Intestines Flashcards
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?
- Uptake of ions
- Uptake of sugars
- Uptake of peptides / amino acids
- Uptake of lipids
- Uptake of vitamin B12
- Recycling of unconjugated bile salts
- Uptake of water
- 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?
- IgA is synthesized as a dimer in plasma cells in lamina propria
- IgA is taken up by receptor-mediated endocytosis at basal surface
- 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 Cholecystokinin Gastric inhibitory peptide Secretin Motilin
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)?
- Immature enterocytes
- Goblet cells
- Enteroendocrine cells
- Paneth cells
- Undifferentiated stem cells
What are paneth cells?
Cells located in base of crypts with prominent acidophilic granules.
Secrete:
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
What is nodular GALT? What are the cell types and what is an example?
Follicular gut-associated lymphatic tissue, covered by a specialized epithelium which includes:
- Microfold cells
- Dendritic cells
- Enterocytes
Peyer’s patches in ileum is a prominent form
How do the three cell types of nodular GALT work together?
Microfold cells - flattened cells with short apical folds of plasma membrane that take up antigens and transport them to underlying lymphocytes
Dendritic cells - APCs that take stuff from lumen
Enterocytes - assist by degrading antigens
What are the lymphoid elements of GALT?
- Lymphatic nodules with B cells and macrophages
- Diffuse lymphatic tissue.
- B cells will become plasma cells to produce antibody response in cooperation with T cells
How is ileum different than jejunum?
Ileum has more lymphatic tissue and goblet cells than jejunum
How does large intestine differ from small intestine?
Large intestine has no villi, but still has crypts of Lieberkuhn. It still functions in absorption of water and electrolytes primarily. It’s mucosal surface is smooth with test-tubes (crypts) stuck in it
What lines the epithelium of the large intestines?
- Enterocytes with primary function of electrolyte and water reabsorption
- Goblet cells - secrete mucus to lubricate feces
What are the differences between large and small intestines in the crypts of Lieberkuhn?
In large intestine: There are no Paneth cells to secrete lysozyme + defensins, and goblet cells are more numerous
What is the outer longitudinal layer of smooth muscle in the muscularis externa of large intestine?
It exists as 3 distinct and separate bands called taenia coli
How can the appendix be distinguished from colon?
Appendix has smaller diameter, and a continuous outer longitudinal layer of smooth muscle rather than taenia coli.
Appendix also has less regularly spaced crypts and more lymphatic infiltrations
How does the rectum differ from the rest of the large intestine?
Similar to colon, but no taenia coli. Crypts have even more goblet cells.
What is the upper anal canal like?
Much like rest of large intestine: simple columnar, simple tubular glands.
What happens in the transition from upper anal canal to lower anal canal?
- Simple columnar to stratified columnar epithelium
- Crypts of Lieberkuhn are lost
- Muscularis mucosae disappears
What happens to the epithelium of the lower anal canal?
Transitions from stratified cuboidal to stratified squamous nonkeratinized.
What happens to the epithelium of the end of the anal canal?
SSNK becomes stratified squamous keratinized of thin skin
What is in the submucosa of the anal canal?
Highly vascularized with extensive plexus of hemorrhoidal veins
What happens to the inner circular layer of smooth muscle in the anal canal?
Becomes internal anal sphincter
What happens to the outer longitudinal layer of smooth muscle in the anal canal?
Terminates in adjacent connective tissue
What forms the external anal sphincter?
Skeletal muscle that forms in line of the outer longitudinal layer which became connective tissue
