Embryology Of The Gut

Question 1

Recap lateral folding of the embryo:

Answer 1

The notochord sends out chemical messengers to the ectoderm and mesoderm causing them to grow

Growth of the ectoderm leads to its folding and finally into a tube that separates from the rest of the ectoderm (this will form the brain and spinal chord).

Mesoderm will form the bones and cartilage near notochord. Further away tissue forms the genitourinary system and the furthest forms the pericardium + pleura etc

Finally the ectoderm will surround the embryo and fuse

Question 2

What parts of the gut does the endoderm and visceral mesoderm form?

Answer 2

Endoderm = epithelium of the bowel (the mucosa), hepatocytes of the liver and the endoderm and exocrine cells of the pancreas

The mesoderm forms the muscle wall and connective tissue - for the pancreas, liver and visceral peritoneum

Question 3

What forms the foregut, midgut and hindgut?

Answer 3

Foregut - starts at the lower end of the hypopharynx — 3rd part of duodenum

Midgut - 3rd part of the duodenum to 2/3 along transverse colon

Hindgut — finishing in the middle 1/3 of the anal canal

Question 4

What are the arteries and nerves of the foregut, midgut and hindgut?

Answer 4

Foregut - coeliac trunk and innervated by the greater splanchnic nerve (T6-T9)

Midgut - superior mesenteric artery
Innervated by the lesser splanchnic nerve (T10-T11)

Hindgut - inferior mesenteric artery
Innervated by the least splanchnic nerve (T12)

Question 5

What is referred pain?

E.g. lungs and heart, diaphragm

Answer 5

Painful stimuli stimulate sensory nerves. These nerves fibres enter the central nervous system through the spinal nerve root

Pain is then felt in the skin (dermatome) by which the same nerve root supplies

T1-T5 = heart and lungs - why heart attack hurts inside arm and chest even though pain is coming from your heart

Diaphragm = C345 therefore pain in neck and shoulder

Question 6

What are the 5 stages of the development of the midgut and hindgut tube?

Answer 6

1. Elongation
   - during 6th week of development midgut elongates rapidly + liver etc
2. Physiological herniation
   - make abdominal cavity too small to contain the primary intestinal loop, thus forced out into the extra embryonic cavity via umbilical chord
3. Rotation
   - it rotates 270 degrees anti-clockwise as viewed from the front. 90 during herniation and 180 during returning - forms coiled loops of SI
4. Retraction
   - during 10th week herniated midgut reruns to abdominal cavity. Jejunum 1st part to return and the decal bud is the last part to return - which spelled down down on the right upper part of the abdomen below liver.

Note the appendix develops at its distal end — appendix has variable length and position

5. Fixation
   - stage that some of the gut mesenteries come to lie against the back of the abdomen and fuse into position. Those that become fixed are called retroperitoneal. - used to be in but now only at front.

Question 7

What is extra and retro peritoneal?

Answer 7

Extra - refers to organs never in the peritoneum

Retro = was in, but now peritoneum only lies at front

Question 8

Are these fixed or mobile?

Duodenum? 
Ascending colon? 
Descending colon? 
Rectum? 
Stomach? 
Rest of SI?
Appendix? 
Transverse colon? 
Sigmoid colon?

Answer 8

D - no - fixed
A - no
D - no
R - no
S - yes - mobile 
R - yes
A - yes
T - yes
S - yes

Question 9

What makes up the foregut?

Answer 9

Oesophagus 
Stomach 
First half of the duodenum (1,2 out of 4) 
Pancreas 
Liver and biliary system 

Dorsal and ventral mesentery (greater and lesser momentum respectively)

Question 10

What parts does the mesoderm and endoderm give rise to?

Answer 10

Visceral Mesoderm = muscle wall, connective tissue for wall, pancreas, liver peritoneum

Endoderm = epithelium of bowl, hepatocytes of liver, endo and exocrine cells of pancreas

Question 11

Steps of foregut development:

Answer 11

Liver, bile, ventral and dorsal pancreatic duct develop and grow

As the liver grows it can’t fit and so rotates the bowl. Also causes a bare patch on the liver as it cant fit in the mesoderm and this is also where the falsified ligament is found.

The ventral pancreatic duct then rotates so that the 2 ducts lie together and then they fuse. They trap the superior mesenteric artery and vein between the 2. Ventral on top, dorsal below.

Note the movement of the liver moves right and the spleen moves left which causes the stomach to rotate

Question 12

Job of the falsified ligament?

Answer 12

It connects the liver to the anterior abdominal wall.

Question 13

What mesentery does the liver and spleen develop in?

Answer 13

Liver in ventral

Spleen in dorsal

Question 14

How is the lesser sac formed?

Answer 14

Fixation of the greater omentum and liver to the posterior wall creates a space behind = less sac

Also note the dorsal edge of the stomach elongates to form the greater curvature

For more see anatomy