GI

Question 0

Describe the development of the muscular and fascial layers of the abdominal wall, including the inguinal canal

Answer 0

Muscular layers develop from the mesoderm of the gut splitting into a somatic layer
Lateral folding causes the two sides to meet in the middle (linea alba)
The inguinal canal is an oblique passage through the layers of the abdominal wall (in males, allows the passing of the testes into the scrotum)

Question 1

Explain how the process of folding in the embryo during the 4th and 5th week creates the primitive gut, abdominal wall and the coelomic cavity

Answer 1

Folding creates a primitive gut tube, lined with endoderm and divisible into foregut, midgut, hindgut
The mesoderm surrounding the gut splits into a somatic layer (develops into muscles and fascia of abdominal wall) and a splanchnic layer (becomes smooth muscle of gut wall)
The coelomic space is created by the space between the layers

Question 2

Explain the developmental basis of umbilical and inguinal hernias

Answer 2

Umbilical - during lateral folding, the two sides of the abdomen come together and meet at the midline (linea alba)
Inguinal - in males, the inguinal canal allows for the passage of the testes into the scrotum, forms the fascial covering of the spermatic cord
Areas of weakness

Question 3

Describe how the coelomic cavity and peritoneal cavity develop

Answer 3

Coelomic cavity - when the mesoderm surrounding the gut splits into the somatic and splanchnic layers, it creates a space
Peritoneal cavity develops due to the ventral and dorsal mesenteries changing shape and position?

Question 4

Describe the fate of the embryonic dorsal and ventral mesenteries

Answer 4

Dorsal mesentery - as stomach rotates, dorsal mesentery is drawn into a sac, the omental bursa, to become the greater omentum
Ventral mesentery - becomes lesser omentum

Question 5

Explain how the greater and lesser omenta and the mesentery of the small intestine develop

Answer 5

Greater omentum - during rotation of the stomach, dorsal mesentery is drawn into a sac, the omental bursa
Lesser omentum - ventral mesentery, connects lesser curvature of stomach to liver
Mesentery of small intestine - jejunum, ileum remain suspended from mesentery and are mobile, ascending and descending colon, duodenum mesentery fuses with peritoneum on posterior abdominal wall to become immobile (retroperitoneal)

Question 6

Explain the embryonic divisions of the gut

Answer 6

Foregut
Midgut (opening continuous with yolk sac)
Hindgut

Question 7

Describe the primitive gut tube

Answer 7

Development begins in late third week
Runs from stomatodeum (future mouth) to proctodeum (future anus) with an opening at umbilicus
Internal endoderm lining
External mesoderm
Suspended in intracoelomic cavity by double layer of splanchnic mesoderm

Question 8

What is a mesentery and why is it necessary?

Answer 8

A double layer of peritoneum suspending the gut tube from the abdominal wall
Allows blood and nervous supply
Allows motility

Question 9

Describe the attachments of the mesenteries to the primitive gut tube

Answer 9

Ventral mesentery - foregut only

Dorsal mesentery - foregut, midgut, hindgut

Question 10

What are omenta?

Answer 10

Specialised regions of peritoneum

Question 11

Describe the rotation of the stomach and its result

Answer 11

Longitudinal rotation - greater curvature lies on left, lesser curvature lies on right
Anteroposterior rotation - cardia and pylorus move horizontally, greater curve pushed inferiorly

Question 12

Explain the consequences of rotation of the stomach

Answer 12

Vagus nerves positioned anteriorly and posteriorly
Shifts cardia and pylorus from midline - stomach lies obliquely
Contributes to moving lesser sac behind the stomach
Creates the greater omentum

Question 13

Explain the difference between retroperitoneal and secondary retroperitoneal

Answer 13

Retroperitoneal - was never in peritoneal cavity, never had a mesentery
Secondarily retroperitoneal - developed in peritoneal cavity but mesentery fused with posterior abdominal wall e.g. duodenum, pancreas

Question 14

Describe the development of the lung bud

Answer 14

In 4th week, a respiratory diverticulum forms in the ventral wall of the foregut at the junction with the pharyngeal gut

Question 15

List organs that develop in the ventral and dorsal mesenteries

Answer 15

Ventral - liver, biliary system, part of pancreas (uncinate process, inferior head)
Dorsal - spleen, pancreas (superior head, neck, body, tail)

Question 16

Describe the development of the liver

Answer 16

Develops from a hepatic bud in ventral mesentery
Occupies a large proportion of abdomen
Contact with diaphragm –> bare area of liver
Fully developed by 6th week

Question 17

Describe the development of the duodenum

Answer 17

Develops from caudal foregut and cranial midgut
Grows rapidly, forming C shaped loop when stomach rotates
In 5th and 6th week, lumen is obliterated then recanalised by end of embryonic period
Rotation of stomach pushes duodenum to right and against posterior abdomen wall

Question 18

Describe the consequences of developmental defects as they relate to the abdominal wall and give relevant examples

Answer 18

Omphalocele - herniation of abdominal viscera through an enlarged umbilical ring, viscera covered by amnion, high rate of mortality
Gastroschisis - protrusion of abdominal contents through body wall directly into amniotic cavity, viscera not covered, excellent survival rate
Meckel’s diverticulum - remnant of yolk sac
Vitelline cyst - intestinal obstruction, strangulation, volvulus
Umbilical fistula - faecal discharge at umbilicus
Divarication of recti - laxity of linea alba –> herniation

Question 19

Relate visceral referred pain to the embryological development of the gut

Answer 19

Visceral pain caused by ischaemia, abnormally strong muscle contraction, inflammation, stretch
Epigastric –> foregut pain (gall bladder, gastric, duodenal)
Periumbilical –> midgut pain (pancreatic, abdominal aorta)
Suprapubic –> hindgut pain (large bowel colic, uterine/ovarian)

Question 20

Describe the role of rotation of the midgut loop in the establishment of the disposition of the abdominal viscera

Answer 20

SMA as its axis
Is connected to the yolk sac by vitelline duct
During 6th week, physiological herniation
During 8th week, midgut rotates 3x 90 degree anticlockwise turns
During 10th week, cranial limb (distal duodenum, jejunum, proximal ileum) returns to abdominal cavity first, moving to left side
Caecal bud descends

Question 21

Describe the consequences of malrotation and give relevant examples

Answer 21

Incomplete rotation - only one 90 degree loop –> left sided colon
Reversed rotation - midgut loop makes one 90 degree rotation clockwise –> transverse colon passes anterior to duodenum –> strangulation

Question 22

Describe the major complications of midgut defects

Answer 22

Volvulus –> strangulation, ischaemia

Question 23

Describe the condition of Meckel’s diverticulum

Answer 23

Rule of 2s - 2% population, 2 feet from ileocecal valve, 2 inches long, detected under 2 years, 2:1 male:female
Can contain ectopic gastric or pancreatic tissue
Similar presentation to appendicitis

Question 24

Describe the developmental basis for common congenital defects of the GI tract, including atresias and stenosis

Answer 24

Atresia - lumen obliterated
Stenosis - lumen narrowed
Most occur in duodenum 
Caused by incomplete canalisation or vascular accidents (impairment of blood supply) 
Duodenum > jejunum = ileum > colon

Question 25

Describe the condition of pyloric stenosis in infants

Answer 25

Hypertrophy of circular muscle in region of pyloric sphincter
Not due to reconciliation
Narrowing of exit from stomach causes characteristic projectile vomiting

Question 26

Describe the partitioning of the cloaca and the development of the anal canal

Answer 26

At the 6th week, a wedge of mesoderm grows down into the cloaca dividing it into the urogenital sinus (anteriorly) and anorectal canal (posteriorly) –> end of 7th week
Cloacal membrane has contact with ectoderm and endoderm, so when the cloacal membrane ruptures, the anal canal has both ectoderm and endoderm

Question 27

Describe the differences above and below the pectinate line in the anal canal

Answer 27

Above:
IMA
S2,3,4 pelvic parasympathetics
Columnar epithelia 
Internal iliac nodes lymph drainage
Below:
Pudendal artery
S2,3,4 pudendal nerve
Stratified epithelium 
Superficial inguinal nodes lymph drainage

Question 28

Describe some hindgut abnormalities

Answer 28

Imperforate anus - failure of anal membrane to rupture
Anal/anorectal agenesis
Hindgut fistulae