Embryology

Question 1

When does the primordial gut begin to form?

Answer 1

4th week

Question 2

What closes the primordial gut at the cranial end?

Answer 2

oropharyngeal membrane

Question 3

What closes the primordial gut at the caudal end?

Answer 3

Cloacal membrane

Question 4

Describe the epithelium of the alimentary canal origin?

Answer 4

The epithelium at the cranial end of the alimentary canal is derived from ectoderm of the stomodeum, while the epithelium at the caudal end is derived from the ectoderm of the anal pit (proctodeum).

Question 5

Where does the muscular, connective tissue and the other layers of the GI tract derive from?

Answer 5

The muscular, connective tissue and other layers of the wall of the alimentary tract are derived from the splanchnic mesenchyme surrounding the primordial gut (digestive tract)

Question 6

What connects the midgut to the yolk sac during development?

Answer 6

The middle part, the midgut, remains temporally connected to the yolk sac via the vitelline duct or the yolk stalk

Question 7

Where is the stroma of the GI tract derived from?

Answer 7

stroma (connective tissue) for the glands is derived from visceral mesoderm

Question 8

What are the derivatives of the foregut?

Answer 8

o	Primordial pharynx and its derivatives
o	Lower Respiratory tract
o	Oesophagus and stomach
o	Duodenum –Part above the opening of bile and pancreatic duct
o	Liver, biliary apparatus and pancreas

Question 9

What is polyhydramnios?

Answer 9

Polyhydramnios is caused by atresia, and is a medical condition where there is excess of amniotic fluid collected in the amniotic sac, as the baby is unable to swallow and recycle the amniotic fluid

Question 10

Describe hypertrophic pyloric stenosis

Answer 10

• In infants, there is marked muscular thickening of the pylorus, the distal sphincteric region of the stomach. The circular and, to a lesser extent, the longitudinal muscles are hypertrophied.
• This results in severe stenosis of the pyloric canal and obstruction of the passage of food into the small intestines. Therefore, the stomach becomes markedly distended and the infant expels the stomach contents with considerable force (projectile vomiting)

Question 11

In what sex is hypertrophic pyloric stenosis more common in?

Answer 11

Males

Question 12

How do you treat hypertrophic pyloric stenosis?

Answer 12

A pyloromyotomy - incision is made in the longitudinal and circular muscles of the pylorus

Question 13

Describe the development of the liver, gallbladder and billiard duct system

Answer 13

• The liver, gallbladder and biliary duct system arise as a ventral outgrowth, the hepatic diverticulum, from the distal part of the foregut early in the 4th week.
• The diverticulum extends into the septum transversum, a mass of splanchnic mesoderm between the developing heart and midgut. The septum transversum becomes the ventral mesogastrium in this region.
• The hepatic diverticulum enlarges rapidly and divides into two parts as it grows between the layers of the ventral mesogastrium
• The larger cranial part of the hepatic diverticulum is the primordium of the liver
• The smaller caudal part of the hepatic diverticulum becomes the gallbladder
• The stalk of the diverticulum forms the cystic duct

Question 14

What % of the population have accessory hepatic ducts?

Answer 14

5%

Question 15

Describe the development of the pancreas

Answer 15

• The pancreas develops between the layers of the mesentery from dorsal and ventral pancreatic buds of endodermal cells, which arise from the caudal end of the foregut
• Most of the pancreas is derived from the larger dorsal pancreatic bud, which appears first and develops a slight distance cranial to the ventral bud
• The smaller ventral bud develops near the entry of the bile duct into the duodenum and grows between the layers of the ventral mesentery.
• As the duodenum rotates to the right and becomes C shaped, the ventral pancreatic duct is carried dorsally with the bile duct. It soon lies posterior to the dorsal pancreatic bud and later fuses with it.

Question 16

What is an annular pancreas?

Answer 16

• If the ventral bud fails to migrate around the duodenum correctly = annular pancreas
• The ring like or annular part of the pancreas consists of a thin, flat band of pancreatic tissue surrounding the descending or second part of the duodenum

Question 17

Describe 2 conditions caused by improper development of the pancreas

Answer 17

Annular pancreas - can wrap around and constrict duodenum to cause duodenal stenosis

Ectopic pancreas - most often located in the wall of the stomach, duodenum or jejunum. It can present with obstruction, bleeding or even as cancer

Question 18

What does the spleen derive from?

Answer 18

Mesenchyme, not endoderm!

Question 19

List the derivatives of the midgut

Answer 19

o Distal part of duodenum (distal to bile duct opening)
o Jejunum
o Ileum
o Caecum
o Appendix,
o Ascending colon
o Proximal 2/3rds of the transverse colon

Question 20

List the 4 main events in the formation of the midgut loop

Answer 20

Herniation of the midgut loop
Rotation of midgut (90 degrees counterclockwise)
Retraction of intestinal loop (further 180-degree counterclockwise rotation)
Fixation of the intestines

Question 21

Describe Meckel diverticulum

Answer 21

• In the human embryo, the vitelline duct is a long narrow tube that joins the yolk sac to the midgut lumen of the developing fetus. It appears at the end of the fourth week, when the yolk sac presents the appearance of a small pear-shaped vesicle (the umbilical vesicle).
• Generally, the duct obliterates, but in 2-4% people a small vitelline duct persists
• Forms Meckel diverticulum, which is situated about two inches above the ileocecal junction and may be attached by a fibrous cord to the abdominal wall at the umbilicus. Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the duct
• May form fistula or vitelline cyst/ligament

Question 22

What is omphalocele?

Answer 22

Omphalocele = herniation of abdominal viscera through an enlarged umbilical ring

Question 23

List some conditions associated with formation of the midgut loops

Answer 23

Meckel diverticulum
Omphalocele
Gastroschicis
Abnormal/reversed rotation of intestinal loops

Question 24

Define gastrulation

Answer 24

Gastrulation is a phase early in the embryonic development of most animals during which the single-layered blastula is reorganized into a trilaminar (three-layered) structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm

Question 25

Describe gastroschicis

Answer 25

Gastroschicis - Herniation of abdominal contents directly through the body wall into the amniotic cavity. Not covered by peritoneum or amnion. Through a weak area right of umbilicus

Question 26

Describe reversed intestinal rotation and its clinical significance

Answer 26

Reversed rotation – rare, midgut loop rotates clockwise rather than counterclockwise. As a result, duodenum lies anterior to the superior mesenteric artery rather than posterior to it, and the transverse colon lies posterior instead of anterior to it. In these infants, pressure from the mesenteric artery may obstruct the transverse colon.

Question 27

List the derivatives of the hindgut

Answer 27

o Distal 1/3rd transverse colon o Descending colon o Sigmoid colon o Rectum and upper part of the anal canal o Endoderm of hindgut also forms the lining of the bladder and urethra

Question 28

What is the cloaca?

Answer 28

Expanded terminal part of the hindgut, a endoderm lined chamber that is in contact with the surface membrane at the cloacal membrane

Question 29

What divides the cloaca into ventral and dorsal parts?

Answer 29

Urorectal septum

Question 30

List 4 developmental abnormalities in the hindgut

Answer 30

Urorectal fistula Rectovaginal fistula Rectoanal atresia Imperforate anus (failure of anal membrane to break down)