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What happens at the pyloric stomach / duodenum junction?
- stomach epithelium → duodenal epithelium
- surface mucous cells → enterocytes
- long pits, short glands → villi extending into the lumen*
- villi = main difference between stomach & duodenum
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Small Intestine
- Duodenum, then
- Jejunum
- Ilium
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Villi / Microvilli
- villi: mucosal surface is thrown up into finger-like projections
- enterocytes (cells that line the villi) have MICROvilli on their surface - critical for digestion
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Plica Circulares
upwellings of the submucosa that greatly increase the surface area of the small intestine in the Jejunum
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Intestinal Glands (Crypts)
contain a number of important cell types
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What's one of the main differences between the small and large intestines?
- there are NO villi in the large intestines
- surface is quite smooth
- both contain Crypts & Goblet Cells (abundant)
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What's a unique feature that makes the LI easy to identify?
- Teniae Coli
- a longitudinal band of smooth muscle (3 of them)
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Brunner's Glands
- glands in the submucosa of the duodenum that make bicarbonate-rich substance to neutralize the acidic chyme that enters from the stomach
- changes the pH to near neutral (~6.0), optimal for further digestive processes
- connected to the lumen by ducts which open into the base of intestinal crypts
- won't see submucosal glands anywhere else in the small or large intestine
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Vili
- can see mucus producing goblet cells & wandering lymphocytes in the simple columnar epithelium
- deep to the villi epithelium is the lamina propria (loose CT)
- can see smooth muscle cells in the lamina propria; these connect to the muscularis mucosa & help vili contract & move
 - there's also a lacteal (small lymphatic) that runs through the core of the vili (lamina propria) to collect lipids via chylomicrons
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What purpose do the smooth muscle cells associated with lacteals in villi serve?
they periodically contract, “milking” the lymph out of the lacteal & moving it into the lymphatic circulation
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Celiac Disease
- normally tight junctions prevent intestinal contents from moving past simple columnar enterocytes - only way to move is to go THROUGH them
- inflammatory response to gluten protein (in wheat) results in disruption of the glycocalyx & microvilli structure
- fragments can now move past enterocytes and come in contact with basolateral surface, get into connective tissue, and cause an inflammatory response
- this results in malabsorption
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What (3) cell types can be found within the intestinal epithelium?
- 1. enterocytes (columnar absorptive cells)
- 2. goblet cells
- 3. entero-endocrine cells
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Microvilli & Epithelial Glycocalyx
- simple columnar epithelial surface enterocytes of the small intestines, colon & appendix that make enterokinase which activates trypsinogen → trypsin, + other zymogen activation
- contain a Glycocaylx which adsorbs pancreatic digestive enzymes & concentrates, therefore:
- DIGESTION HAPPENS AT APICAL ENDS OF THESE CELLS
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Glycocalyx
- contains carbohydrates & many anchored enzymes:
- trypsin (pancreatic enzyme that activates zymogens), chymotrypsin, elastase, nucleases
- enterokinase converts trypsinogen to trypsin
- these enzymes help break down products in the chyme for absorption into enterocytes & eventual transport to the liver
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Crypts (Crypts of Lieberkuhn, Intestinal Glands)
- gland-like structures that project DOWN into the lamina propria
- where new intestinal epithelial cells are made (from stem cells in the bottom of each crypt which migrate up & differentiate into intestinal absorptive/goblet cells)
- stem cells generate all 4 types of cells: enterocyte, goblet, enteroendocrine & paneth
- crypt lumen is continuous with the gut lumen
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X-Section of Crypts v. Villi
 - inside of villi is the lamina propria
- what surrounds the crypt is the lamina propria
- a clear space in the villi is a lacteal
- the clear space in a crypt is its lumen
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What are the cells contained in Crypts?
- Goblet cells
- Paneth cells
- Enteroendocrine cells
- Stem cells
- [as well as Enterocytes & M (microfold) cells]
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Goblet Cells
- unicellular glands with a lot of lipid
- so much lipid that the cell's nucleus is forced toward its base
- it rests on the basal lamina
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How does the number of goblet cells reflect one's location in the intestinal tract?
- the farther down the intestinal tract, the MORE goblet cells there are
- they are MOST numerous in the colon
- more mucus is needed to move contents through the intestine as the contents become thicker
- there are 0 submucosal glands to assist with the process

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- a type of epithelial cell located at the base of intestinal crypts
- have bright red eosinophilic secretory granules that contain antimicrobial products (lysozyme, α-defensins)
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What is the role of Paneth cells?
it is thought that they protect stem cell population from bacterial infection because they contain lysozyme + other antibacterial (microbial) substances
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Enteroendocrine Cells
 - require special stains (eg. Ig against specific hormone being produced)
- exist at bottom & sides of Crypts
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What are the 2 types of enteroendocrine cells?
- 1. Open: regulated by luminal contents; it has a surface exposed to lumen of the gut

- 2. Closed: regulated by paracrine & neural mechanisms - doesn't see lumen
 - both types' basal end will contain secretory granules because there's no point in secreting hormones INTO gut (will just be broken down)
- hormone secreted basally into ECM when it will be picked up by fenestrated capillaries
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Where is the only place enteroendocrine cells can secrete hormone containing granules?
- basally (or they're closed) - if they were to secrete into the lumen (apically), the hormones would just be digested
open type regulated by luminal contents- closed type regulated by neural & paracrine mechanisms
- products: gastrin, glucagon, serotonin
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Stem Cells
- enterocyte epithelium turns over frequently (~5-6 days)
- there are slowly dividing cells that exist near the base of the crypts: STEM CELLS
- daughter cells work their way in both directions: up toward the villi (to become enterocytes or goblet cells) & down toward the crypt (to become paneth or enteroendrocrine cells)
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Why does chemotherapy & radiation compromised gut functions?
because such therapies kill rapidly proliferating (tumor) cells, & intestinal epithelial stem cells turn over quickly also
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Plicae Circulares
- are not restricted to the jejunum, but are most abundant there
- large infolding of the submucosa that project INTO the lumen
- involve both mucosa (villi) & submucosa (core)
- greatly increase the surface area available for digestion/absorption of nutrients
- notice there are no glands in the submucosa, because those cease to exist beyond the duodenum
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- this is a submucosa esophageal glands
- can tell because of the lighter stained secretory regions contrasted with the darker stained ducts
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What is this picture showing?
- muscularis externa - two layers of smooth muscle
- between them is a MYenteric plexus (muscle)
- darker portions of the muscle region = skeletal muscle, so this must be in the MIDDLE 1/3rd of the esophagus (both skeletal & smooth)
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Ileum
- can see some plicae and masses of lymphocytes
- Peyer's patches are abundant in the Ileum

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Most of the cells that cover Peyer's Patches are enterocytes, but what kinds of cells are scattered on the surface?
- M (microfold) Cells
- all of which have a deep invagination that allows immune cells (B/T cells) to get as close to the gut lumen as possible
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Enterocytes take up which antibody secreted by plasma cells into the intestinal lumen?
- IgA - technically secretory IgA as the plasma cells attach glycoprotein to the IgA to prevent it from being degraded
- provides protection against intestinal microbes/pathogens
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- Peyer's Patches - nodular lymphoid tissue
- contains Microfold (M) cells which are specialized epithelial cells for antigen sampling
- take up antigens from the lumen
- presented to other APCs if immune response is needed
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What covers most of the small intestine?
- Serosa (Visceral Peritoneum)
- except for where there's mesentary...
 - the Large Intestine is covered by both a Serosa (anteriorly) & Adventitia (posteriorly)
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Colon
- no villi - smooth surface
- no plicae circulares
- few Paneth cells
- many Crypts
- abundant goblet cells
 - teniae coli
- enteroendocrine cells
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Taenia Coli
- the outer longitudinal muscularis externa/propria layer is gathered into 3 thick bands

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