Lecture #34: Lower GI Histology I

Question 1

What 3 sections does the small intestine consist of?

Answer 1

1. duodenum
2. jejunum
3. ileum

Question 2

What are the layers of the intestinal wall (inside to outside)?

Answer 2

• mucosa with lamina propria and muscularis mucosa
• submucosa
• muscularis layers
• serosa

Question 3

What are the histologic characteristics of duodenum?

Answer 3

• villi with wide spatulate or “leaflike” distal shape
• deep crypts of Lieberkuhn
• Brunner’s glands with excretory ducts in submucosa layer

Question 4

look at the micrograph of the duodenum and identify:

• Brunner’s Glands
• Crypt of Lieberkuhn
• Villi with wide spatulate or “leaflike” distal shape

Question 5

What are the histologic characteristics of the jejunum?

Answer 5

• Villi longer and more narrow than duodenum
  
  • finger-like shape
• Crypts of Lieberkuhn present
• No glands in submucosa

Question 6

What is the histologic characteristics of the ileum?

Answer 6

• villi longer and more narrow than jejunum
• crypts of Lieberkuhn present
• No glands in submucosa layer
• Peyer’s patches

Question 7

Describe the hitology of a crypt of Lieberkuhn and distinguish the various cell types and their functions.

Answer 7

• are simple tubular glands within the intestinal mucosa
  
  • open between adjacent villi and extend to muscularis mucosa
  • cells of the crytps:
    
    • enteroendocrine cells
    • paneth cells
    • enterocytes
    • goblet cells

Question 8

What are the characteristics of enterocytes in the crypts of Lieberkuhn?

Answer 8

• columnar epithelial cells
  
  • basal nucleus
  • perinuclear Golgi
  • apical brush border
• produce disaccharideases
• involved in absorption of:
  
  • proteins
  • carbohydrates
  • lipids
• enzymes present on apical brush border are important in carbohydrate digestion
• produce enteropeptidase (enterokinase) which is necessary forthe activation of pancreatic zymogens and proenzymes

Question 9

True or False:

Enterocytes in the crypts of Lieberkuhn do not produce disaccharideases or enteropeptidase (enterokinase).

Answer 9

False - produce disaccharidases and enteropeptidase (enterokinase)

Question 10

What is enteropeptidase (enterokinase), that is produced by enterocytes of the crypts of Lieberkuhn, necessary for?

Answer 10

For the activation of:

• pancreatic zymogens
• proenzymes

Question 11

What are the enzymes that are present on the apical brush border of enterocytes important in?

Answer 11

carbohydrate digestion

Question 12

What do paneth cells of the crypts of Lieberkuhn produce? Also, what do they stain as?

Answer 12

• Produce = lysozyme to control intestinal flora
• Stain = prominent eosinophilic granules
• Found = basal crypt cells

Question 13

What do enteroendocrine cells stain with, produce, and what are they formed from?

Answer 13

• stain = chromium (chromaffin)
  
  • agyrophilic (respond to silver stains)
• produce = peptide hormones and serotonin
  
  • may produce paracrine or autocrine secretions
• formed = from endodermal stem cells in all regions of the GI tract

Question 14

What do enteroendocrine cells of the crypts of Lieberkuhn produce?

Answer 14

produce

• peptide hormones
• serotonin

may produce

• paracrine or autocrine secretions

Question 15

What are Brunner’s glands and where are they found?

Answer 15

• found in submucosa of duodenum
• responsible for:
  
  • formation of bicarbonate and mucus
    
    • supplements bicarbonate from the pancreas
    • necessary to neutralize gastric acid

Question 16

What are the functions of the large intestine?

Answer 16

• secretion of mucus for lubrication
  
  • goblet cell is the prominent cell of the large intestine
• absorption of fluid
• formation of fecal mass
• continuation of digestion initiated in small intestine

Question 17

Fill in the Blank:

In the large intestine, ______ are present but _____ are not.

Answer 17

crypts; villi

Question 18

What type of epithelium lines the large intestine?

Answer 18

simple columnar epithelium

Question 19

Where are paneth cells found?

Answer 19

bases of mucosal glands

Question 20

What are the characteristics of a paneth cell?

Answer 20

• basophilic basal cytoplasm
• supranuclear Golgi complex
• large, intensely acidophilic apical secretory granules

Question 21

What do paneth cells secrete?

Answer 21

• lysozyme to increase permeability of bacteria by degrading peptidoglycan coat
• defensins to increase membrane permeability of target organisms
• tumor necrosis factor - alpha (proinflammatory cytokine)
• may phagocytize some microorganisms and help regulate intestinal flora

Question 22

What are toll-like receptors ?

Answer 22

• Toll-like receptors are found on surface of enterocytes.
  
  • type of pattern recognition receptor
  • recognize structurally conserved molecules broadly shared by pathogens but distinguishable from host molecules (pathogen associated molecular patterns (PAMPs))

** refer to figure 16-11 **

Question 23

What is the role of IgA in the immune defense of the GI tract?

Answer 23

• IgA is the primary immunoglobulin produced by GALT
  
  • synthesized and secreted by plasma cells in the lamina propria of the gut
  • picked up at the basal surface of enterocytes and transported across the cell
• While in the cell, slgA is linked to a protein called secretory component that inhibits degradation of the slgA by proteolytic enzymes in the GI lumen
• Secretory IgA, unlike IgG, does not stimulate the complement system, but functions by coating microorganisms, thus inhibiting microorganism binding to the epithelium

Question 24

What are M cells and what is their role in defense?

Answer 24

• Take up antigens and replaces the brush border by short microfolds.
• Differentiate from enterocytes when stimulated by membrane-bound lymphotoxin present on local B-cells.
• The mucosal covering of the dome of the Peyer’s patches includes specialized M (microfold) cells:
  
  • they sample particulate antigen and present it to antigen-presenting cells (APCs) in the underlying lamina propria

Question 25

What are Peyer's patches ?

Answer 25

* dome shaped lymphoid structures under the mucosal surface of the ileum * contain B- and T-cell dependent areas * possess high endothelial venules (HEVs), which facilitate entrance of lymphocytes into lumphoid organs from the bloodstream * the mucosal covering of the dome of the Peyer's patches includes specialized M (microfold) cells: * the antigen-presenting cells (APCs) and macrophages phagocytize the antigen and present it to helper and cytotoxic T-cells as wel as B-lymphocytes

Question 26

What is GALT?

Answer 26

* gut associated lymphoid tissue * the bulk of the body's immune defenses is centered in the GALT * components: * transitory aggregations of lymphocytes, neutrophils, eosinophils * permanent structures: * appendix * Peyer's patches * mesenteric lymph nodes * primary immunoglobulin produced by GALT is IgA

Question 27

True of False: Rapid cell turnover is a protective mechanism used by the GI epithelium.

Answer 27

True

Question 28

How often does the lining of the GI tract turnover?

Answer 28

lining replaced every 5 days in humans

Question 29

How does cell turnover in the esophagus and anus happen?

Answer 29

new cells are formed in basal layer of stratified epithelium and migrate through sub-basal levels to be sloughed off into the lumen

Question 30

How does cell turnover occur in the small intestine?

Answer 30

new cells are formed in the crypts from undifferentiated cells

Question 31

Since there is no villi in the large intestine, unlike the small intestine, how does cell turnover occur?

Answer 31

there are no villi, so proliferative compartments are found at the base of the crypts

Question 32

What do the stem cells found in the neck of the gastric glands differentiate into?

Answer 32

* surface and neck mucus cells * turn over most rapidly * enteroendocrine cells * parietal cells * chief cells

Question 33

List the major features that provide a defensive mechanism for segments of the GI tract.

Answer 33

The bulk of the body's immune defenses is centered in the GALT. Components: * transitory aggregations of lymphocytes, neutrophils, eosinophils * permanent structures: * appendix * Peyer's patches * mesenteric lymph nodes

Question 34

Look at lecture and figure 16-13 to cover the immune surveillance system mechanism

Answer 34

hang in there everyone, the semester is almost over

Question 35

Intestinal Tight Junction Barrier

Answer 35

* study figure 16-11 and note that epithelial cells of the GI tract are held together by occluding junctions containing the transmembrane proteins claudins and occludins, which function to prevent paracellular transport * why are these junctions especially important with regard to the GI tract epithelium?

Question 36

Why is the intestinal tight junction barrier important to GI epithelium?

Answer 36

* a defect in the intestinal tight junction barrier enables he unrestricted passage of antigens to the lamina propria * unregulated proinflammatory cytokines signal enterocytes to increase the passage of antigens across leaky tight junctions, from the lumen to the lamina propria, thus amplifying the inflammatory reaction. This mechanism can lead to an intestinal inflammatory disease.