Embryology of the GI

Question 1

Define gastrulation.

Answer 1

The bilaminar germ disk is covered into the trilaminar disk through the following process;

1. Cells flow over primitive streak between two existing layers (ecto- and endoderm).
2. A third embryonic germinal layer is formed, called the mesoderm.
3. The ectoderm now lies dorsally, while the endoderm lies ventrally.

Question 2

What occurs in Week 1 of embryonic development?

Answer 2

Fertilisation and implantation of the embryo in the uterine wall.

Question 3

What occurs in Week 3 of embryonic development?

Answer 3

Trilaminar germ disk formation (gastrulation), CNS induction and neurulation.

Question 4

Name two factors that can cause major abnormalities if present during organogenesis.

Answer 4

Drugs and viruses.

Question 5

What occurs in Week 8 of embryonic development?

Answer 5

Organogenesis.

Question 6

What does the embryonic ectoderm form?

Answer 6

Skin (epidermis, hair, nail), CNS, PNS, sensory epithelia of the eye, ear, nose & other structures (pituitary, mammary, sweat glands, enamel of teeth), and adrenal medulla.

Question 7

What does the embryonic mesoderm form?

Answer 7

Muscle, bones, cartilage, dermis, vascular system (most of the CVS connective tissues, blood cells and bone marrow, vessels associated with organs and tissues), urogenital system except bladder, spleen, liver mesenchyme, and suprarenal gland cortex.

Question 8

What does the embryonic endoderm form?

Answer 8

Epithelial lining of respiratory system, GIT, glands, liver (hepatocyte and biliary epithelium), pancreas, thyroid, parathyroid parenchyma, and bladder.

Question 9

Name the components of the foregut.

Answer 9

Mouth, pharynx, oesophagus, stomach and upper duodenum.

Question 10

Where does the foregut end?

Answer 10

The second section of the duodenum.

Question 11

Is the duodenum intra- or retroperitoneal?

Answer 11

The first (superior) section is intraperitoneal while the other three sections are retroperitoneal.

Question 12

Name the components of the midgut.

Answer 12

The distal duodenum, jejunum, ileum, caecum, appendix, ascending colon, and the proximal 2/3rds of the transverse colon.

Question 13

Where does the midgut end?

Answer 13

Proximal to the end third of the transverse colon.

Question 14

Describe the process of regulating gut formation.

Answer 14

SHH (Sonic Hedgehog) transcription factor is secreted from the gut endoderm –> induces HOX gene in gut mesoderm –> molecular regulation of craniocaudal organisation of gut

Question 15

What structure does the foregut develop from?

Answer 15

The cranial part of the primitive gut tube.

Question 16

What structure develops from the ventral wall of the foregut?

Answer 16

Laryngo-tracheal diverticulum (lung bud).

Question 17

Describe the formation of the stomach.

Answer 17

Forms in the 4th week of embryonic development.

1. Forms greater (GC) and lesser curvature (LC).
2. Undergoes a 90° clockwise rotation around its own longitudinal axis (GC now on left side).
3. Undergoes an anteroposterior to create final adult position.

Question 18

Describe the formation of the omental bursa.

Answer 18

The stomach is attached to the dorsal and ventral wall by a mesentery called the mesogastrium.
Rotation along the longitudinal axis pulls the dorsal mesentery to the left, creating the mental bursa.

Question 19

Define the transverse mesocolon.

Answer 19

The mesentery that attaches the transverse colon between the colic flexures..

Question 20

What structure(s) does the duodenum form from?

Answer 20

The caudal part of the foregut and the cranial part of the midgut.

Question 21

What arteries supply the duodenum?

Answer 21

Coeliac trunk and superior mesenteric artery.

Question 22

What happens to the duodenum what the stomach rotates?

Answer 22

It become C-shaped and also rotates.

Question 23

In what week of embryonic development is the liver bud formed?

Answer 23

Week 3.

Question 24

What structure does the liver and biliary apparatus form from?

Answer 24

An outgrowth of endoderm (epithelia) in the distal end of the foregut.

Question 25

Describe the formation of the liver and biliary apparatus.

Answer 25

1. Liver bud grows into the septum transversum.
2. Liver bud grows and connection between it and duodenum narrows –> forms bile duct.
3. Ventral outgrowth from bile duct forms gallbladder and cystic duct.

Question 26

When does the spleen form? What tissue type is it derived from?

Answer 26

Week 5 and is a mesodermal derivative (not parenchymal).

Question 27

When do leukocytes colonise the spleen? What type of leukocytes are present?

Answer 27

Weeks 15-17: T-lymphocytes.
Week 23: B-lymphocyte precursors.
After this, the spleen can begin its lymphoid function.

Question 28

Describe the formation of the pancreas.

Answer 28

1. Forms from 2 bus that grow from endodermal lining of the duodenum (Week 5).
2. As the duodenum rotates, the ventral bud also moves to lie close to the dorsal bud (Week 6).
3. Dorsal bud –> inferior part of head of pancreas, ventral bud –> forms the rest.

Question 29

Name all structures that develop from the foregut.

Answer 29

Oesophagus, lungs (bud), stomach, omental bursa, duodenum, liver and biliary apparatus, spleen and pancreas.

Question 30

If the tracheo-oesophageal septum deviates incorrectly, then there is incomplete separation of laryngo-tracheal tube (lung bud). What abnormalities can result from this?

Answer 30

Oesophageal atresia (most common 90%) and trachea-oesophageal fistula.

Question 31

Describe annular pancreas and its consequences.

Answer 31

The result of the ventral bud failing to migrate correctly.
This can cause duodenal stenosis or for pancreatic tissue to form in other areas of the foregut.

Question 32

Describe the early development of the midgut.

Answer 32

Cephalic part --> distal duodenum, jejunum, proximal ileum
Caudal part (below bile duct) --> distal ileum, caecum, appendix, ascending colon, proximal 2/3rds of transverse colon
Both must undergo rotation and physiological herniation to reach adult pattern.

Question 33

Describe the rotation of the midgut.

Answer 33

• In Week 6.
• Initial rotation pf primary intestinal loop occurs around the axis of superior mesenteric artery.
• 90° rotation anti-clockwise (think stomach formation).
• Cranial part of midgut carried to the right.

Question 34

Describe how the physiological herniation of the midgut.

Answer 34

• Gut tube herniates into extra embryonic cavity in umbilical cord.
• Allows growth of gut tube (not enough room due to liver).
• Intestinal loops begin to move from umbilical cord back into abdominal cavity.

Question 35

Describe omphalocele.

Answer 35

• Midgut anomaly.
• Herniation of intestinal loop through an enlarged umbilical ring.
• Viscera covered by layer of amnion.

Question 36

Describe the midgut anomaly, remnants of vitelline duct.

Answer 36

• Small vitelline duct persists (2-4%)

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