Lecture 32: Lower GI Histology

Question 1

Small Intestine

Answer 1

• The small intestine consists of:
• The duodenum
• The jejunum
• The ileum
• Layers of the intestinal wall (inside to outside):
• Mucosa with lamina propria and muscularis mucosa
• Submucosa
• Muscularis layers
• Serosa
• See Slide 6-7

Question 2

Histologic characteristics of the duodenum:

Answer 2

• Villi with wide spatulate or “leaflike” distal shape
• Deep crypts of Lieberkuhn
• Brunner’s glands with excretory ducts in submucosa
• See Slide 9

Question 3

Crypts of Lieberkuhn

Answer 3

Simple tubular glands within the intestinal mucosa:

• Open between adjacent villi and extend to muscularis mucosa
• Cells of the crypts:
• Goblet cells
• Enterocytes:
• • Columnar epithelial cells with the following characteristics:
• – Basal nucleus, perinuclear Golgi, apical brush border
• – Produce disaccharidases
• – Involved in absorption of proteins, carbohydrates, lipids
• – Enzymes present on apical brush border are important in carbohydrate digestion.
• – Produce enteropeptidase (enterokinase) which is necessary for the activation of pancreatic zymogens and proenzymes.
• Panethcells:
• • Basal crypt cells
• • Prominent eosinophilic granules
• • Produce lysozyme to control intestinal flora
• Enteroendocrine cells:
• • Stain with chromium (chromaffin)
• • Agyrophilic (respond to silver stains)
• • Produce peptide hormones and serotonin
• • May produce paracrine or autocrine secretions
• • Analogous to unicellular goblet cells
• • Formed from endodermal stem cells in all regions of the GI tract.

Question 4

Glands of Brunner

Answer 4

• Found in submucosa of duodenum
• Responsible for:
• Formation of bicarbonate and mucus
• • Supplements bicarbonate from the pancreas
• • Necessary to neutralize gastric acid

Question 5

Histologic Characteristics of the Jejunum

Answer 5

• Villi longer and narrow
• Crypts of Lieberkuhn present
• No glands in submucosa

Question 6

Histologic characteristics of the ileum

Answer 6

• Villi longer and narrow
• Crypts of Lieberkuhn present
• No glands in submucosa
• See Slide 26

Question 7

Large Intestine

Answer 7

• Epithelium:
• Simple columnar epithelium
• Crypts but no villi
• Functions:
• 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
• See Slides 28-33

Question 8

Intestinal Tight Junction Barrier

Answer 8

• 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?
• See Slide 36

Question 9

Paneth Cells

Answer 9

• Found in bases of mucosal glands (Refer to Figure 16-15) .
• Characteristics:
• Basophilic basal cytoplasm
• Supranuclear Golgi complex
• Large, intensely acidophilic apical secretory granules.
• Secrete:
• Lysozyme to increase permeability of bacteria by degrading peptidoglycan coat.
• Defensins to increase membrane permeability of target organisms
• Tumor necrosis factor-α (proinflammatory cytokine)
• May phagocytize some microorganisms and help regulate intestinal flora.
• See Slide 38

Question 10

Mucosal Layer

Answer 10

• Mucosal layer coats intestinal epithelial luminal surface.
• Secreted by goblet cells
• Made up of two layers:
• • Outer layer contains microorganisms
• • Inner layer contains:
• – Antimicrobial proteins (from Paneth cells, enterocytes, and goblet cells) that resist microbial penetration.

Question 11

Immune Surveillance System

Answer 11

• 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))

See Slides 41-42

Question 12

GALT

Answer 12

• The bulk of the body’s immune defenses is centered in the GALT.
• Components:
• Transitory aggregations of lymphocytes, neutrophils, eosinophils
• Permanent structures:
• • Appendix
• • Peyer’spatches
• • Mesenteric lymph nodes
• The primary immunoglobulin produced by GALT is IgA (secretory IgA, sIgA,).
• Synthesized and secreted by plasma cells in the lamina propria of the gut
• Picked up at the basal surface of enterocyes and transported across the cell
• While in the cell, sIgA is linked to a protein called secretory component that inhibits degradation of the sIgA 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.
• See Slide 46-48

Question 13

Peyer’s Patches

Answer 13

• Dome-shaped lymphoid structures under the mucosal surface
• Contain B-and T-cell dependent areas
• Possess high endothelial venules (HEVs), which facilitate entrance of lymphocytes into lymphoid organs from the bloodstream
• The mucosal covering of the dome of the Peyer’s patches includes specialized M (microfold) cells:
• Sample particulate antigen and present it to antigen-presenting cells in the underlying lamina propria
• The antigen-presenting cells (APCs) and macrophages phagocytize the antigen and present it to helper and cytotoxic T cells as well as B lymphocytes.

Question 14

Rapid Cell Turnover

Answer 14

• Rapid cell turnover is a protective mechanisms used by the GI epithelium:
• True of most of GI tract
• • Particularly true of stomach and small intestine
• • Lining replaced every 5 days in humans
• In esophagus and anus:
• • New cells are formed in basal layer of stratified epithelium and migrate through sub-basal levels to be sloughed off into the lumen.
• In small intestine (Refer to Figure 16-16) :
• • New cells are formed in the crypts from undifferentiated cells.
• In large intestine:
• • There are no villi, so proliferative compartment is found at the base of the crypts.
• In stomach:
• • Stem cells are found in the neck of the gastric glands.
• • Differentiate into:
• – Surface and neck mucus cells (turn over most rapidly)
• – Enteroendocrine cells
• – Parietal cells
• • Chief cells
• See Slide 50

Question 15

Anal Canal

Answer 15

• Epithelium of anus returns to stratified squamous epithelium.
• Skeletal muscle returns as external anal sphincter.
• See Slide 54