Development of the Alimentary Canal and associated Secondary Organs

Question 1

[8-minute video]: Liver, Gall bladder and Pancreas Development - Lecturio

Answer 1

📝

Question 2

Click on Answer to view diagrams and/or images pertaining to this topic.

Answer 2

[Diagram 1] [Diagram 2] [Diagram 3] [Diagram 4]

Question 3

The embryo folds in both the longitudinal (cranio-caudal/sagittal) and the transverse (lateral) planes/axes. State the effect of embryonic folding on the yolk sac.

Answer 3

✓ The ventral part of the yolk sac shrinks and involutes (initially, as the vitelline duct).
✓ The dorsal part of the yolk sac becomes incorporated into developing embryo to form the primordial gut (primitive gut tube; primitive alimentary canal).
[Diagram 1] [Diagram 2]

Question 4

State the derivatives of the endoderm in the alimentary canal.

Answer 4

(a) Epithelial lining of the gut wall
(b) Exocrine glands: both intrinsic (e.g. gastric glands, Brunner’s glands etc.) and extrinsic (e.g. biliary system, liver, pancreas)

Question 5

State the derivatives of splanchnic mesoderm in the alimentary canal.

Answer 5

(a) Connective tissue elements of the gut
(b) Smooth musculature of the gut wall [muscularis mucosa and muscularis externa]
(c) Mesentery

Question 6

State the derivatives of the neural crest cells in the alimentary canal.

Answer 6

[They migrate and invade the developing gut tube.]
They give rise to the enteric nervous plexus of the gut wall. [Myenteric nerve plexus and the submucosal nerve plexus.]
[Diagram]

Question 7

The primordial gut has three parts: foregut, midgut and hindgut. State the derivatives of the foregut.

Answer 7

Gut components: pharynx, esophagus, stomach, duodenum [proximal part only]
Extrinsic glands: liver, pancreas and biliary tree
[also the lower respiratory system…]

Question 8

The primordial gut has three parts: foregut, midgut and hindgut. State the small gut and large gut derivatives of the midgut.

Answer 8

Small gut: distal part of duodenum, jejunum, ileum
Large gut: caecum, appendix, ascending colon, transverse colon [right 1/2 to 2/3]

Question 9

The primordial gut thas three parts: foregut, midgut and hindgut. State the derivatives of the hindgut.

Answer 9

right part of the transverse colon, descending colon, sigmoid colon, rectum, anal canal [Diagram]

Question 10

Name the artery of the:
(a) Foregut
(b) Midgut
(c) Hindgut

Answer 10

(a) Celiac artery
(b) Superior mesenteric artery
(c) Inferior mesenteric artery
[Image 1]

Further notes:
The regions where these three arteries anastomose represent the regions which have dual origin in terms of the gut segments e.g. the duodenum and transverse colon.

Question 11

Summarize the steps in the development of the esophagus.
[Hints: it is tied to the development of the lower respiratory tract…]

Answer 11

⚚ The respiratory diverticulum forms from the ventral aspect of the foregut.
⚚ The foregut undergoes septation to separate the esophagus and the trachea.
⚚ Elongation takes place.
⚚ The esophagus enters the solid stage of development. [This is due to high rate of proliferation of endodermal cells, hence obliteration of the lumen. This is a universal process in the development of all tubular structures lined by endoderm.]
⚚ Recanalization takes place.
[Diagram]

Question 12

Name A to E: [Image].

Answer 12

A: esophageal atresia without tracheoesophageal fistula
B: esophageal atresia with proximal tracheoesophageal fistula
C: esophageal atresia with distal tracheoesophageal fistula
D: esophageal tresia with proximal and distal tracheoesophageal fistula
E: tracheoesophageal fistula without esophageal atresia

Further notes:
📝 Clinical presentations of A: 1. frothing at the mouth, 2. vomiting of breast milk, 3. inability to advance a nasogastric tube
📝 Type B presents similarly to A with choking.
📝 Type C presents similarly to A with distension of the abdomen due to entry of gases.
📝 Type E is diagnosed late.

Question 13

Name two embryological defects caused by inadequate recanalization of the esophagus.

Answer 13

(a) Esophageal stenosis [Image]
(b) Esophageal web [Image 1] [Image 2]

Question 14

Summarize the steps in the development of the stomach.

Answer 14

☑ It begins as a fusiform dilatation in the distal parts of the foregut.
☑ Its posterior border grows faster than its anterior border, leading for curvature formation.
☑ The developing stomach then undergoes rotation in the vertical axis and in the anteroposterior axis.
[Diagram]

Question 15

Discuss the rotation of the stomach in the vertical axis and state the consequences of this rotation.

Answer 15

☛ Rotation in the vertical axis occurs 90° in the clockwise direction [if viewed anteriorly].
☛ This results in the greater curvature facing the left side, and the lesser curvature facing the right.
☛ The left vagus nerve becomes the anterior vagal trunk and the right vagus nerve becomes the posterior vagal trunk.
☛ Ventral mesogastrium moves to the left, dorsal mesogastrium moves to the right.

Question 16

Discuss the rotation of the stomach in the antero-posterior axis and state the consequences of this rotation.

Answer 16

☛ about 60-70 degrees
☛ The fundus goes downwards and to the left side.
☛ The pylorus goes upwards and to the right side.

Question 17

Click on Answer to view some developmental anomalies of the stomach.

Answer 17

Hypertrophic pyloric stenosis: hypertrophy of the pylorus hence narrowing of the pyloric canal [Diagram 1]
Gastric malrotation

Question 18

Summarize the steps in the development of the small intestine.

Answer 18

☯︎ The intestines arise from all the segments of the primordial gut, but mainly from the midgut and the hindgut.
☯︎ Rapid elongation of the midgut causes the primary intestinal loop to form.
☯︎ The loop has two limbs: the cephalic limb (small gut) and the caudal limb (large gut).
☯︎ The apex of the loop (corresponding to the distal ileum) is still attached to the involuting yolk sac via the vitelline duct.
☯︎ The intestinal loop herniates through the umbilicus, for the purpose of achieving further development in a greater space.
☯︎ This event, termed the physiological umbilical herniation occurs around the 6th week of gestation.
☯︎ During herniation, the gut rotates 90° anticlockwise around the axis of the superior mesenteric artery.
☯︎ Development continues within the hernial sac for about four weeks.
☯︎ Hernia reduction occurs around the 10th week of gestation, with the duodenum leading the process.
☯︎ During the return, large gut rotates 180° anticlockwise around the axis of the superior mesenteric artery.
☯︎ Gut fixation occurs by regression of the dorsal mesenteries.

Question 19

Identify the congenital anomaly and state its embyological basis: [Image].

Answer 19

omphalocele: failure of reduction of the physiological hernia

Further notes:
Here are some differences between gastroschisis and omphalocele:
(a) location
Gastroschisis: the opening is typically located to the right of the umbilicus [paramedian]
Omphalocele: the opening is at the umbilicus, with the umbilical cord at the centre of the defect

(b) hernia sac
Gastroschisis: absence of a hernia sac
Omphalocele: presence of a hernia sac

Question 20

Identify the developmental anomaly and state its cause: [Image].

Answer 20

Gastroschisis: caused by a defect in the anterior abdominal wall

Further notes:
Here are some differences between gastroschisis and omphalocele:
(a) location
Gastroschisis: the opening is typically located to the right of the umbilicus [paramedian]
Omphalocele: the opening is at the umbilicus, with the umbilical cord at the centre of the defect

(b) hernia sac
Gastroschisis: absence of a hernia sac
Omphalocele: presence of a hernia sac

Question 21

Identify the congenital anomaly and state its embryological basis: [Image].

Answer 21

midgut malrotation: anomal rotation of the midgut

Question 22

Briefly summarize the development of the liver and gall bladder.

Answer 22

The liver originates from the hepatic diverticulum, a small outgrowth of the ventral foregut. The hepatic diverticulum develops from around the fourth week of intrauterine life and it gives rise to the hepatocytes and biliary ducts.
The gallbladder also develops from an outpouching of the hepatic diverticulum known as the cystic diverticulum. It can hence be inferred that the cystic duct is a derivative of the cystic diverticulum.
[Diagram 1] [Diagram 2]

Question 23

Briefly summarize the development of the pancreas.

Answer 23

✓ The pancreas develops from two distinct buds originating from the duodenal part of the foregut: the ventral pancreatic bud and the dorsal pancreatic bud. Each bud connects to the foregut through a duct.
✓ Due to rotation, the ventral and dorsal buds eventually align and fuse into a single organ.
✓ The ventral bud contributes to the uncinate process and the inferior part of the head of the pancreas, whereas the dorsal bud gives rise to the body and tail of the pancreas.
✓ [Diagram 1] [Diagram 2]