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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
- Duodenum, then
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
upwellings of the submucosa that greatly increase the surface area of the small intestine in the Jejunum
Intestinal Glands (Crypts)
contain a number of important cell types
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)
What's a unique feature that makes the LI easy to identify?
- Teniae Coli
- a longitudinal band of smooth muscle (3 of them)
- 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
- 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
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
- 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
What (3) cell types can be found within the intestinal epithelium?
- 1. enterocytes (columnar absorptive cells)
- 2. goblet cells
- 3. entero-endocrine cells
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
- 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
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
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
What are the cells contained in Crypts?
- Goblet cells
- Paneth cells
- Enteroendocrine cells
- Stem cells
- [as well as Enterocytes & M (microfold) 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
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
- 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)
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
- require special stains (eg. Ig against specific hormone being produced)
- exist at bottom & sides of Crypts
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
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
- 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)
Why does chemotherapy & radiation compromised gut functions?
because such therapies kill rapidly proliferating (tumor) cells, & intestinal epithelial stem cells turn over quickly also
- 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
- this is a submucosa esophageal glands
- can tell because of the lighter stained secretory regions contrasted with the darker stained ducts
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)
- can see some plicae and masses of lymphocytes
- Peyer's patches are abundant in the Ileum
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
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
- 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
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)
- no villi - smooth surface
- no plicae circulares
- few Paneth cells
- many Crypts
- abundant goblet cells
- teniae coli
- enteroendocrine cells
- the outer longitudinal muscularis externa/propria layer is gathered into 3 thick bands