L20 Development of the GI Tract

Question 1

Where do the liver, gallbladder and pancreas arise from?

Answer 1

There are outpouchings of the foregut which give rise to the liver, gallbladder and pancreas. The epithelium lining of the gut tube is derived from the endoderm and so the majority of the organs are derived from the endoderm. These develop from diverticulum of the cranial half of the duodenum.

Question 2

When does the liver develop?

Answer 2

Week 3

Question 3

What is the septum transversum?

Answer 3

The primitive diaphragm

Question 4

Where does haemotopoietic cells, kupffer cells and connective tissue arise from?

Answer 4

Mesoderm of the septum transverse

Question 5

Where does the liver originate from?

Answer 5

Endoderm (and mesoderm) - The endodermal cells go onto form the liver parenchyma, hepatocytes. Since the liver develops in the septum transversum, it develops Kupffer cells and hemopoietic cells which are derived from mesoderm

Question 6

How does the liver form?

Answer 6

Begins in week 3. It is an outpouching of endoderm. The bile duct is connected to the gut tube. The liver begins as an endodermal plate from the gut tube around the cranial part of the duodenum. The development of the liver bud develops as a thickening of the endodermal plate to form a diverticulum hepatic diverticulum (liver bud). This contains cells that will rapidly divide. Due to the location, it becomes encompassed within the septum transversum. This is of mesodermal origin and will go onto form the diaphragm. Since it is still developing, it is a condensement of cells. Since the liver is developing at the same time, in the same region, some of the mesodermal cells becomes encompassed within the liver. Mesoderm therefore both surrounds the liver and is compassed within it. The connection between the gut tube and the liver bud narrows to become the bile duct. This is the connection between the liver and the gut tube.

Question 7

How does the function of the liver in utero differ from the function in an adult?

Answer 7

In neonates, and foetus, the main function of the liver is hemopoieses. All of the filtering of the blood was previously done by the mother as a child and so the liver can change function as the liver does not need to filter the blood as a child. Once born, the liver must take on this responsibility.

Question 8

When does the gall bladder and cystic duct form?

Answer 8

Week 3

Question 9

What is meconium?

Answer 9

• The foetus swallows amniotic fluid to help tone respiratory muscles. There is amniotic fluid going through the gut tube. We release bile into the gut tube. The bile has no where to go until the child first defecates. As a result the first bowel movement of the new-born (meconium) is dark green. This is a sign that the child’s gallbladder and biliary system has formed correctly.

Question 10

What is a main cause of neonatal jaundice?

Answer 10

In 60% the liver does not have enough enzymes to conjugate some of the bilirubin leading to hyperbilirubinemia.

Question 11

What is a treatment for neonatal jaundice?

Answer 11

Light therapy

Question 12

What is binary atresia?

Answer 12

The biliary system undergoes rapid proliferation and recanalization. The cells proliferate very quickly and fill it so it is solid; under apoptosis we form holes which join together to form a lumen. Failure of a duct to canalise can lead to biliary atresia. This is a narrowed bile duct. The liver continues to secret and synthesis bile. The hepatic duct is distended due to the build up. This leads to jaundice. The bilirubin goes into the bile and GIT eventually. We cannot get enough into the GIT if there is a blockage. Initially the neonate will respond to light therapy but light therapy is a transient cure (it is done for a short period of time for the liver to make the enzyme). The child will then get jaundice again.

Question 13

How can duplication of the gall bladder occur?

Answer 13

There can be more than one endodermal thickening. There can even be a triplicate of the gallbladder (in week 5 and 6). The gallbladder is an endodermal thickening from the gallbladder. Usually this is asymptomatic.

Question 14

How does the pancreas form?

Answer 14

The pancreas is an endodermal outgrowth. The pancreas develops first from 2 buds: a ventral bud and a dorsal bud. The dorsal bud begins to develop first. The stomach undergoes 90 degree rotation; since the buds are connected to the GIT they rotate with the stomach. The ventral duct rotates clockwise and fuses with the dorsal bud. The ventral bud moves the most (90 degrees clockwise) and fuses with the inferior edge. In week 6, the ventral bud has moved around. As well as the buds fuses together, there is a change in configuration of the duct system. The dorsal bud gives rise to the head, body and tail of the pancreas. The ventral bud from the uncinate process. The dorsal duct system fuses with the ventral duct system to form the main pancreatic duct. Usually the last little part of the dorsal bud system degenerates and we have formed the minor pancreatic duct from the pat of the dorsal bud.

Question 15

Where does the effect of the rotation of the stomach on the pancreas?

Answer 15

Fusion of the ventral duct to the dorsal duct

Question 16

What is the effect of ectopic pancreatic tissue?

Answer 16

Tissue from the pancreas, the glandular tissue can end up in other areas of the GIT. Where pancreatic tissue in areas where is shouldn’t is usually asymptomatic but there can be a problem where it secretes enzymes, leads to degradation of the gut wall leads to large lesions and ulceration. A lot of scar tissue formed from these enzymes can lead to an obstruction.

Question 17

How does the spleen develop?

Answer 17

The spleen is not connected to the gut tube. It develops at the same time in a similar area. It is not of endodermal origin. It is derived from mesoderm that appears at week 5 as a mesenchymal condensation in the dorsal mesentery. The spleen develops in the dorsal mesentery. The rotation of the stomach brings the spleen over the left hand side. It comes from mesenchyme - “undifferentiated mesoderm soup.” Cellular signals cause the “soup” to differentiate into a spleen. The dorsal mesentery between the stomach and the spleen is now called the gastrosplenic ligament and the dorsal mesentery between the spleen and kidney (or posterior wall) is called the lienorenal (splenorenal) ligament.

Question 18

How is the bare area of the liver formed?

Answer 18

With the growth of the liver, it is not contained in the septum transversum and grows caudally into the ventral mesentery. As it moves caudally, the cranial part of the liver is still in contact with the septum transversum. Since so closely related it has not got a peritoneal covering and so is known as the bare area of the liver.

Question 19

What is an annular pancreas?

Answer 19

An extra endodermal thickening can lead to two buds. When it rotates, one can sweep clockwise and the other anticlockwise either side of the duodenum, this leads to an obstruction leading to narrowing of the duodenum.