Peritoneal and GI development

Question 1

Are the events which will be described below sequential, or concurrent ?

Answer 1

Almost concurrent.

Question 2

Identify the main steps in GI development.

Answer 2

Folding of the embryo
Development of the foregut
Development of the stomach
Development of the duodenum
Development of the liver
Development of the gallbladder
Development of the pancreas
Development of the pancreatic ducts
Development of the spleen
Development of the midgut
Development of the hindgut
Development of mesentery derivatives

These are all more or less concurrent

Question 3

Describe the main steps in the folding of the embryo.

Answer 3

FOLDING OF EMBRYO
The primitive yolk sac develops into the endodermal digestive tract or the gut tube in week 4
The connection between the gut tube and the secondary yolk sac will become the yolk stalk and then the Vitelline duct
Intraembryonic cavity (coelom) develops into the abdominopelvic cavity
The gut tube is suspended by dorsal mesentery
Visceral layer of lateral plate mesoderm will develop into visceral peritoneum

Question 4

Describe the main steps in the development of the foregut.

Answer 4

FOREGUT

• The gut tube consists of a blind-ended cranial foregut (defined as part of gut tube extending from the mouth to just distal of the developing liver, terminating at the oropharyngeal/ buccopharyngeal membrane), a blind- ended caudal hindgut (terminating at the cloacal membrane), and a midgut that opens to the yolk sac through the vitelline duct
• Foregut gives rise to the Oesophagus (which, in turn gives rise to Tracheo-bronchial tree), Stomach, Proximal duodenum, Liver and gall bladder, Pancreas and spleen
• By week 5, the thoracic and abdominal portion of the foregut is visibly divided into the pharynx, oesophagus, stomach, and proximal duodenum

Question 5

What is the innervation of the abdominal foregut ? As a result, where is pain from here referred ?

Answer 5

Coeliac trunk (T7 to T9, so pain referred to epigastrium)

Question 6

Describe the main steps in the development of the stomach.

Answer 6

STOMACH
- By Week 4, part of the foregut which will become the stomach starts to dilate and become fusiform
- Differential growth of the stomach walls (the dorsal wall of the stomach growing faster than the ventral wall) results in formation of the greater curvature, fundus and cardiac notch
- Deformation of the ventral stomach wall forms the lesser curvature
- During weeks 6-7, Stomach rotates 90° around its longitudinal axis
(while also rotating around its sagittal axis). As a result of rotation around the longitudinal axis, left side faces anteriorly, the lesser curvature faces to the right and the greater curvature faces to the left. As a result of slight rotation around the sagittal axis, lesser curvature faces slightly upwards

Question 7

Describe the main steps in the development of the duodenum.

Answer 7

DUODENUM
♦ The duodenum forms from the foregut and beginning of midgut
♦ Initially it is found in the midline but the rotations of the stomach also:
- rotate the duodenum
- cause the duodenum get into C shape
- displace the duodenum to the right until it lies against the dorsal body wall and becomes partially retroperitoneal

Question 8

Describe the main steps in the development of the liver.

Answer 8

LIVER
• On about day 22, a small endodermal thickening, the hepatic plate, forms on the ventral side of the duodenum (hence in the ventral mesentery)
• Hepatic plate develops into the hepatic diverticulum (liver bud)
• Hepatic diverticulum gives rise to
- the inferior region of the septum transversum (which will become the diaphragm)
- hepatoblasts

1) Hepatoblasts become hepatocytes (parenchyma), bile canaliculi of the liver
(hepatic ducts)
2) Liver sinusoids (stroma) develop from septum transversum
3) The ventral mesentery around the liver becomes its visceral peritoneum. Ventral mesentery also reflects onto the diaphragm and the area between these reflections is the bare area of the liver is where it contacts the diaphragm.

Question 9

Describe the main steps in the development of the gallbladder.

Answer 9

GALLBLADDER
♣ By day twenty-six, an endodermal thickening forms on the ventral side of the duodenum just caudal to the hepatic diverticulum and grows into the ventral mesentery
♣ This cystic diverticulum will form the gallbladder and cystic duct

Question 10

Describe the main steps in the development of the pancreas.

Answer 10

PANCREAS
♠ On day 26 dorsal pancreatic bud begins to grow into the dorsal mesentery just opposite the hepatic diverticulum

♠ Another endodermal diverticulum, the ventral pancreatic bud, develops and grows into the ventral mesentery just caudal to the developing gallbladder
♠ This ventral pancreatic bud will give rise to:
- the ventral pancreas → the uncinate process
- the common bile duct

♠ The rotation of the duodenum causes the ventral bud to migrate around to lie behind and fuse with the dorsal bud so that the adult pancreas lies in the curve of the duodenum

Question 11

Describe the mains steps in the development of the pancreatic ducts.

Answer 11

PANCREATIC DUCTS
♦ When the ventral and dorsal pancreatic buds fuse, their ductal systems also become interconnected.
♦ The proximal portion of the duct of dorsal pancreas degenerates. The accessory duct is the remnant of the duct of the dorsal bud
♦ The ducts of the dorsal and ventral buds unite to form the main pancreatic duct
♦ The main pancreatic duct and the common bile duct meet and drain into the 2nd part of duodenum at the major duodenal papilla or ampulla of Vater.

NB: SOMETIMES the proximal dorsal pancreatic duct persists as an accessory pancreatic duct that empties into the duodenum at a minor duodenal papilla

Question 12

Describe the main steps in the development of the spleen.

Answer 12

SPLEEN
♦ a mesenchymal condensation develops within it near the body wall.
♦ This condensation differentiates during the fifth week to form the spleen
♦ This is a mesodermal derivative, not a product of the gut tube endoderm
♦ However, rotation of the stomach and growth of the dorsal mesogastrium translocate the spleen to the left side of the abdominal cavity
♦ The portion of the dorsal mesentery between the spleen and the stomach is called the gastrosplenic ligament.

Question 13

Describe the main steps in the development of the midgut.

Answer 13

MIDGUT
• The future ileum elongates more rapidly and by the fifth week the midgut makes the primary intestinal loop

• Pressure of growing abdominal organs (especially the liver) forces the primary intestinal loop to herniate into the umbilicus during the sixth week
• As the primary intestinal loop herniates, it rotates around the axis of the superior mesenteric artery by 90 degrees counterclockwise (as viewed from the ventral side) so that the future ileum lies in the right abdomen and the future large intestine lies in the left abdomen
• The small intestine keeps on elongating to form jejunal-ileal loops
• The caecum expands and vermiform appendix is also formed

• During the tenth week, the midgut retracts into the abdomen
• During the eleventh week, the retracting midgut does another 180° of rotation (a total of 270° rotation)
- the caecum is positioned just inferior to the liver.

• The caecum is then displaced inferiorly, pulling down the proximal hindgut to form the ascending colon.
• The descending colon is simultaneously fixed on the left side of the posterior abdominal wall. The jejunum, ileum, transverse colon, and sigmoid colon remain suspended by mesentery.

Question 14

Describe the main steps in the development of the hindgut.

Answer 14

HINDGUT
♦ Just superior to the cloacal membrane, the primitive gut tube forms an expansion called the cloaca.
♦ A slim diverticulum of the cloaca called the allantois extends into the yolk stalk
♦ During the fourth to sixth weeks, a coronal urorectal septum divides the cloaca
− Anteriorly the cloaca develops into the urogenital sinus → urogenital structures
− Posteriorly the cloaca develops into the anorectal canal (proximal 2/3 anal canal)
♦ As the tip of the urorectal septum approaches the cloacal membrane, the anal part of the membrane sinks into the anal pit and the cloacal membrane ruptures
− As a result the urogenital sinus and dorsal anorectal canal open to the exterior.

Question 15

What structures does the midgut form ?

Answer 15

The mid gut forms the distal duodenum, jejunum, ileum, caecum, ascending colon, and proximal two thirds of the transverse colon

Question 16

What structures does the hindgut form ?

Answer 16

Hindgut forms the distal third of the transverse colon, the descending and sigmoid colon, and the upper two thirds of the anorectal canal.

Question 17

What is the significance of the pectinate line ?

Answer 17

The pectinate line marks the boundary between hindgut endoderm and ectoderm, hence the boundary between different sources of blood and nerve (somatic vs autonomic)

Question 18

Describe the main steps in the development of mesentery derivatives.

Answer 18

FALCIFORM LIGAMENT
♦ The remnant of ventral mesentery connecting the liver to the anterior body wall becomes the falciform ligament
♦ The free caudal margin of falciform ligament carries the umbilical vein from the body wall to the liver

LESSER OMENTUM and HEPATODUODENAL LIGAMENT
♦ The ventral mesentery between the liver and lesser curvature of the stomach forms the lesser omentum.
♦ The caudal border of the lesser omentum, connecting the liver to the developing duodenum, is called the hepatoduodenal ligament

HEPATOGASTRIC LIGAMENT
♦ The region of the lesser omentum between the liver and the stomach is
called the hepatogastric ligament.

Question 19

What are the contents of the hepatoduodenal ligament ?

Answer 19

The portal vein, the proper hepatic artery and the common bile duct

Question 20

Identify congenital anomalies of the midgut.

Answer 20

1) Gastroschisis
2) Exomphalos
3) Umbilical hernia
4) Intestinal malrotation (can be partial or complete)
5) Meckel’s diverticulum
6) Vitelline fistula/patent vitelline duct:
7) Annular pancreas
8) Hirschsprung Disease

Question 21

Are umbilical hernias always congenital ?

Answer 21

No, usually acquired, but sometimes congenital.

Question 22

What proportion of people have a Meckel’s diverticulum ?

Answer 22

2%

Question 23

Define Gastroschisis.

Answer 23

In lateral folding of embryo, lateral folds may not fuse properly, so intra-abdominal contents may herniate through those defects in anterior abdominal wall. (Umbilucus is present, organs are herniating on one side of umbilicus, or both).

Question 24

Define Exomphalos.

Answer 24

Umbilical cord has not closed, so intra-abdominal organs (especially SI) go into the umbilical cord.

Question 25

Define Umbilical hernia.

Answer 25

The linea alba (part of anterior abdominal wall) is weak behind the umbilicus so organs herniate into umbilicus when intra-abdominal P is increased.

Question 26

Define intestinal malrotation.

Answer 26

“Any deviation from the normal 270° counterclockwise rotation of the midgut during embryologic development.”

Question 27

Define Meckel’s diverticulum.

Answer 27

“Ileal diverticulum, that results from the failure of the vitelline duct to obliterate during the fifth week of fetal development (i.e. remnant of the vitelline duct). Can ulcerate and cause symptoms mimicking appendicitis”

Question 28

Define Vitelline fistula/patent Vitelline duct.

Answer 28

“Vitelline duct remains patent over its entire length, thus, connects the umbilicus and intestinal tract. It may lead to fecal discharge at the umbilicus.
One may find the duct closed at both ends and the formation of a vitelline cyst or enterocystoma in its midportion”

Question 29

Define annular pancreas.

Answer 29

The ventral pancreas may consist of two lobes, which is they migrate around the duodenum in opposite directions to fuse with the dorsal pancreatic bud, form an annular process (strangulates duodenum)

Question 30

Define Hirschsprung disease.

Answer 30

Lack of normal development of the colonic innervation (i.e. no peristalsis, no movement of feces), leading to a constricted aganglionic segment of bowel with a distended segment proximally the innervation of which is normal (normal peristalsis, movement of feces)

Question 31

Identify and define congenital abnormalities of the hindgut.

Answer 31

“-Urorectal fistula: defect in the urorectal septum resulting in passage of urine through the rectum.

• Rectovaginal fistula: failure of proper segmentation, resulting in colonic contents passing into vagina
• Rectoperineal fistula: a misplaced anal passage that is often narrowed
• Imperforate anus: absence of the normal opening of the rectum” (part of cloacal membrane that separates skin from hindgut, does not rupture posteriorly)

Question 32

Identify and define tracheo-esophageal folds defects.

Answer 32

1) Tracheo-esophageal fistulas:
♣ Tracheo- esophageal folds do not fuse, resulting in communication between the trachea and esophagus (i.e. tracheo-esophageal fistula)
♣ Usually accompanied by esophageal atresia (failure of the full esophageal lumen to develop)
♣ Usually (90% of the time), trachea connected to the distal part of esophagus (rather than the entire esophagus or the proximal part of the esophagus). In that case, the proximal part of the esophagus is blind ended.
♣ Consequences:
-After birth, when the baby attempts to feed, milk enters the trachea, causing choking and possible development of pneumonitis and pneumonia.
-Also the possibility of Retrograde passage of gastric acid from the GI tract into the respiratory system.