S2: introduction to embryology of the GI tract & abdominal wall hernias

Question 1

Describe the anatomy of the inguinal canal in the male

Answer 1

Anterior wall – aponeurosis of the external oblique, reinforced by the internal oblique muscle laterally
Posterior wall – transversalis fascia
Roof – transversalis fascia, internal oblique & transversus abdominis
Floor – inguinal ligament, thickened medially by the lacunar ligament
Two openings – superficial (exit) and deep (entrance) rings

Question 2

What is a hernia?

Answer 2

The protrusion of an organ or fascia through the wall of cavity that normally contains it

Question 3

Describe the development of the inguinal canal and how inguinal hernias develop embryologically

Answer 3

Gubernaculum attaches the inferior portion of the gonad to the future scrotum or labia and guides them during the descent
Inguinal canal = pathway by which the testes leave the abdominal cavity and enter the scrotum
Processus vaginalis normally degenerates, but if it fails to do so – indirect inguinal hernia, a hydrocele or interferes with the descent of the testes
Gubernaculum becomes a small scrotal ligament, tethering the testes to the scrotum and limiting their movement

Question 4

Explain how indirect inguinal hernias develop

Answer 4

Indirect inguinal hernias – caused by the failure of the processus vaginalis to regress
-peritoneal sac enters the inguinal canal via the deep inguinal ring -> degree to which the sac herniates depends on the amount of processus vaginalis still present

Question 5

Describe the clinical presentation of an inguinal hernia

Answer 5

Fullness/swelling 
Increase in size
Discomfort
Nausea/vomiting
Necrotic bowel -> peritonitis

Question 6

Compare direct and indirect inguinal hernias

Answer 6

Indirect are more common & males are more commonly affected
Indirect hernias – exit the abdomen through the deep inguinal ring & enter the inguinal canal to a variable distance, some pass through to the scrotum
->pass laterally to the inferior epigastric vessels
Direct hernias – pass directly through the abdominal wall in an area of potential weakness called Hesselbach’s triangle
->pass medially to the inferior epigastric vessels

Question 7

Describe the relationship between the femoral canal and the inguinal ligament

Answer 7

Inguinal ligament forms the anterior border of the femoral canal

Question 8

Explain how femoral hernias develop and the clinical presentation

Answer 8

Part of the small intestine protrudes through the femoral ring
Presents as a lump situated inferolateral to the pubic tubercle -> more common in women, due to wider bony pelvis
NB: borders of femoral canal are tough -> can compress the hernia, interfering with its blood supply = strangulated hernia

Question 9

Describe the anatomy and location of umbilical hernias

Answer 9

Commonly found in infants
Hernia at the site of the umbilicus – not usually painful
-defect in the linea alba -> umbilical cord passes through the umbilical ring
80-90% close by age 3 (no treatment unless it is painful or very large)

Question 10

Describe the common incisional sites used for abdominal surgery

Answer 10

1) Midline incision -> through linea alba, extendable, particularly uncomfortable post-operatively
2) Paramedian -> poor cosmetics, can damage nerves/structures
3) Grid iron -> appendicectomy
4) Pfannenstiel -> obstetric & urology
5) Kocher -> open cholecystectomy (removing gallbladder)

Question 11

Explain how the primitive gut, the abdominal wall and the coelomic cavity are formed

Answer 11

Folding of the embryo creates a primitive gut tube, lined with endoderm -> divisible into three regions: foregut, midgut and hindgut (week 3)
Mesoderm surrounding the gut splits into layers – somatic (develops into the abdominal wall) & splanchnic (smooth muscles of the gut wall)
Space created by the split = coelomic cavity

Question 12

Describe the development of the coelomic cavity and peritoneal cavity

Answer 12

Coelomic cavity begins as one large cavity – the forerunner of the pleural cavity and peritoneal cavity
Parietal peritoneum is formed by the somatic portion of lateral plate mesoderm
Visceral peritoneum is formed by the splanchnic portion of lateral place mesoderm
The greater and lesser sacs of the peritoneal cavity are formed by the rotation of the stomach

Question 13

Describe the fate of the embryonic dorsal and ventral mesenteries

Answer 13

The dorsal and ventral mesenteries become the various peritoneal folds and reflects and give passage to vessels and nerves in the adult
Greater omentum is derived from the dorsal mesentery
Lesser omentum is derived from the ventral mesentery

Question 14

What are mesenteries?

Answer 14

Mesentery = double fold of peritoneum that suspends the gut tube from the abdominal wall
Dorsal mesentery – developing gut is attached to the roof of the abdominal cavity
Ventral mesentery – foregut is attached to the floor

Question 15

Explain the basic development of the stomach

Answer 15

When the stomach develops the dorsal border develops faster giving the stomach its characteristic shape (greater curve)

Question 16

Explain the basic development of the liver

Answer 16

The liver grows into the ventral mesentery dividing it into two parts – falciform ligament and the lesser omentum
Develops from an anterior bud off the foregut
-cranial portion: liver
-caudal portion: gallbladder
Liver’s connection to foregut is via the common bile duct

Question 17

Explain the basic development of the pancreas

Answer 17

Foregut structure which develops from two portions
1) Dorsal portion (larger) forming most of the gland
2) Ventral portion (smaller) forming most of the duct system
Rotation of the foregut causes the ventral bud to be forced round the back & inferior to the dorsal bud -> eventually fuse and ductal systems fuse from both
Nestles in the developing C-shape of duodenum

Question 18

Explain the basic development of the duodenum

Answer 18

Duodenum’s lumen is obliterated in the 5th and 6th weeks of its development and only recanalized by the end of the embryonic period

Question 19

Describe the borders of Hesselbach’s triangle

Answer 19

Medial – lateral border of rectus abdominis
Lateral – inferior epigastric vessels
Inferior – inguinal ligament

Question 20

Describe the borders of the femoral ring

Answer 20

Lateral – femoral vein

Medial – lacunar ligament

Question 21

What are para-umbilical hernias?

Answer 21

Affects adults – hernia through the linea alba
Risk factors – obesity, greater incidence in females
Risk of strangulation

Question 22

What is an incisional hernia?

Answer 22

Herniates through previous incision
Majority remain asymptomatic
Risk factors: previous surgery (especially emergency), obesity, midline, wound infection

Question 23

What are the three sections of mesoderm and their derivatives?

Answer 23

Paraxial - forms somites -> skeletal muscles, vertebrae and cartilage 
Intermediate - kidneys & gonads 
Lateral plate 
-splanchnic: viscera 
-somatic: body walls

Question 24

What is the vitelline duct?

Answer 24

Only connection from the midgut to another structure

Connects the midgut to the yolk sac

Question 25

What does the splanchnopleuric & somatopleuric mesoderm form? What does the intraembryonic coelom form?

Answer 25

Splanchnopleuric mesoderm - forms the viscera
Somatopleuric mesoderm - forms the body walls and dermis
Intraembryonic mesoderm - thoracic and abdominal cavities

Question 26

Describe the blood supply to the three sections of the gut

Answer 26

Foregut - coeliac artery
Midgut - superior mesenteric artery
Hindgut - inferior mesenteric artery

Question 27

How does the trachea and oesophagus separate?

Answer 27

Lung bud off the foregut on the anterior surface
Tracheoesophageal septum separates the respiratory bud and oesophagus
Problems: blind-end oesophagus and tracheoesophageal fistula

Question 28

What is the foramen of Winslow? Describe what the left and right vagal fibres form

Answer 28

Small space which connects the greater and lesser sacs
Left vagal fibres - anterior vagal trunk
Right vagal fibres - posterior vagal trunk

Question 29

Describe the peritoneal ligaments (anterior to posterior)

Answer 29

1) Falciform ligament - anterior abdominal wall to liver
2) Lesser omentum - liver to stomach
3) Gastro-splenic ligament - stomach to spleen
4) Spleno-renal ligament (lieno-renal) - spleen to kidney

Question 30

Explain how direct inguinal hernias develop

Answer 30

Direct inguinal hernias – acquired, usually in adulthood, due to weakening in the abdominal musculature

• peritoneal sac bulges into the inguinal canal via the posterior wall medial to the epigastric vessels & can enter the superficial inguinal ring
• > sac is NOT covered with the coverings of the contents of the canal