Structures of Lower GI Tract

Question 1

What are the three regions of the small intestine?

Answer 1

Duodenum
Jejunum
Ileum

-These are all histologically similar but minor differences permit their identification.

Question 2

Name 3 modifications of the small intestine that increase its surface area to aid in absorption.

Answer 2

1. Plicae circulares
2. Villi
3. Microvilli
   * * These three specialized modifications increase the intestinal surface area by 400-600 fold

Question 3

Permanent mucosal folds present in the duodenum, jejunum and proximal ileum – increase surface area by 2-3 fold

Answer 3

Plicae circularis

Question 4

Finger-Like projections of epithelium-covered lamina propria that increase surface area by 10-fold

Answer 4

Villi

Question 5

Extensions of the epithelial cell apical plasma membrane that increase surface area 20-fold

Answer 5

Microvilli

Question 6

Villi are extensions of the….

Answer 6

Lamina propria

They contain a central lymphatic channel and several capillaries

Question 7

Epithelial covering of the villi are composed of:

Answer 7

Absorptive cells (enterocytes) 
Goblet cells
DNES (enteroendocrine) cells

Question 8

Extend from the base of the villi into the lamina propria

Answer 8

Crypts of Lieberkuhn

Contain: DNES (enteroendocrine) cells, stem cells, paneth cells

Question 9

What is the most abundant cell of the small intestine?

Answer 9

Absorptive cells/enterocytes – they are columnar epithelial cells with an elongated nucleus

Question 10

What is the principal function of the enterocytes?

Answer 10

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

Question 11

What is in the brush border of the luminal surface of each enterocyte?

Answer 11

Microvilli covered in glycocalyx – has protective role but also involved in digestions of disaccharides and dipeptides

Question 12

What do Goblet cells secrete?

Answer 12

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

Question 13

In the Crypts of Lieberkuhn, explain how cell type differs between the upper and lower regions?

Answer 13

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

Question 14

How often is the intestinal epithelium replaced?

Answer 14

Every 3-6 days

Question 15

What do Paneth cells secrete?

Answer 15

Lysozyme and defensins

*Paneth cells are long lived and do not participate in upward migration of epithelial cells

Question 16

Highly vascularized to accept the products of absorption – also contains abundance of lymphoid cells that protect the intestinal lining from microbial invasion

Answer 16

Lamina propria

Question 17

Contains an inner circular and outer longitudinal layer of smooth muscle fibers – during digestion these fibers contract causing shortening of the villus

Answer 17

Muscularis mucosae

Question 18

Lamina propria’s extensive immune defense system

Individual lymphocytes within the mucosa and epithelium or regions of coalesced lymphoid nodules

Answer 18

GALT = gut-associated lymphoid tissue

Consists of individual lymphoid nodules and Peyer’s patches

Question 19

How does the duodenal submucosa differ from the rest of the GI tract?

Answer 19

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

Question 20

What controls the release of secretions from the intestines?

Answer 20

Submucosal (Meissner’s) Plexus

Question 21

What lies between the smooth muscle layers of the muscularis external?

Answer 21

Myenteric (Auerbach’s) Plexus – peristaltic movement

Question 22

How do the 3 regions of the small intestine differ?

Answer 22

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

Question 23

Components of the large intestine

Answer 23

Cecum, appendix, colon (ascending, transverse, descending and sigmoid), rectum and anus

Question 24

How does the colon differ from small intestine?

Answer 24

Not fold into plicae circularis and villi are absent from the mucosa giving it a smooth surface

Also no Paneth cells

Question 25

Crypts of Lieberkhun in the Large intestine

Answer 25

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

Question 26

How does the muscularis mucosa in the colon differ from that in the small intestine?

Answer 26

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

Question 27

Blind-ended tube extending from the cecum distal to the ileocecal junction

Answer 27

Appendix – has masses of lymphoid tissue in mucosa and submucosa, which often forms follicles with germinal centers

Question 28

What change happens at the end of the rectum where it joins the anal canal?

Answer 28

Epithelium transitions from simple columnar to stratified squamous
Crypts disappear and are replaced by large circumanal glands

Question 29

Venous plexi found in the submucosa of the anal canal

Answer 29

1. Internal hemorrhoidal plexus

2. External hemorrhoidal plexus

Question 30

Duodenum

Answer 30

Brunner’s glands in submucosa

Question 31

Ileum

Answer 31

Villi and crypts

Peyer’s patches

Question 32

Colon

Answer 32

Just Crypts

No paneth cells

Question 33

Appendix

Answer 33

Lymphoid tissue

Question 34

Anus

Answer 34

Squamous epithelium

Glandular