Embryology of the Gut tube 1

Question 1

What is the function of the yolk sac?

Answer 1

• Generation of blood cells (in wall of sac) particularly red cells.
• Origin of primordial germ cells.

Question 2

Describe body folding?

Answer 2

• Longitudinal folding (cranio-caudal)
• Communication with yolk sac narrows – vitelline duct.
• Transverse folding: Surface ectoderm and amniotic cavity fold around laterally. Development or incorporation of embryonic body cavity (coelum).

Question 3

What is the function of the dorsal mesentery?

Answer 3

• Suspends gut tube from dorsal body wall.
• Given regional names, i.e. dorsal mesogastrium, dorsal mesocolon and dorsal mesoduodenum.

Question 4

Where is the ventral mesentery located?

Answer 4

• Only present superiorly.
• Divided in two by the developing liver:
  1. Falciform ligament anteriorly.
  2. Lesser omentum between liver & stomach.
• Derived from septum transversum

Question 5

What is septum transversum?

Answer 5

• Mesodermal tissue.
• Extends from thorax to stalk of yolk sac.
• Derivatives: Ventral mesentery diaphragm and contributes to Liver.

Question 6

Where does the diaphragm develop?

Answer 6

• Initially cervical region, then descends.
• Formed by: septum transversus, pleuroperitoneal folds, oesophageal mesentery and ingrowth of muscle cells.

Question 7

What is a diaphragmatic hernia?

Answer 7

• Abnormal opening in the diaphragm – multiple sites & causes.
• Gut herniation may cause restriction on lung growth & function.
• Hernia of Bochdalek.
• Hernia of Morgagni

Question 8

What is a hernia of Bochdalek.

Answer 8

• Absence of pleuroperitoneal membrane, more common on left side because right side closes earlier in development.

Question 9

What is a hernia of Morgagni?

Answer 9

• A diaphragmatic hernia between sternal & costal heads.

Question 10

What is the foregut derivative of the lungs?

Answer 10

• The tracheo-bronchial diverticulum.

Question 11

Discuss lung development.

Answer 11

• Respiratory diverticulum buds off foregut
• Tracheo-oesophageal ridges deepen
• Trachea & oesophagus separate (remain joined only at larynx)
• Diverticulum splits to form two lung (bronchial) buds (split into lobes).

Question 12

What is a tracheoesophageal fistula?

Answer 12

• Defect in communication of esophagus with trachea.
• Fluid runs into lungs when feeding, infants cough or cyanose when feeding.

Question 13

What is oesophageal atresia?

Answer 13

• There is no communication between proximal and distal ends of oesophagus.
• Fluid has no entry into stomach.
• Often esophagus forms fistula with trachea (distal part is more common).

Question 14

Discuss lung maturation?

Answer 14

• Lungs no needed in utero, develop late.
• Pre-mature babies: poorly developed lungs with minimal (or no) surfactant (develops 24 – 37 / 40 ) – use of antenatal corticosteroid.
• May result in respiratory distress syndrome

Question 15

Discuss the development of the stomach.

Answer 15

• Rapid cell growth – tube – sac
• More rapid posteriorly, along greater curvature.
• Cells at outflow grow to form pyloric sphincter.
• Stomach rotates 90 degrees.

Question 16

What happens when the stomach rotates 90 degrees?

Answer 16

• Lesser curve rotates to the right, greater curve rotates to the left.
• Dorsal mesogastrium & spleen move left (e.g. gastro-splenic ligament)
• Ventral mesogastrium & liver move right (e.g. lesser omentum)
• Lesser sac now lies posterior to stomach.

Question 17

What happens in pyloric stenosis?

Answer 17

• Hypertrophy of pyloric sphincter
• Results in projectile vomiting!!!
• Usually presents at 6 – 8 weeks*
• Tx: Surgical pyloromyotomy (Ramstedt’s procedure).

Question 18

What does the ventral mesogastrium form?

Answer 18

• Grows from lesser curvature.
• Lesser omentum & falciform ligament.

Question 19

What does the dorsal mesogastrium form?

Answer 19

• Grows from greater curvature.
• Greater omentum & gastrosplenic ligament.

Question 20

When does obliteration of the duodenum occur and what follows?

Answer 20

• At 6 weeks.
• Endodermal cells lining the tube proliferate & obliterate the lumen.
• Re-canalisation up to 9 weeks.

Question 21

What happens when the duodenum fails to re-canalise?

Answer 21

• Duodenal atresia (or stenosis)
• Abdominal distension, vomiting and absent bowel movements.
  *Vomiting may be bilious or non bilious depending on level.

Question 22

Where is the spleen formed?

Answer 22

• Within dorsal mesentery.
• Moves to left as stomach & dorsal mesogastrium rotate.

Question 23

What is an accessory spleen?

Answer 23

• Splenunculus: where splenic tissue is found outside the normal spleen.
• <10% of population.
• Relevant post-splenectomy.