Lecture 31 Flashcards
Mucosa differences in GI tract
Muscularis mucosa, submucosa, muscularis externa and adventitia are relatively similar in GI tract. BUT the mucosa (epithelium and loose CT) changes depending
on section of GI tube
Esophagus has stratified squamous non keratin ep
Stomach has pits and glands
Small intestine has large finger-like protrusions protruding into lumen of the gut
In colon, no finger-like projections; arrangement is similar to that in the stomach
Anatomical, histological, cellular and molecular ways to increase SA for food absorption
Small intestine is large, up to 7m long, highly coiled, large surface for absorption
Wall of small intestine is folded and runs in circulatory fashion, called plica circularis, which is a fold containing a core of submucosa
The mucosa of the plica circularis has finger-like projections called intestinal villi to increase SA. The mucosa forms the intestinal villi. Each intestinal villus has loose CT or lamina propria inside. There are also intestinal glands right below and in between intestinal villus that produces certain materials
The ep covering each villus has columnar cells called enterocytes that absorb material. The enterocytes have microvilli on its apical PM
Each microvillus has bundle of actin filaments in the middle and glycocalyx (transmembrane proteins that form a tree-like arrangement of molecules that aid in absorption) on apical surface
OVERALL: PLICA CIRCULARIS to VILLUS to MICROVILLUS (on enterocyte) to GLYCOCALYX (on microvilli)
Intestinal villus
Intestinal villus is covered by simple columnar ep with goblet cell; The ep absorbs food.
The mucosa of the small intestine has villi with a core of lamina propria and has intestinal glands (Crypts of Lieberkühn)
Each intestinal villus has 3 vessel types:
- Arteriole brings in blood and forms capillary network within the intestinal villus
- Venule is where the blood leaves
- Lacteal is a blind ending lymphatic vessel. The
early anatomist opened the gut and saw little white villi, thinking there was milk in there, but it was lipids, not milk. If the gut absorbed lots of lipids, the lipid will be transported away through the lymphatic system through the lymphatic vessels. Surrounding the lacteal is loose CT, lamina propria, highly cellular.
Often can find bundles of smooth muscle too, which means the intestinal villi has ability to change their length a little bit and respond to incoming food
Food goes to basal end and delivers the material into CT, which is full of capillaries and lacteals. AA, sugars, fatty acids enter post capillary venules and then veins,
which then transports to liver; There is portal vein that receives this material from GI tract. Intestinal villus transports nutrients from intestine to liver bc intestinal villus contains portal vein
Fatty acids are reassembled into larger structures called
chylomicrons, which is assembly of lipids and proteins that is large and taken up by lacteal. Fat is transported through lymphatic system and enters cardiovascular system at thoracic duct
Intestinal epithelium and enterocytes
Intestinal villus is covered by simple columnar ep with goblet cell. The ep is made of enterocytes. The enterocytes have microvilli
Enterocyte has brush border (next to terminal bar) that has microvilli that protrude from apical cell surface. Microvilli is anchored in a meshwork of cytoskeletal elements that contain actin, intermediate filaments. It is called terminal web or terminal bar
Enterocytes absorb food from surface
Goblet cells make film of mucus that covers the surface cells for protection, lubrication, solubilization of food
Goblet cells and enterocytes are at surface of villi
Intestinal glands
Intestinal glands are the fingerlike indentations in b/w the villi. These intestinal glands also called Crypts of Lieberkühn.
There are 2 cell types here: entero-endocrine cells (secrete cholecystokinin and secretin, release determined by hormones) and paneth cells (have large granules containing lysozymes, lysozyme is component of saliva but also present in small intestine)
Brush border of enterocytes
Enterocytes have microvilli that contain actin inside. Each microvillus has a meshwork of membrane proteins called glycocalyx sticking up from apical surface
Brush border all has same length, cilia has diff length
The actin is anchored in electron dense material called terminal web, seen as dark line underneath brush border. The terminal web anchors microfilaments, has junctional complexes and is a woven matt of microfilaments & intermediate filaments
Protein absorption
Sequence of events that take place to absorb protein. Done by enzymes that are released into the small intestine. The enzymes come from the exocrine pancreas
Enzymes are inactive, unless required to digest food. Otherwise, you digest yourself
The digestive precursor enzymes get activated by another enzyme called enterokinases (they are not kinases, but are enzymes that make the digestive enzymes active and are found within glycocalyx)
Pathway:
- Inactive Enzymes
•Trypsinogen
•Chymotrypsinogen
•Proelastase
- Enterokinases located on microvilli
- Active Enzymes
•Trypsin
•Chymotrypsin
•Elastase - Proteins —> AA
Cells in intestinal glands - crypt
Enteroendocrine cell
• Secretin
• Cholecystokinin
Paneth cell
• Lysozyme
These 2 cell types located at bottom of one of those tubular intestinal glands. Identified at higher mag by presence of large magenta/red coloured secretory granules also called zymogen granules. zymogen granules contain lysozymes (enzyme that attacks bacteria, dissolves bacterial walls, creating antigens so our immune
system can recognize bacterial proteins
Paneth cells
Found at bottom of intestinal glands
Secrete lysozymes, have zymogen granules. Secrete into lumen Lysozyme is antimicrobial enzyme
Enteroendocrine cells
Do not secrete into lumen or surface, but secrete into blood vessels; They secrete hormones secretin and cholecystokinin
Have secretory granules sitting on the south side of the nucleus, not towards the apex
Hormones picked up by fenestrated capillaries in GI tract
Cell renewal in stomach
Bidirectional cell flow
Stem cells are found in neck region of the gastric glands. The progeny cells migrate in 2 directions (down and up).
Replacement of surface cels are faster than those at the bottom.
Rapid surface-cell renewal of 4-7 days
Slow gland-cell renewal of weeks
Cell renewal in intestines
Unidirectional cell flow
High turnover of cells. Stem cells located amongst the paneth cells at bottom of intestinal glands, meaning that all the resulting cells have to migrate up into one direction, quite rapid
Cell renewal is 3-6 days
Distinguishing features of the 3 intestinal regions
Duodenum
- Within submucosa of duodenum, there are Brunner’s glands, only present in duodenum
Jejunum
Ileum
- Has lymphatic nodules/tissue called peyer’s patches, which is diffused lymphatic tissue
Duodenum
Brunner’s glands in submucosa make mucus and deliver to surface
First part of duodenum highly involved in breaking down food, so needs lots of lubrication
Lymphoid nodule present in submucosa, but this is present throughout GI tract
Ileum
Peyer’s Patches: Permanent Lymphoid nodules in lamina propria and submucosa
- Large masses of lymphocytes within the mucosa and some of submucosa; The lymphocytes penetrate into epithelium
Peyer’s patches detect antigen; In order to do so, the gut allows lymphocytes to come close to the lumen
Plasma cell in lamina propria
Transcytosis of IgA
- IgA is picked up, transported and released back into gut lumen by a mechanism called transcytosis. Occurs in GI
tract, but also in salivary glands
- IgA DIMERS secreted by ‘plasma cells in the lamina propria’ bind to receptors on the basal side of enterocytes and get transported to the apical intestinal lumen.
There are certain additional proteins that hook the antibodies together: J chain, secretory component
Colon
- flat and smooth surface
- No Villi.
- Intestinal Glands (includes white material, which is mucus containing goblet cells)
- Absorption of electrolytes and water
- Abundant Goblet cells
Tenia Coli: Thick bands of longitudinal Smooth Muscle
- Outer longitudinal layer of muscularis externa
becomes very thick and forms elongated bands called tenia coli.
- As these smooth muscle bundles pull, it sacculates the large intestine, creating reaction compartments so food stuff can hang around for quite some time, which allows water to be removed from it. This causes your poop to be solid/semi-solid
- Tenia coli create haustra, which are pockets where your shit is located in
- Epithelium forms indentations, aka intestinal glands
Overall:
- Looks like abs
- The ab bubbles are haustra
- The midline is Tenia coli
- The horizontal separations of haustra is Plica Semilunaris
Enterocytes only found at the upper surface, but most of the glands contain goblet cells. As breakdown product of our food becomes more solid, we need to lubricate more. The enterocytes are involved in removing sodium chloride and water
