Development of the gut

Question 1

Name all of the foregut structures.

Answer 1

The foregut technically starts in the oral cavity and includes the pharynx and proximal oesophagus however in some cases this isnt counted.

The true structures of the foregut include the distal half of the oesophagus, the stomach, the liver, the pancreas, the gall bladder (biliary apparatus) and the proximal half of the duodenum.

Question 2

Name all of the midgut structures.

Answer 2

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

Question 3

Name all of the hindgut structures.

Answer 3

The distal 1/3 of the transverse colon, the descending colon, the sigmoid colon, the rectum and the proximal 2/3 of the anal canal.

Question 4

What are the two layers of the bi-laminar disk of the developing embryo? What migrates through to become the 3rd layer?

Answer 4

The epiblast and the hypoblast. The mesoderm migrates as the 3rd layer.

Question 5

What layer of the embryo does the gut tube form from?

Answer 5

The hypoblast develops into endoderm which then forms the gut tube. Any tissue that secretes mucus is endoderm in origin.

Question 6

Explain what happens during longitudinal and lateral folding to form the gut tube.

What does the endoderm become?
What does the mesoderm become?

Answer 6

The ectoderm and mesoderm fold down and inwards to pinch off the yolk sac (hypoblast) to form the gut tube.
Endodermal cells then line the inside of the tube to become the precursor mucosal lining of the gut.

Mesoderm forms the outer layer of the gut tube which will go on to form all of the muscles and blood vessels.

The mesoderm directly surrounding the gut tube forms the visceral mesoderm whilst the mesoderm that is further out forms the parietal mesoderm.

The space in between the the two mesodermal layers in the body cavity/the peritoneal cavity.

Question 7

How do all of the organs come from the gut tube?

Answer 7

They “bud off” from the gut tube throughout different periods of the development.

The midgut remains in contact/open to the yolk sac.

Question 8

What is the blood supply to the foregut and what spinal level does this come from?

Answer 8

The Coeliac trunk at the level of T12.

Question 9

What is the blood supply to the midgut and what level does this come from?

Answer 9

The superior mesenteric artery at the level of L1

Question 10

What is the blood supply to the hindgut and what level does this come from?

Answer 10

The inferior mesenteric artery at the level of L3.

Question 11

What is the respiratory diverticulum and what does it form?

Answer 11

The respiratory diverticulum is a ventrl outgrowth of the endoderm of the gut tube during early development, just caudal to the pharynx.

This diverticulum goes on to form the trachea and the lung buds.

The oesophagus eventually partitions from the trachea and is initially very short.

Question 12

What is a diverticulum?

Answer 12

Can either be described as a blind tube leading from a cavity or as an abnormal sac formed in a tract.

Question 13

What is an oesphageal atresia? How could it be recognized in a baby?

Answer 13

An atresia means there is a blockage and in this case it means the oesophagus is a blind ended tube. (the distal end failed to open and is disconnected from the distal oesophagus and rest of the GI tract).

In a baby it could be indentified as after the feeding the baby would consistently vomit the milk up as there is nowhere for it to go.

Question 14

What is a tracheo-esophageal fistula?

Answer 14

A fistula is a connection that shouldnt occur. (a fist punching into something it shouldnt). In this case the oesophagus is connected into the trachea (there is an imcomplete partition).

Question 15

What is a congenital hiatal hernia?

Answer 15

The oesophagus fails to lengthen and pulls the stomach up through the diaphragm into the thorax.

Question 16

What is a hernia?

Answer 16

a condition in which part of an organ is displaced and protrudes through the wall of the cavity containing it (often involving the intestine at a weak point in the abdominal wall).

Question 17

What is a mesentary/mesogastrum?

Answer 17

A double layer of peritoneum that attaches the stomach, small intestine, pancreas, spleen, and other organs to the posterior wall of the abdomen.

Question 18

What is the dorsal mesogastrium?

Answer 18

The portion of mesentery attached to the greater curvature of the stomach is named the dorsal mesentery (or dorsal mesogastrium, when referring to the portion at the stomach), and the part which suspends the colon is termed the mesocolon.

The dorsal mesogastrium develops into the greater omentum.

Question 19

What is the ventral mesogastrium?

Answer 19

The lesser omentum is formed, by a thinning of the mesoderm or ventral mesogastrium, which attaches the stomach and duodenum to the anterior abdominal wall.

By the subsequent growth of the liver this leaf of mesoderm is divided into two parts, viz., the lesser omentum between the stomach and liver, and the falciform and coronary ligaments between the liver and the abdominal wall and diaphragm.[1]

Question 20

Describe the development of the foregut.

Answer 20

The foregut develops from a cranial region of endoderm created after the initial cephalocaudal folding of the embryo. Starting at the stomodeum, a rapid expansion of the primitive gut forms the esophagus, from which the respiratory bud branches off.[1]During early foregut development, the esophagus lengthens considerably, reaching its proportional postnatal size. Simultaneously, the stomach begins to expand in width dorsally and ventrally in an asymmetric manner. This asymmetric expansion creates two curvatures, with the ventral side creating the lesser curvature and the dorsal side creating the greater curvature.[1]The expanding dorsal stomach wall then rotates the on its transverse plane, pulling its caudal portion upward and forcing the upper duodenum into a C shape. This rotation positions the left vagus nerve anteriorly and right vagus nerve posteriorly.[1] While the hindgut and midgut are only attached dorsally to the body wall by a fold of peritoneum, the foregut also has a ventral attachment. Its two attachments are commonly referred to as the dorsal mesogastrium and the ventral mesogastrium. As the stomach rotates during early development, the dorsal and ventral mesentery rotate with it; this rotation produces a space anterior to the expanding stomach called the greater sac, and a space posterior to the stomach called the lesser sac. After this rotation the dorsal mesentery thins and forms the greater omentum, which is attached to the greater curvature of the stomach. The ventral mesentery forms the lesser omentum, and is attached to the developing liver. In the adult, these connective structures of omentum and mesentery form the peritoneum, and act as an insulating and protective layer while also supplying organs with blood and lymph vessels as well as nerves.[1] Arterial supply to all these structures is from the celiac trunk, and venous drainage is by the portal venous system. Lymph from these organs is drained to the prevertebral celiac nodes at the origin of the celiac artery from the aorta.

Question 21

What is connecting the foregut, midgut and hindgut to the body wall?

Answer 21

The foregut has both a ventral and dorsal mesentary (the dorsal and ventral mesogastrium) while the midgut and hindgut have only a dorsal mesogastrium.

Question 22

What is congenital hypertrophic pyloric stenosis and how can it be indenitfied?

Answer 22

congenital = a disease of abnormailty present from birth 
hypertrophic = enlargement of an organ or tissue as a result of enlargement of the size of the cells
pyloric = the pylorus is the part of the stomach that empties into the small intestine
stenosis = abnormal narrowing of a passage in the body 

This is due to a thickening of the muscular wall of the pylorus which prevents emptying of the stomach contents into the duodenum (first part of the small intestine). This is characterized by muscle spasms that induce vomitting and the vomit is often tested because if it dosent contain bile then it shows that the contents have been unable to pass out of the stomach.

Question 23

What is the biliary tree?

Answer 23

The biliary tree is a system of vessels that directs secretions from the liver, gallbladder and pancreas through a series of ducts into the duodenum.

Question 24

What is the hepatic diverticulum and what forms from it?

Answer 24

A ventral outgrowth of the duodenum that grows and rotates to become dorsally positioned.

The hepatic diverticulum buds into the liver and the bile ducts.

Question 25

Where in relation to the hepatic diveritculum does the pancreas arise and how is the main pancreatic formed?

Answer 25

The pancreas is formed from two buds fusing. The dorsal pancreatic duct which is fairly large and the ventral pancreatic duct which is much smaller and originates from the hepatic duct.

As the duodenum rotates (the ventral duodeunum becomes dorsal duodenum) and the ventral pancreatic bud fuses posteriorly to the dorsal pancreatic bud to form the pancreatic duct.

The accessory duct (original duct between the large dorsal duct and the duodenum) is no longer needed and often disappears.

Question 26

How does the midgut rotate?

Answer 26

The midgut creates a ‘U” shaped loop around the superior mesenteric artery which creates two limbs (the caudal and the cranial limbs). The loop then undergoes a normal herniation and moves out into the umbilical cord. The midgut then rotates counterclockwise and the limbs become left and right of the superior mesenteric artery. It continues to rotate through 270 degrees before the herniation retracts back into the abdomen and the caecum and appendix rotate down to the lower abdomen.

Question 27

What is an umbilical herniation/fistula?

Answer 27

A failure of the umbilical cord to close properly so the gut herniates through weakened region of the body wall. (eg see some babies with guts on outside).

Question 28

What is the cloaca of the hindgut?

Answer 28

The cloaca is the expanded distal part of the hindgut which is divided into dorsal and ventral parts by the mesenchymal urorectal septum. Eventually the septum grows right down to seperate the gut tube (rectum) from the urogenital sinus (urinary tract).

Question 29

What happens in the rectum/anal canal?

Answer 29

There is a transition from inner endoderm and outer ectoderm. The boundary is formed by the pectinate line.

Question 30

What is megacolon (hirshprungs disease)?

Answer 30

An abscence of ANS ganglion cells means there is a lack of innervation to the colon and so it cannot perform peristalsis so food gets built up.

Question 31

What is renal atresia AND imperforated anus?

Answer 31

A