Digestive System

Question 0

Generic layers of GI tract

Answer 0

Mucosa - epithelium (usu simple columnar)
- lamina propria (connective)
- muscularis mucosa (thin)
Submucosa - connective
Muscularis externa - circular and longitudinal
Serosa (visceral peritoneum + connective) or adventitia (loose CT)

Question 1

Functions of digestive system

Answer 1

Ingestion
Preparation and digestion
Absorption
Excretion of waste
Fluid and electrolyte balance

Question 2

Histology of esophagus

Answer 2

Non-keratinized strat squam (abrasion, no absorption)
- abrupt transition at gastroesophageal junction
Serous glands in submucosa
Muscularis externa = skeletal muscle (sup 1/3) -> smooth (inf 1/3)
Adventitia (serosa in ant and lateral abdominal portion)

Question 3

Functions of stomach

Answer 3

Storage (up to 1.5 L)
Hormones (gastrin, leptin)
Mechanical mixing
Begin breakdown - acid env’t -> pepsin, lipase

Question 4

Overall histology of stomach

Answer 4

Lumen - lined by columnar and surface mucous cells
Gastric pit ->
Neck - stem cells, mucous (different kind)
Gastric gland - parietal/oxyntic, chief/zymogenic, enteroendocrine
Muscularis externa - thickened, three layers (+ pyloric sphincter)

Question 5

Gastric mucous

Answer 5

Required due to acidic env’t
Sticky, alkaline (HCo3-) released by surface mucous cells
Different mucous released by neck mucous cells

Note: parietal cells release HCo3 due to carbonic anhydrase -> H+ secretion, capillaries carry up to surface mucous

Question 6

Parietal cells of stomach

Answer 6

“Oxyntic”
Located in gastric glands
Produce HCl and intrinsic factor (for B12 absorption)
Produce H+ via carbonic anhydrase (release HCo3- to capillaries)
Pump H+ into extensive cannaliculi (requires ATP)
Many mitochondria
Appears pink/red due to mitochondria, tubulovesicular (inactive) -> cannaliculi

Question 7

Chief cells of stomach

Answer 7

“Zymogenic”
Produce pepsinogen (-> pepsin in acid env’t)
Also produce lipase, leptin
In gastric glands
Appear blue (base, ribosomes) -> red (apical vesicles of protein)

Question 8

Enteroendocrine cells of stomach

Answer 8

Produce gastrin (in antrum -> H+ secretion by parietal of body)
Produce serotonin, VIP, somatostatin, etc
Within gastric glands, release into capillary via secretory granules

Question 9

Regions of stomach

Answer 9

Fundus = superior
Cardia - around esophageal opening
Body
Antrum - distal portion
Pylorus

Question 10

Gross histology of small intestine

Answer 10

1500-fold increase in surface area!
Plicae circularis - visible folds in surface
Villi - projections of epithelium
Microvilli - cellular projections from apical surface
Glands/crypts throughout - continuous epithelium, burrows into lamina propria
Muscularis mucosa - moves villi
Muscularis externa -> peristalsis (antero and retrograde)

Question 11

Duodenum

Answer 11

Pyloric sphincter - duodenojujunal junction (ligament of Treitz)
Submucosal glands of Brunner -> alkaline
Common bile duct - bile from liver/gallbladder -> fat breakdown
Pancreatic duct - secretions - alkaline, breakdown protein and carbs

Submucosal glands + simple columnar = duodenum!

Question 12

Intestinal villi

Answer 12

Projections of epithelium
Core = lamina propria with capillaries, lymphatics, nerves, smooth muscle, connective tissue/plasma cells
Epithelium - enterocytes with microvilli, also goblet cells
(Muscularis mucosa is only at base of villus, everything else is within lamina propria)

Question 13

Intestinal enterocytes

Answer 13

Simple columnar layer
Contain microvilli
Absorption across apical membrane -
- fat = cholymicrons -> lacteal (lymphatic)
- protein, carbs -> capillary
- both are aided by glycocalyx/glycoproteins
Appearance - microvilli -> brush border, junctional complexes -> terminal bars between cells

Question 14

Intestinal glands

Answer 14

Crypt of Leiberkuhn
Epithelium is continuous with surface
Regeneration of cells -> migrate out -> slough off after 3-6 d
Paneth cells - secrete lysozyme (anti-bacterial)

Question 15

Ileum

Answer 15

Immune function - lots of lymphoid tissue

• Peyer’s patches = big
• lymphoid follicles in lamina propria and submucosa
  
  • epithelium M cells endocytose -> macrophage, lymphocytes -> lymph nodes -> plasma/IgA response

Also absorption of B12 (requires intrinsic factor)
Reabsorption of bile -> portal vein (enterohepatic circulation)
Less absorption overall - fewer plicae circularis, villi

Question 16

General histology of colon

Answer 16

No villi - enterocytes still have microvilli
Many goblet cells -> “gland”
External (longitudinal) muscularis externa -> 3 bands = taeniae coli with haustra (bulges/sacculations)
Lymphoid tissue in lamina and submucosa

Question 17

Functions of colon

Answer 17

Absorption of water and electrolytes -> formed stool

Small intestine absorbs greatest quantity of water overall! (8500 vs 400)

Question 18

Appendix

Answer 18

“Vermiform”

Large increase in lymphoid tissue -> immune fx?

Question 19

Rectum

Answer 19

12 cm long
Longitudinal muscularis externa is once again circumferential (no taeniae)
Ends at pelvic diaphragm - longitudinal folds = columns of Morgagni

Question 20

Anus

Answer 20

4 cm
Abrupt epithelial changes: simple columnar -> non-keratinized strat squam -> keratinized strat squam
Circumferential muscularis externa -> thickens = internal sphincter

Question 21

Enteric nervous system

Answer 21

10>8 - outnumbers CNS neurons
Autonomic with SNS and PNS modulation
Meissner’s plexus - submucosa
Auerbach’s plexus - between layers of muscularis externa = “myenteric”

Question 22

Enteroendocrine system

Answer 22

Enteroendocrine cells throughout epithelium
Gastrin: pylorus -> acid secretion
Secretin: small intestine -> HCO3- and H20 from pancreas, bile
Cholecystekinin (CCK): small intestine -> pancreatic enzyme and bile secretion
Somatostatin: pylorus and duodenum -> inhibit other hormones

Question 23

Lymphoid tissue in GI

Answer 23

MALT -
- M = “microfold”, M cell at apex, transcytosis of viruses to lymphoid below -> plasma cells -> IgA release into bloodstream
Also GALT…

Question 24

GI cell renewal

Answer 24

Stomach: stem cells as isthmus of glands
-> epithelium ever 4-8 days
-> glands over weeks
Small intestine: stem cells deep in crypts
-> apex/slough off 3-6 days (fastest in body)

Question 25

Surface area of intestine

Answer 25

Total of 1500x increase in surface area in jejunum

Plicae circularis - 6x
Villi - 8x
Microvilli - 30x