The Digestive Tract

Question 1

What are the components/organs of the gastrointestinal tract?

Answer 1

Gastrointestinal tract (muscular tube):

• oral cavity
• pharynx
• oesophagus
• stomach
• small intestine
• large intestine
• rectum

Question 2

What are the organs associated with the digestive tract/formed by it during development?

Answer 2

Associates organs:
-from oesophagus: 
   lungs
-from duodenum:
   liver
   gall bladder
   pancreas

Question 3

What germ layer(s) form the digestive tract?

Answer 3

all three germ layers form digestive tract:

• ectoderm
• mesoderm
• endoderm

Question 4

What is the general internal structure of the digestive tract?

Answer 4

centre to periphery:
(-inside=lumen)
-mucosa
-submucosa
-muscularis externa
-adventitia

Question 5

What is the submucosal plexus (Meissner’s plexus)?

Answer 5

• between submucosa and circumferential layer of muscle (muscularis externa)
• 1st layer of ganglia of enteric nervous system (controlling contraction of muscle layers)

Question 6

What is the myenteric plexus (Auerbach’s plexus)?

Answer 6

• between inner circumferential and outer longitudinal smooth muscle layers (of muscularis externa)
• 2nd layer of ganglia of enteric nervous system (controlling contraction of muscle layers)
• smooth + skeletal muscle (can be voluntary)

Question 7

Describe the anatomy of the oesophagus

Answer 7

• similar to skin but not keratinised (however in rodents may be keratinised as food more coarse)
• stratified squamous epithelium in oral cavity, pharynx, and oesophagus (coarse foods passing down)
• new cells continuously produced in basal layer, become strat. squamous epithelium as they move to the lumen
• at lumen: cells soughed off as food passes down

Question 8

Describe the anatomy of the stomach

Answer 8

-simple columnar epithelium = secretion
-closely packed tubular glands that open into lumen via gastric pits
-oesophagal + phyloric sphincters keep acid chyme in stomach
-secreting cells in glands:
-closest to lumen (neck): mucous (then lines surface
of
cells in epithelium - stops acid from digesting
stomach)
-middle: acid secreting parietal cells (=low pH)
-at base: pepsin secreting chief cells (degrades
protein in stomach, only works at low pH)
(at very bottom: enteroendocrine cells: secrete
hormones regulating acid + pepsin production;
also mitotic stem cells: replace others)

Question 9

Describe the anatomy/function of the small intestine

Answer 9

-3 parts:
-duodenum: digestion, iron absorption
(duodenal glands secrete bicarbonate: increase pH
from 2 to 8)
-jejunum: food absorption
-ileum: vitamin B12 and bile salt absorption
(more lymphoid aggregates)
-simple columnar epithelium:
absorptive enterocytes + mucus-secreting goblet cells
-circular folds (plicae circulares) + villi
=increased surface area for absorption

Question 10

Describe the functions of the liver, gall bladder and pancreas in digestion

Answer 10

Pancreas:
-pancreatic acini (acinar cells = exocrine) secrete bicarbonate (neutralise pH of chyme) and enzymes (break down proteins - ex: pancreatic lipase)

Liver/gall bladder:

• bile formed by liver, then stored in gall bladder, then release into duodenum
• aids digestion by pancreatic lipase by emulsifying fat

Question 11

What is bile?

Answer 11

Bile: an emulsifier, forcing fats/lipids into smaller aggregates (micelles) = easily digested by pancreatic lipases

Question 12

What are exocrine vs endocrine glands? What do these correspond to in the pancreas?

Answer 12

-exocrine: secrete onto surface, have ducts
(acini: acinar cells - secrete proteases/enzymes +
bicarbonate)
-endocrine: secrete hormones into blood, don’t have
ducts
(islet - secrete hormones such as insulin, regulates
glucose level in blood)

Question 13

What is the anatomy/function of the large intestine?

Answer 13

3 parts:
-cecum (appendix)
-colon (water absorption, production of faeces)
-rectum (storage of faeces)
-simple columnar epithelium
-made up of tubular glands
(water-absorbing enterocytes and mucus-secreting goblet cells = protection from increasingly solid faeces)
-outer muscle layer is discontinuous in large intestine (missing from most of colon), forming three longitudinal bands = taenia coli

Question 14

How do the 3 germ layers contribute to the development of the digestive tract?

Answer 14

• ectoderm: NCCs become enteric nervous system of digestive tract
• mesoderm: connective tissue layers of digestive tract
• endoderm: linings of the digestive tract

Question 15

What are the embryonic foldings that occur?

Answer 15

• rostral-caudal (longitudinal)

- ventral (lateral)

Question 16

What does rostral-caudal folding create?

Answer 16

• head fold first - foregut
• then folding at rostral end - hindgut
• middle region still stays open to underlying yolk sac (open region=where midgut will form)

Question 17

What does ventral folding create?

Answer 17

• endoderm turned into a tube
• lateral plate mesoderm fuses above gut tube (dorsal mesentery - persists) and beneath it (ventral mesentery- lost everywhere except foregut)

Question 18

What is omphalocoele?

Answer 18

congenital malformation in which gut extrudes from abdomen through the umbilicus

Question 19

How is the enteric nervous system formed?

Answer 19

Submucosal + myenteric neurons are formed by NCCs that migrate from dorsal neural tube to developing gut

• vagal NNCs (neck region) migrate: invade foregut, midgut, and then hindgut where they differentiate
• NCCs from sacral region invade most caudal regions of hindgut but do not migrate rostrally

Question 20

What is Hirschsprung disease?

Answer 20

-posterior migration of NCCs not complete
=aganglionic potion of colon (no enteric NS)
-leads to expanded segment of colon above constricted aganglionic segment of colon (untreated=megacolon) - build up of bowel contents due to absence of peristaltic movements
-treated by removal of aganglionic portion