Week 2: Lower Gi tract, intestines Microanatomy

Question 1

Mechanisms used to increase surface area in lower GI tract.

Answer 1

Features of small intestines

1. small intestine is 20 feet long
2. Plica circulares- permanent folds of mucosa and submucosa. Increases surface area 3x.
3. Villi: finger like projections of epithelium and lamina propria
4. Microvilli on columnar absorptive cells, increases surface area 20x.

Question 2

Schematic/cells of intestinal gland (Crypts of Lieberkuhn) in small intestines

Answer 2

1. surface absorptive cell (columnar absorptive cell)
2. goblet cell
3. enteroendocrine cell
4. stem cell. Immature columnar cells located near base of crypt. Renewal rate 3-5 days.
5. Paneth cell: towards bottom of crypt of lieberkuhn. Makes Zn.potential phagocytic activity
6. M cell

Question 3

Cell types of villi of small intestines

Answer 3

1. Columnar absorptive cell
   - extensive microvillous border, rich glycocalyx
   - rich in organelles
   - real tight junctions with space between basolateral side
   - enzymes in membrane, e.g. alkaline phosphatase, disaccharidases, peptidases, in mosaic pattern
   - efficient absorption system, 50-60% protein has been absorbed by end of duodenum
2. Mucous goblet cell: 10% of surface epithelium.

Question 4

Immune surveillance in small intestines

Answer 4

• M cells bring in intact bacterial/antigens to lymphocytes and macrophages below it
• columnar absorptive cell interact with certain types of bacteria and produce cytokines, e.g. Il-8, which act as chemotactic factors for PMNs
• lymphocytes activated and enter lymph vessels and join systemic circulation and home back to original site. E.g. B cells–>plasma cells producing IgA

Question 5

Regional variations in small intestines

Answer 5

• further along the small intestines, Villi and plica circulares decrease
• Brunner’s mucosa glands present in some parts of duodenum
• Peyer’s patches particularly prominent in ileum.

Question 6

Protection and renewal of epithelial surface of intestines

Answer 6

• rapid turnover of digestive enzymes, 2-3hrs
• mucus for lubrication and protection
• biological escalator: of immature columnar cells
• columnar and goblet cells sloughed at villous tip. Smooth muscle within lamina propria helps with sloughing cells

Question 7

Micro anatomy features of large intestines

Answer 7

• lacks villi and plica circulares
• smoother surface
• thicker mucosa, deeper crypts
• increasing amount of goblet cells and decreasing amounts of columnar absorptive cells
• also present: stem cells, EEC, M cells, lymphocytes, macrophages
• few Paneth cells
• lower cell turnover than small intestines
• longitudinal layer of mucularis external is organized into taenia coli, bands of longitudinal muscle that results in haustra (sacculations)
• surface epithelial cells and not crypts are major sites of Na pumps/water absorption, with aldosterone receptors
• Colon absorbs ~300-400mLday compared to 9L/day of small intestines, but still important

Question 8

Micro anatomy of the appendex

Answer 8

• blind ended narrow appendage
• mucous membrane similar to colon
• lamina propria has more lymphatic tissue
• occlusion or blockage makes susceptible to bacterial infection and appendicitis

Question 9

Rectum and anal canal micro anatomy

Answer 9

Rectum
-epithelium mostly all goblet cells. Thick mucosa with deep crypts
-not well developed muscular mucosa. Merged lamina propr. and submucosa
-thin walled veins without valves in submucosa (hemorrhoid formation)
Recto-anal junction
-epithelium changes to stratified squamous epithelium. Lacks crypts
-circular smooth muscle forms internal anal sphincter
-skeletal muscle forms external anal sphincter
-longitudinal smooth muscle tapers away blending into CT