Structures of Lower GI Tract Flashcards
What are the three regions of the small intestine?
Duodenum
Jejunum
Ileum
-These are all histologically similar but minor differences permit their identification.
Name 3 modifications of the small intestine that increase its surface area to aid in absorption.
- Plicae circulares
- Villi
- Microvilli
* * These three specialized modifications increase the intestinal surface area by 400-600 fold
Permanent mucosal folds present in the duodenum, jejunum and proximal ileum – increase surface area by 2-3 fold
Plicae circularis
Finger-Like projections of epithelium-covered lamina propria that increase surface area by 10-fold
Villi
Extensions of the epithelial cell apical plasma membrane that increase surface area 20-fold
Microvilli
Villi are extensions of the….
Lamina propria
They contain a central lymphatic channel and several capillaries
Epithelial covering of the villi are composed of:
Absorptive cells (enterocytes) Goblet cells DNES (enteroendocrine) cells
Extend from the base of the villi into the lamina propria
Crypts of Lieberkuhn
Contain: DNES (enteroendocrine) cells, stem cells, paneth cells
What is the most abundant cell of the small intestine?
Absorptive cells/enterocytes – they are columnar epithelial cells with an elongated nucleus
What is the principal function of the enterocytes?
Terminal digestion
Absorption of water and nutrients
They re-esterify fatty acids into triglycerides, form chylomicrons an transport the bulk of the absorbed nutrients into the lamina propria for distribution
What is in the brush border of the luminal surface of each enterocyte?
Microvilli covered in glycocalyx – has protective role but also involved in digestions of disaccharides and dipeptides
What do Goblet cells secrete?
Mucin = high MW glycoprotein macromolecules that have 20% protein and 80% carbohydrate
After releases mucin granules become hydrated and expand several hundred fold in volume – this mucus lubricates and protects the intestine
In the Crypts of Lieberkuhn, explain how cell type differs between the upper and lower regions?
Upper: surface absorptive and Goblet cells
Basal: No surface absorptive cells and only a few Goblet cells, most of the cells are regenerative, DNES cells and Paneth cells
How often is the intestinal epithelium replaced?
Every 3-6 days
What do Paneth cells secrete?
Lysozyme and defensins
*Paneth cells are long lived and do not participate in upward migration of epithelial cells
Highly vascularized to accept the products of absorption – also contains abundance of lymphoid cells that protect the intestinal lining from microbial invasion
Lamina propria
Contains an inner circular and outer longitudinal layer of smooth muscle fibers – during digestion these fibers contract causing shortening of the villus
Muscularis mucosae
Lamina propria’s extensive immune defense system
Individual lymphocytes within the mucosa and epithelium or regions of coalesced lymphoid nodules
GALT = gut-associated lymphoid tissue
Consists of individual lymphoid nodules and Peyer’s patches
How does the duodenal submucosa differ from the rest of the GI tract?
It has a large number of Brunner’s glands – secrete alkaline mucus that neutralizes the pH of gastric chyme and protects the duodenum
Also contains epidermal growth factor that may be involved in stem cell proliferation in the crypts
What controls the release of secretions from the intestines?
Submucosal (Meissner’s) Plexus
What lies between the smooth muscle layers of the muscularis external?
Myenteric (Auerbach’s) Plexus – peristaltic movement
How do the 3 regions of the small intestine differ?
Duodenum: shortest segment; villi are broader, taller and more numerous per unit area; fewer goblet cells; Brunner’s glands in submucosa
Jejunum: villi are narrower, shorter and sparser than the duodenum; number of goblet cells per unit area is greatest
Ileium: villi are sparsest, shortest and narrowest of all three regions; lamina propria houses Peyer’s patches in the wall
Components of the large intestine
Cecum, appendix, colon (ascending, transverse, descending and sigmoid), rectum and anus
How does the colon differ from small intestine?
Not fold into plicae circularis and villi are absent from the mucosa giving it a smooth surface
Also no Paneth cells
Crypts of Lieberkhun in the Large intestine
Crypts are longer and highly glandular with lots of Goblet cells
Open into the lumen at the surface of the colon and can be seen as a regular array of holes across the mucosa
How does the muscularis mucosa in the colon differ from that in the small intestine?
Better developed with clear circular and longitudinal fibers
Outer longitudinal muscle is arranged in 3 distinct bands called taenia coli that remain partially contracted and cause the intestine to be puckered into sacculations called haustra coli
Blind-ended tube extending from the cecum distal to the ileocecal junction
Appendix – has masses of lymphoid tissue in mucosa and submucosa, which often forms follicles with germinal centers
What change happens at the end of the rectum where it joins the anal canal?
Epithelium transitions from simple columnar to stratified squamous
Crypts disappear and are replaced by large circumanal glands
Venous plexi found in the submucosa of the anal canal
- Internal hemorrhoidal plexus
2. External hemorrhoidal plexus
Duodenum
Brunner’s glands in submucosa
Ileum
Villi and crypts
Peyer’s patches
Colon
Just Crypts
No paneth cells
Appendix
Lymphoid tissue
Anus
Squamous epithelium
Glandular
