Lecture 30: microstructure GI Flashcards
Four layers of GI tract
1. mucosa (innermost lining) epithelial lining cells lamina propria muscularis mucosae 2. Submucosa (supports the mucosa) loose tissue; contains blood vessels, nerves, lymphatics 3. Muscularis propria (muscle proper) inner circular muscle, outer longitudinal muscle 4. Serosa (thin outer lining)
What are the functions of the two layers of muscularis propria
inner circular muscle –> Contracts food
outer longitudinal muscle –> propels muscle
myenteric plexus vs. submucosal plexus
Nerves are clustered in two different areas of the tract, differentiated by function.
Myenteric (also called auerbach’s plexus) between two muscle proper layers. Well represented throughout the gut, muscles needed in all of alimentary canal.
Submucosal/Meissners above submucosa, mostly seen in small intestine in large intestine, more functional in absorption/digestion.
Enteric Nervous System
Own nervous system of gut. Nerves arise in gut, communicate with other nerves. Can work without communication from CNS, seen paraplegic patients with spinal cord injuries.
parasympathetic vs. sympathetic innervation of the gut
sympathetic go through pre-vertebral ganglion, while the parasympathetics communicate directly through one nerve system.
Key features histo of exophagus
stratified squamous epithelium (non-keratinized)
muscularis propria varies –> upper 1/3 skeletal, middle 1/3 mixed, lower 1/3 smooth
No serosa here.
Squamo-columnar junction
Between stomach and esophagus, the squamous epithelium abruptly transitions to the columnar epithelia of the stomach.
But only the epithelia transitions, the lamina propria and rest of mucosa stays the same.
key structural features of the stomach
Epithelium:
Parietal cells –> secrete acid; intrinsic factor
chief cells –> secrete pepsinogen
G cells –> secrete hormone gastrine
Mucous cells –> protect, lubricate mucosa
Muscularis propria –> 3 layers, additional oblique layer
Part of stomach that is reservoir
Fundus and body, also where acid is added
Part of stomach that does mechanical digestion
Mixing and Grinding of food happens in antrum
Three types of glands in stomach
Fundic –> in fundus and body of stomach
Pyloric –> antrum and pyloric
Cardiac –> proximal stomach
What are in fundic glands
long branched glands with mucous, parietal, chief, and endocrine glands
Contents of Pyloric glands
Mucous and endocrine glands, long branched glands
Contents of cardiac glands
short branched tubular glands, mostly mucuous glands
Gastric pits
The pit is leading to the gland, seen in the folds of the stomach. Lined with mucous producing cells to protect the stomach from acid produced by stomach.
Foveolar cells
Located at surface line the gastric pits, mucous cells with faint, pink cytoplasm; rectangular shape
Mucous neck cells
located at base of gastric pits; pale mucous vacuoles
parietal cells
located lower in fundic gland; eosinophilic cytoplasm with central nucleus (“fried egg” appearance); lots of mitochondria lead to v. pink cytoplasm. PM lots of invaginations
Chief cells
Located near parietal cells, pyramidal cells with more basophliic cytoplasm (lots of RER), have granules
Endocrine cells
Seen with special IHC stains, eosinophilic granules
Stem Cells
located in mucous neck region
Small Intestine, key structural features
Highly adapted for maximal srface area
very long organ (15-18 feet)
folds of mucosa/submucosa (not muscularis propria)
mucosa has finger-like projections (villi)
epithelial cells have microvilli
Villi vs. Crypts
Villi are extensions up of tissue, crypts are folds into tissue. both are present in small intestine. Usually 4:1 height ratio of villi:crypt
Villi features
Finger like extensions into lumen of small intestine. Site of absorption. The lamina propria is rich in blood vessels, lymphatics, nerves.
Central core of lamina propria has lacteal (lymphatic channel), tendons of muscularis mucosae (give villi structural integrity). Things can be absorbed directly into veins or lymphatics.
Crypt
Are invaginations that are continuous with villi in small intestine.
Site of secretion of fluids and electrolytes.
Paneth cells found at base –> make lots of defensive substances against dangerous substances in lumen, especially the nearby stem cells.
Stem cells found in crypt, cells born here and mature as they migrate up the crypt.
Can villi move?
Yes have muscles (muscularis mucosae) which also give it structural integrity.
what makes up the brush border
Microvilli membrane, found on columnar cells of the small intestine. have actin filaments inside. Have enzymes critical for digestion of certain food (lactose).
enterocyte
Other word for columnar absorptive cells of small intestine.
Have microvilli.
Tall cells with basal membrane.
Junctional complexes –> attach cells
intercellular clefts –> spaces between enterocytes, at base of cells.
Chylomicrons
Encapsulate fat, go into lymphatic channels.
Panneth cells in histo
v. obvious pink granules
duodenum functions
neutralize acidic chyme that exits the stomach through secretions of the brunners glands, which secrete alkaline mucus from submucosal cells.
Also the site of absorption of most minerals
Jejunum function
absorption of most nutrients
ileum function
absorption of many nutrients.
absorb B12 and bile proteins.
Peyers patches (lymphoid aggregates) seen here
Why does colon need to be strong?
Bc it has to move solidified stool. Tenia coli are the longitudinal muscles seen in the colon, are now arranged in rings. instead of longitudinal to increase strength.
Function of colon
Absorb water, solidify the stool
Histo structures of colon
No villi, only crypts in mucosa. Mostly goblet cells. Muscularis propria has tenia colia (3 layers) of longitudinal muscle.
Cell types of colon
Mostly goblet cells Columnar absorptive cells rare enteroendorcine cells rare paneth cells (even though there are lots of bacteria in the colon) Stem Cells seen in base of crypt
anal canal junction
squamo-columnar junction. Anal canal has squamous stratified
rectum has columnar
Anal Canal functions
Controls continence
Key structural feature of anal canal
mucosa has stratified squamous epithelium
two parts of anal sphincter
external anal sphincter –> skeletal muscle, coluntary
internal anal sphincter –> smooth muscle, continuous with colonic muscle, involuntary.
Villus vs. Crypt cells of small intestine
Villus cells are columnar, have microvilli/terminal web, Golgi/ER
Crypt Cells are cuboidal, no microvilli/terminal web/golgi-ER
