GI Tract II

Question 1

Length and segments of the small intestine

Answer 1

• 6 meters long
• Duodenum: 25cm
• Jejunum: 2.5m
• Ileum: 3.5m

Question 2

Major functions of the small intestine

Answer 2

1. Move chyme
2. Absorb nutrients
3. Immunodefense

Question 3

What three structures increase the lumenal surface area of the small intestines, and by how much?

Answer 3

1. Plicae circularis (valves of Kerckring): permanent transverse circular folds of mucosa/submucosa –> 2-3x increase
2. Villi: finger-like projections of surface epithelium and lamina propria –> 10x increase
3. Microvilli: projections of apical plasma membrane –> 20x increase
   * Total surface area increase is 400-600 fold

Question 4

Crypts of Lieberkuhn

Answer 4

• Intestinal glands

- Simple tubular glands that extend from epithelium down to the muscular mucosae

Question 5

6 major cell types in the epithelium

Answer 5

1. Absorptive cell (enterocyte)
2. Goblet cell
3. Enteroendocrine cell
4. Paneth cell
5. Stem cell
6. M (microfold) cell

Question 6

Absorptive cells

Answer 6

• Tall columnar cells with striated border
• Microvilli: 1 micron tall, 0.1 micron diameter
• Secrete disaccharidases and peptidases - bind microvilli and produce monosaccharides and amino acids for absorption
• Lipids absorbed, reesterified to TG, converted to chylomicrons, released from lateral surfaces in lamina propria, absorbed by central lacterals

Question 7

Goblet cells

Answer 7

• Unicellular glands
• Produce acid glycoproteins (mucin type) –> lubricate and protect small intestine
• Less abundant in duodenum, increase towards ileum

Question 8

Enteroendocrine cells

Answer 8

• Unicellular glands
• Synthesize hormones and vasoactive peptides
• Most occur in the basal portion of the crypts of Lieberkuhn

Question 9

Paneth cells

Answer 9

• Exocrine cells
• Found in small groups at base of crypts
• Replaced every 30 days
• Apical cytoplasm filled with large, eosinophilic granules (lysozyme with antibacterial activity, controls intestinal flora)

Question 10

Stem cells in the small intestine

Answer 10

• Located near the base of crypts

- Move upwards repopulating the crypts and villi in 3 to 6 days

Question 11

M (microfold) cells

Answer 11

• Only found in ileum
• Specialized epithelial cells, cover Peyer’s patches
• Discontinuous basement membrane

Question 12

Gut-associated lymphoid tissue (GALT)

Answer 12

• Prominent in lamina propria of the small intestine
• 25% of mucosa
• Peyer’s patches: in ileum, groups of 20-40 lymphatic nodules covered by M cells –> take up antigens and deliver to lymphocytes in Peyer’s. IgA syn. by plasma cells
• Secretory component: glycoprotein made by absorptive cells, inserted in membrane. IgA dimers bind –> enter by endocytosis –. exit apical PM with SC bound, forming sIgA
• sIga binds antigens, toxins, microorganisms; *major role in host defense

Question 13

Brunner’s glands

Answer 13

• In submucosa of duodenum
• Mostly mucous type, secrete and alkaline mucous that helps protect against stomach acid and provides optimum pH for pancreatic enzymes

Question 14

Regional differences between duodenum, jejunum, and ileum

Answer 14

• Duodenum: Brunner’s glands in submucosa, adventitia, villi are squared off (resemble thick cactus leaves)
• Jejunum: Most highly developed plicae circularis, villi are longer/more finger-like than duodenum, more goblet cells, has serosa
• Ileum: longest villi, highest number of goblet cells, *Peyer’s patches (unique to ileum)

Question 15

Five gross subdivisions of the large intestine

Answer 15

1. Colon
2. Cecum (don’t need to distinguish)
3. Appendix
4. Rectum
5. Anal canal

Question 16

Major functions of the larger intestine

Answer 16

1. Absorption of water, salts, minerals, and vitamins
2. Mucus secretion
3. Storage and elimination of fecal material

Question 17

Colon and cecum

Answer 17

• Plicae sumilunares: temporary folds of mucosa/submucosa
• No villi
• Crypts lined with simple columnar epi with a striated border
• Many large goblet cells
• Unique muscularis externa: outer longitudinal smooth muscle is subdivided into 3 bands (taenia coli). Thin layer of longitudinal smooth muscle between bands –> pucker colon into haustra
• Transverse and sigmoid colon have a serosa
• Ascending/descending colon and cecum have adventitia

Question 18

Vermiform Appendix

Answer 18

• Epithelium resembles colon, but fewer and shorter crypts
• Lamina propria - many lymphoid cells and lymph nodules that often interrupt muscularis mucosae. Submucousa frequently edematous (fluid)
• Muscularis externa has two complete thin layres and no taenia
• There is a serosa

Question 19

Upper rectum

Answer 19

• Resembles the colon
• Less lymphiod tissue
• No taenia coli

Question 20

Anal canal

Answer 20

• 2.5-4cm long
• Simple columnar epithelium of rectum changes to stratified cuboidal –> to stratified squamous nonkeratinized epithelium
• No crypts of muscularis mucosae
• Circumanal glands (apocrine sweat glands) found in ano-cutaneous area
• Inner anal sphincter formed from thickening of inner smooth muscle layer
• External anal sphincter form by outer longitudinal layer being replaced by skeletal muscle