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
- 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:
- Epithelium: Stratified squamous (esophagus and anus), is protective. The remainder is simple columnar. It secretes enzymes and absorbs nutrients. Some specialized cells:
- 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.
- 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.
- 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
- Inner circular fibers and outer longitudinal fibers (except in stomach where 3 layers exist)
- Mixes, crushes, and propels food along by peristalsis
- 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.
- Parasympathetic – acetylcholine increases motility in GI
- Sympathetic – norepinephrine relaxes the gut
- 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
- 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
- Dense fibroelastic CT, has esophageal glands
- Serous glands: secrete pepsinogen and lysozyme
- Muscularis Extera
- Upper region: All skeletal
- Middle region: Skeletal and smooth
- Lower region: All smooth
- Adventitia / serosa: Has adventitia to diaphragm
- 4 Regions
- Esophagus enters into cardia
- Top = Fundus
- Body = main portion. Has the same histology as the fundus.
- Chemical digestion
- Hydrochloric acid – main chemical for digestion
- 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
- 1. Gastric glands: 3 portions
- Base: blind end at the bottom
- Isthmus: junction to the gastric pit
- Neck: short connection between the base and the isthmus
Note that the gland is almost the same length as the gastric pit.
- has 6 cell types
- surface lining cells and mucous neck cells produce mucous. This is not the same as goblet cells.
- 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.
- Parietal cells: hydrochloric acid secretion. Gastric intrinsic factor is synthesized and secreted along with the HCl. This factor is required for absorption of B12.
- 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.
- 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.
- gastrin: a hormone from G cells
- releases more gastric juice
- increases gastric motility
- 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.
- 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.
- 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.
- Villi are narrow, shorter, less dens
- Many goblet cells
- 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.
- 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
- 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.
- 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.