, regenerative cells, EC cells, paneth cells = secrete lysozyme.

Submucosa of Duodenum:

• Brunner’s Glands: branched tubulo-alveolar glands. Secretes mucous-alkaline fluid to neutralize chyme.  This is protection of the duodenum.
• If submucosa contains glands and it isn’t the esophagus (tell by epithelium) you are in the duodenum.

Specialization of Ileum: Peyer’s patches.

There are no specializations in the jejunum, it has the same structure as the ileum.

Large intestine:

• 5 ft long by 2 inches in diameter
• Rectum = last 8 inches
• Anal canal = Last 1 inch of GI tract
  • Internal sphincter = smooth muscle and involuntary
  • External sphincter = skeletal muscle and voluntary control

Structure:

• There are no villi in the large intestine, only crypts.
• Crypts have same kind of cells, but are largely taken over by goblet cells. Have a large number of aborptive cells to take in water, electrolytes.  There are EC cells present at base of crypt.  Crypts lack paneth cells.
• A structure in GI without villi and only crypts must be large intestine.
• Muscular layer:
  • Has normal circular layer
  • Unusual longitudinal muscle. Outer longitudinal layer forms three fascicles =  taenia colià form haustra coli by maintaining constant tonus (mild contraction).  Constant tonus causes sacculations AKA haustra coli.
• Serosa = Visceral peritoneum.
• Beneath the adventitia = Investment of fat.
• All of the specializations are meant to hold feces in place and compact it.

Rectum and anal canal: crypts are fewer and deeper than colon. Have an increased number of goblet cells:

• Rectal anal juncton: epithelium becomes stratified squamous. Branched circumanal glands.
• Epithelium at external anal sphincter is keratinized stratified squamous epithelium.

Anus:

• Submucosa thickens
• The outer longitudinal muscle disappears where puborectalis muscle meets GI.
• The internal circular layer of muscle forms the internal anal sphincter.
• The external anal sphincter is formed by skeletal muscle.
• Epithelium has changed from cuboidal to keratinized hairy skin.
• The junctional line for epithelium change = pectinate line.
• Have column of internal folds of mucosa = columns of morgangi which form internal valves (anal valves) = give structural support for feces.  Have a series of anal glands that secrete watery mucous which aids in defecation.
• Have both internal and external hemorrhoidal plexus of vessels.