13 Intenstines

• 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
• Jejunum
• Ilium

• 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

contain a number of important cell types

• there are NO villi in the large intestines
• surface is quite smooth
• both contain Crypts & Goblet Cells (abundant)

• 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

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

• 1. enterocytes (columnar absorptive cells)
• 2. goblet cells
• 3. entero-endocrine cells

• 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

• 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

• 
• 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

• 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

• 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)

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

• 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

• 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)

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

• 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
• 

• 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

• 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

• 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
•