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What are some ways in which the alimentary canal protects the body from pathogens?
- 1. Specialized epithelial cells (M-cells) for antigen sampling
- 2. Large aggregations of diffuse and nodular lymphoid tissues (Peyer's patches in ileum)
- 3. Mucus coating traps potential pathogens (eg. bacteria)
GI Tract Organization
- 1. Lumen →
- 2. Mucosa (epithelium, lamina propria, muscularis mucosa)
- 3. Submucosa (may have glands, nerves)
- 4. Muscularis Externa
- 5. will either have an Adventitia or Serosa, depending on where in GI tract you are
- lined by epithelium
- covered by the Lamina Propria: loose CT
- then covered by the Muscularis Mucosa: smooth muscle layer
- all together = MUCOSA (epithelium, loose CT, SM muscle)
- another muscular layer comprised of three components
- 1. muscularis mucosa: thin layer of smooth muscle that can assist in emptying glands
- 2. lamina propria: loose CT that contains immune cells
- 3. luminal epithelium: epithelial layer covering entire luminal surface of GI tract
- contains many of the structural and functional specializations of the different regions of the GI tract
- (eg. intestinal villi, crypts, gastric pits or glands)
- a connective tissue layer that lies between the muscularis externa and the muscularis mucosa
- contains blood vessels, nerves, parasympathetic ganglia, & lymphatics
- muscular layer deep to the serosa that consists of two layers of smooth muscle – an outer one that is longitudinal and inner one that is circular
- wave-like contractions of these opposed muscle layers (peristalsis) results in the efficient propulsion of food
- contractions are regulated by parasympathetic nerves & ganglia (myenteric plexus) that lie between the muscle layers
- outermost layer along most of the GI tract
- consists of two components: 1) simple squamous epithelium (mesothelium) that forms a slippery surface allowing intestines to slide over one another & 2) adventitia made of loose CT
- the serosa contains blood vessel, nerves & lymphatics
What is the sole conduit for blood vessels/nerves to enter AND exit the intestines, and for lymphatics to exit?
- the outermost layer connecting the GI tract to the body wall in regions where there is NO serosa, due to the absence of an epithelium
- most of the esophagus & part of the gall bladder
- In the esophagus it is a stratified squamous epithelium
- throughout the remainder of the tract it is a simple columnar epithelium (gastric epithelium)
- this epithelium is also continuous with the epithelial lining of the ducts and glands of the accessory digestive organs (the pancreas, liver & gall bladder)
Where is the Serosa layer located in the esophagus?
- the outer esophagus lining = Adventitia until it crosses the diaphragm
- in the Thorax the outer layer = Adventitia
- within the abdominal cavity it's outermost region consists of a Serosa
Where might skeletal muscle be found in the esophagus?
in the upper 1/3
Where might both skeletal & smooth muscle be found in the esophagus?
in the middle 1/3
Where is only smooth muscle found in the esophagus?
How does the muscularis mucosa run in the esophagus?
located in between muscle layers of the gut & esophageal wall of the Muscularis Externa
- nerve plexus in the submucosa that innervates the muscularis mucosa to control it’s contraction
- further down in the GI system it controls the emptying of glands
Where are regions of the GI tract where the submucosa contains glands?
1. esophagus: producing a lubricating/protective mucous
visually the only place there's a submucosal gland deep to a SSNKE = esophagus
2. duodenum (small intestine): neutralizes partially digested acidic chyme (food from the stomach)
What happens to the luminal epithelium at the esophageal-stomach junction (squamo-columnar)?
- it undergoes an abrupt change from the stratified squamous epithelium (of the esophagus) to the simple columnar epithelium (the gastric epithelium) of the stomach (and subsequently the intestines)
- when acidic contents of the stomach chronically bypass the lower esophageal sphincter (one-way valve), the esophageal SSNKE epithelial cells can undergo a transition to simple columnar epithelium
- associated with the subsequent development of esophageal adenocarcinoma (lethal)
Regions of the Stomach
- Cardiac region: short pits & glands; mucous & enteroendocrine cells
- Fundus/Body region: short pits, long glands; thick surface mucous, mucous neck (clear), parietal, chief, & enteroendocrine cells
- fundus also has SM muscle cells running through mucosa to contract it, forcing gland contents into lumen
- Pyloric region: long pits, short glands; mucous & enteroendocrine cell
Where is intrinsic factor synthesized?
- in the stomach by parietal (fried egg) cells
- made in the stomach but used in the gut for the absorption of vitamin B12
- issues with stomach lining or parietal cells can cause Vitamin B12 deficiency & eventually pernicious anemia
Where is there a third, oblique muscle layer present in the Muscularis Externa of the GI tract?
- in the stomach where food is also subjected to a grinding action
- outer (longitudinal), middle (circular) & inner (oblique)
What are the 5 types of cells in the glands of the stomach (including the pits)?
1. surface mucous: simple columnar; protective against acid made in stomach
2. mucous neck
4. parietal: make HCl & intrinsic factor
5. gastric chief: make pepsinogen --> pepsin in acid
the cardiac & pyloric glands contain ONLY the first 3 cell types
the fundic/gastric glands contain all five
What are the general turnover rates for gastric cells as a result of stem cell proliferation (derived from base of gastric glands)?
- mucous: 3-5 days
- endocrine: weeks
- chief & parietal: 90-200 days (months)
- PAS stained surface mucus & mucous neck cells
- mucus cells making thick bicarbonate mucus that serves to protect the stomach lining
- Parietal cells
- make HCl & Intrinsic Factor
- look like red (eosinophilic) fried eggs
- packed with mitochondria (red dots)
- clear areas in cell represent Intracellular Canaliculus - implies cells were active (secreting HCl)
- purple cells = chief cells
- basophilic because of abundant RER making pepsinogen
- typical protein secreting cells
- have a ton of zymogen granules that contain inactive enzymes: pepsinogen, lipase, chymosin
- when released into acid environment of gut from chief cells, the low pH (<5) causes autocatalytic activity, turning pepsinogen into pepsin
- when pepsin gets to neutral pH of duodenum, it's inactivated
- chronic inflammation of the stomach mucosa leading to loss of gastric glandular cells & their eventual replacement by intestinal & connective tissue
- ability for stomach to secrete substances is impaired (deficiency in intrinsic factor leads to vitamin B12 deficiency, pernicious anemia)
- can be caused by autoimmunity or by Helicobacter pylori infection
- will see lymphocyte infiltration
H. Pylori Infection
- bacteria convert urea to ammonia via urease
- this neutralizes acid of the stomach, & allows bacteria to penetrate into mucosal lining
- bacteria have proteins that look similar to stomach lining itself, so when the stomach makes antibodies against the bacteria, it damages self (autoimmune attack to the stomach lining)
- if chronic this can lead to Gastritis
- an cell abundant in the GI tract that requires special staining
- make numerous products including Gastrin, Somatostatin, Ghrelin, Enterochromaffin
- here they're visualized in the pyloric region (end) of the stomach (can see long pits, short glands)
a peptide hormone secreted by Eneteroendocrine (G) cells that stimulates secretion of HCl by parietal cells
- inhibits Gastrin release
- also inhibits release of CCK & Secretin, inhibiting pancreatic & gall
- bladder secretions
- the last part of the stomach overlies the pyloric sphincter, a thickened part of the Muscularis Externa that closes the stomach & only opens when digestion within the stomach is sufficient (material is liquid)
- as the sphincter opens it squirts that fluid into the duodenum (1st part of the SI)