Lecture 32: Lower GI Histology Flashcards Preview

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Flashcards in Lecture 32: Lower GI Histology Deck (15)
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Small Intestine

* 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


Histologic characteristics of the duodenum:

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

- See Slide 9


Crypts of Lieberkuhn

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.


Glands of Brunner

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


Histologic Characteristics of the Jejunum

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


Histologic characteristics of the ileum

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

- See Slide 26


Large Intestine

* 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


Intestinal Tight Junction Barrier

* 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


Paneth Cells

* 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


Mucosal Layer

* 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.


Immune Surveillance System

* 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



* 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


Peyer's Patches

* 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.


Rapid Cell Turnover

* 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


Anal Canal

* Epithelium of anus returns to stratified squamous epithelium.
* Skeletal muscle returns as external anal sphincter.

- See Slide 54