Flashcards in Intestines Deck (47)
What are plicae circulares and how are they different from rugae?
They are submucosal folds in the intestines that are PERMANENT, rather than the temporary submucosal folds of rugae
What are villi? Where are they found?
Finger-like mucosal folds that project into the lumen, greatly increasing surface area
They are found only in the small intestine
What are crypts of Lieberkuhn? What type are they?
Intestinal mucosal glands. They are simple tubular glands that open at the base of villi and empty into lumen between neighboring villi.
What are the features of the lamina propria of the intestines?
It is the delicate CT that forms the core of the villus and surrounds the glands, containing lymphatic vessels (i.e. central lacteal) and capillary network that runs just under the epithelium. May have diffuse or nodular lymphatic tissue
What is a central lacteal? What could cause it to swell?
A blind-ending vessel that begins near tip of villus and runs through the central core of the villus, draining into a plexus in the lamina propria and submucosa. Lots of fat absorption will cause it to swell.
What are Brunner's glands?
Submucosal glands which are present in the duodenum only. Ducts drain into crypts of Lieberkuhn
Compound branched tubular glands producing alkaline mucus to buffer the acidic chyme from the pyloric stomach.
What are the actions of the two different orientations of muscle in the muscularis externa of the intestines?
Inner circular layer - contraction results in segmentation
Outer longitudinal layer - contraction results in peristalsis
What are enterocytes and what is their surface called?
Primary cell type covering the villi - columnar absorptive cell
Apical surface is covered with microvilli called the striated border
What crosslinks actin and actin to the membrane in microvilli?
Between actin: Villin + fimbrin
Between actin and membrane: myosin I and calmodulin
What anchors actin + cytokeratin intermediate filaments in terminal web?
The junction complex, including zonula occludens, zonula adherens, and macula adherens
What is the function of the proteins in the enterocyte glycocalyx?
Contains oligosaccharidases, disaccharidases, and peptidases which break down carbohydrates for uptake of sugars. This allows monosaccharides to be absorbed by active transport and move to basement membrane, and taken up by fenestrated capillaries.
What are the functions of enterocytes?
1. Uptake of ions
2. Uptake of sugars
3. Uptake of peptides / amino acids
4. Uptake of lipids
5. Uptake of vitamin B12
6. Recycling of unconjugated bile salts
7. Uptake of water
8. Release of secretory IgA
What causes lactose intolerance?
Genetic defect in lactase in enterocyte glycocalyx
What enzymes are involved in the uptake of peptides via enterocytes?
Enterokinase in in glycocalyx, which activates trypsinogen from pancreas to form trypsin.
Trypsin activates other pancreatic proenzymes, which cleave proteins to amino acids that can be uptaken by enterocytes
How are lipids uptaken by enterocytes? What is the role of smooth ER?
Pancreatic lipase in the presence of bile salts breaks down lipids to free fatty acids and monoglycerides which diffiuse into enterocytes.
Smooth ER resynthesizes triglycerides and transfers them to Golgi complexes for further processing and packaging to chylomicra
What are chylomicra and where are they released?
Vesicles formed in Golgi of enterocytes, released in intercellular space in lateral margin. They move towards lacteals in center of villus
How is vitamin B12 uptaken by enterocytes?
Gastric intrinsic factor (IF) is produced by parietal cells in gastric glands in the fundic stomach, which binds to B12 in intestinal lumen, which can be taken up by enterocyte.
It is liberated in the enterocyte and transported across the basal membrane
How do enterocytes help the liver?
They recycle unconjugated bile salts, which are returned to liver hepatocytes
How is water absorbed by enterocytes?
Na+ / K+ ATPase in lateral plasma membrane moves Na+ ions into intercellular space, and water follows sodium. Space is enlarged during active absorption via aquaporins
How is secretory IgA released via enterocytes?
1. IgA is synthesized as a dimer in plasma cells in lamina propria
2. IgA is taken up by receptor-mediated endocytosis at basal surface
3. Complex is transported through the cell and released at margin with receptor still attached.
Complex is less easily degraded in intestinal lumen
What are goblet cells in the epithelium of villi and what do they release?
They are unicellular exocrine glands. Their apical cytoplasm is packed with granules of the glycoprotein mucinogen, which is released to form a coat of mucus
What is the relative abundance of the enterendocrine cells in villi? What are their 4 main types?
1% of cell population, which is significant
Gastric inhibitory peptide
What is the function of CCK enteroendocrine cells?
Responds to lipid content to stimulate gall bladder contraction and pancreatic secretion
Gastric inhibitory peptide - inhibits acid secretion from parietal cells
What is the function of gastric inhibitory peptide enteroendocrine cells?
Inhibits acid secretion from parietal cells
What is the function of secretin enteroendocrine cells?
Responds to low pH, inhibits gastric acid secretion and causes pancreatic ducts to release bicarbonate ion buffer
What is the function of motilin enteroendocrine cells?
Stimulates GI motility
What are the 5 cell types of crypts of lieberkuhn (intestinal glands)?
1. Immature enterocytes
2. Goblet cells
3. Enteroendocrine cells
4. Paneth cells
5. Undifferentiated stem cells
What are paneth cells?
Cells located in base of crypts with prominent acidophilic granules.
Lysozyme - degrade bacterial surface coats
Defensins - increase membrane permeability of parasites and bacteria
How are cells in villi regenerated?
Stem cells undergo mitosis within the glands, and migrate towards extrusion zone at tips
Epithelium is replaced every 5-6 days, paneth + enteroendocrine cells are replaced more slowly