Development Of The GI System 1&2

Question 1

when does embryonic folding take place? How does it fold and what does that create?

Answer 1

in the 4th week

Laterally
-> ventral body wall, primitive gut becomes tubular

Craniocaudally -> cranial and caudal pockets from yolk sac endoderm

Question 2

What is the primitive gut tube?

Answer 2

Endoderm lined tube which runs the length of the body

Blind pouches at the head and tail ends (not open)

Opening at the umbilicus (->midgut)

Splanchnic mesoderm covering

Formed from definitive yolk sac during folding - blind diverticula cranially (buccopharyngeal membrane) & caudally (cloacal membrane)

Embryonic divisions - different blood supply, innervation, Lymph systems

Question 3

What do the somatic and splanchnic mesoderm become and which is more sensitive to Pain?

Answer 3

Somatic mesoderm -> parietal pleura - more innervation so more sensitive to pain

Splanchnic mesoderm -> visceral pleura - poorly innervated Ill- defined pain

Question 4

What does the foregut become and what blood vessel supplies them?

Answer 4

Oesophagus -> duodenum (before bile duct)

Oesophagus 
Stomach 
Pancreas
Liver 
Gallbladder 
Duodenum (proximal) 

Celiac trunk branch of abdominal aorta

Question 5

What does the midgut become and what blood vessel supplies them?

Answer 5

Duodenum (after bile duct) -> last 2/3 transverse colon

Duodenum (distal) 
Jejunum
Ileum
Caecum 
Appendix
Ascending colon 
Proximal 2/3 transverse colon 

Superior mesenteric artery branch of abdominal aorta

Question 6

What does the hindgut become and which blood vessel supplies them?

Answer 6

(Distal 1/3) Transverse colon-> internal lining of bladder and urethra

Transverse colon (distal) 
Descending colon 
Rectum 
Upper anal canal 
Internal lining of bladder and urethra 

Inferior mesenteric artery branch of abdominal aorta

Question 7

Which structures have mixed blood supply and why?

Answer 7

Structures which develop close to the junction between foregut and midgut (duodenum and pancreas)

Duodenum (proximal to bile duct) gastroduodenal artery and superior pancreaticoduodenal artery (celiac trunk)

Duodenum (distal to bile duct) inferior pancreaticoduodenal artery (superior mesenteric artery)

Pancreas superior pancreaticoduodenal artery (celiac trunk) and inferior pancreaticoduodenal artery (superior mesenteric artery)

Question 8

what is the intraembryonic coelom? How does the intraembryonic coelom get divided and into what?

Answer 8

Subdivided by the future diaphragm (septum transversum) into abdominal and thoracic cavities

Intraembryonic coelom suspends primitive gut tube and is a double layer of splanchnic mesoderm

Question 9

What does the membrane lining the intraembryonic cavity become?

Answer 9

Pericardium, pleural membrane of lungs and peritoneum of peritoneal cavity (all have parietal and visceral pleura)

Question 10

What is a Mesentery and what’s its function?

Answer 10

Double layer of peritoneum suspending the gut tube from the abdominal wall

Allows a conduit for blood and nerve supply and mobility where needed

Formed from a condensation of splanchnic mesoderm which surrounds the gut

Question 11

What are the two gut mesenteries?

Answer 11

Dorsal - suspends the entire gut tube from the dorsal body wall

Ventral - only in the region of the foregut

Question 12

What do the dorsal and ventral mesenteries in the foregut create and what do they become?

Answer 12

Divide the cavity into:

Left sac contributes-> greater sac

Right sac becomes -> lesser sac (comes to lie behind the stomach and allows for distension)

Form:
Specialised regions of peritoneum

Dorsal Mesentery -> greater omentum (first structure seen when abdo cavity opened anteriorly)

Ventral Mesentery -> lesser omentum (free edge conducts the portal triad)

Question 13

How does the stomach rotate and what does this mean for the position of the certain structures? How is the greater curve formed?

Answer 13

The stomach enlarged and tips on its axis

Dorsal Mesentery grows faster and creates the greater curve, it folds to form the greater omentum

The vagus N goes from right and left to anterior and posterior

Cardiac and pylorus (opening to duodenum) shifted from midline

Ventral Mesentery becomes the lesser omentum

Question 14

What is peritoneal reflection?

Answer 14

The peritoneum changes direction so

parietal peritoneum becomes Mesentery

and then Mesentery becomes visceral peritoneum

and that becomes Mesentery etc.

Question 15

What are the meaning of the words retroperitoneal and secondarily retroperitoneal? What are examples of structures?

Answer 15

Retroperitoneal - never in the peritoneal cavity and never had a Mesentery e.g. kidneys, aorta, IVC

Secondarily retroperitoneal - began development invested by peritoneum and had a Mesentery but Mesentery is lost through fusion at posterior abdominal wall (replaced by fusion fascia) e.g. pancreas, duodenum bar cap, ascending and descending colon, distal 1/3 rectum

Question 16

What does the foregut extend to and from in development?

Answer 16

Foregut extends from the lung bud (4th week respiratory diverticula forms in ventral wall at junction with pharyngeal gut) to the liver bud

Question 17

What separates the developing GI and respiratory tracts? What are some consequences of abnormal positioning of this structure?

Answer 17

Respiratory diverticulum and foregut separated by a tracheoesophageal septum

There are a variety of oesophageal abnormalities e.g. proximal blind-end of oesophagus, tracheoesophageal fistula (connection)

Question 18

What do the foregut-derived glands develop from (liver, biliary system, pancreas)?

Answer 18

Liver and biliary systems (liver, gallbladder, bile ducts) - ventral Mesentery

Pancreas
Uncinate process and inferior head - ventral Mesentery
Superior head, neck, tail, body - dorsal

Question 19

Peritoneal reflections of the liver from 6th week

Answer 19

Whole liver covered in peritoneum bar ‘bare area’ on diaphragmatic surface - rough, dull areas

Falciform ligament to left

Lesser omentum to right

Question 20

What does the duodenum develop from and what determines its shape?

Answer 20

Develops from caudal foregut & cranial midgut

Shape determined by rotation of stomach: pushes duodenum to the right and against posterior abdo wall -> secondarily retroperitoneal bar cap

Question 21

What is the primary intestinal Loop?

Answer 21

Midgut elongates - runs out of space so makes a loop that has:
Superior mesenteric artery at axis
Connected to yolk sac via vitelline duct

Has cranial and caudal limbs

Question 22

What is the normal physiological herniation that occurs in the 6th week of development?

Answer 22

Growth of the primary intestinal loop is v rapid, liver is also growing rapidly so abdo cavity too small for both -> intestines herniate into umbilical cord

Question 23

Midgut rotation

Answer 23

during herniation turns 90d

After herniation cranial limb first returns to abdo cavity by rotating 90d to left (so cranial limb now inferior)

Third 90d rotation creates transverse colon (cranial limb now left, SMA middle, caudal limb right)

Question 24

How does the ascending gut develop?

Answer 24

Caecal bud descends

Question 25

What are some malrotations of the midgut and what problems are associated with these?

Answer 25

~ incomplete rotation- midgut loop makes one 90d rotation = left sided colon

~ reversed rotation- midgut loop makes one 90d rotation clockwise = transverse colon passes posterior to duodenum

Most complications in neonatal period -> volvulus (loop of intestine twists around itself and Mesentery that supports it) -> strangulation (bowel obstruction) & ischaemia

Question 26

What are 3 abnormalities associated with the vitelline duct persisting (remnants of the yolk sac)? whats a main problem that could be as a result of all three?

Answer 26

• vitelline cyst
  Forms fibrous strands between midgut and umbilicus

-Vitelline fistula
Communication between umbilicus and intestines (contents can leak)

-Meckel’s diverticulum
A.k.a ileal diverticulum, 2% population, 2:1 Male:Fm, 2 feet from ileocaecal valve (62cm), normally picked up before 2yrs

All could contain ectopic gastric or pancreatic tissue -> can become inflamed

Question 27

In which structures does the lumen become at least partially obliterated and why? What does the body do to restore the lumen? What occurs if this is unsuccessful?

Answer 27

Gut structures: oesophagus, bile duct, small intestine

Cell growth becomes so rapid

Recanalisation occurs to restore lumen

If wholly/ partially unsuccessful -> atresia (no lumen) /stenosis (narrowed)

Question 28

What is pyloric stenosis? What’s a characteristic symptom?

Answer 28

Region between stomach and duodenum, hypertrophy of circular muscle in sphincter

Not a recanalisation failure

Common abnormality in stomach of infants

-> projectile vomiting

Question 29

What is gastroschisis?

Answer 29

Failure of closure of the abdominal wall during embryonic folding

Leaves gut tube and derivatives outside the body cavity, no peritoneal layer

Caesarean section but normally fine

Question 30

What is an omphalocoele?

Answer 30

Persistence of physiological herniation, have covering of peritoneum in umbilical cord

(An umbilical hernia differs as covered in skin and subcutaneous tissue)

Often associated with other genetic defects so mortality rate is high

Question 31

What separates the two regions of the anal canal? What is different about these regions?

Answer 31

Divided by the pectinate line into distinct superior (part of hindgut) - Inferior mesenteric artery, S2-4 pelvic parasympathetic, columnar epithelium, internal iliac nodes - sensation of stretch only bc made from endoderm

Pectinate line

and inferior (not part of hindgut), pudendal artery, S2-4 pudendal N, stratified epithelium, superficial inguinal nodes, sensations of touch/ temperature & pain bc made from ectoderm

indicates differences in arterial supply, venous and lymphatic drainage & innervation

Question 32

What do different locations of visceral pain mean?

Answer 32

Poorly localised bc splanchnic innervation

Epigastric pain (top abdominal cavity) -> foregut and derivatives

Periumbilical (belly button) -> midgut

Suprapubic (pelvic region) -> hindgut

Parietal peritoneum- more localised as somatic innervation

Question 33

What is imperforate anus?

Answer 33

Failure of anal membrane to rupture

Question 34

What is anal/ anorectal agenesis?

Answer 34

Never develops

Question 35

What is a hindgut fistulae?

Answer 35

Connection between:

Bladder and rectum - recto-vesical

Vagina and rectum-
Recto-vaginal

Recto-uretheral

Question 36

What will the cranial and caudal limb of the primary intestinal loop become?

Answer 36

Cranial: distal duodenum, jejunum, proximal ileum

Caudal: distal ileum, caecum, appendix, ascending colon, proximal 2/3 transverse colon