Development of the GIT - GIT Associated Organs Flashcards
What organs are derivatives of the foregut?
- outpocketings of the foregut give rise to the organs that assist digestion
- these include:
- liver
- gallbladder
- pancreas
- they develop from the cranial half of the duodenum
When does the liver primodium first appear?
What structure does it form and how does the bile duct develop?
- the liver primordium appears in week 3 as an out-pocketing of the endodermal epithelium of the future duodenum
- this out-pocketing is the hepatic diverticulum (liver bud)
- the liver bud contains rapidly proliferating cells that penetrate the septum transversum (future diaphragm)
- whilst cells continue to proliferate and penetrate the septum, the connection between the hepatic diverticulum and duodenum narrows to form the bile duct
What is formed from the bile duct following its formation?
- the bile duct forms a small ventral outgrowth that gives rise to the gallbladder and cystic duct
What will epithelial liver cords mix with and differentiate into?
What cells are formed from mesoderm of the septum transversum?
- epithelial liver cords intermingle with the vitelline and umbilical veins, which form hepatic sinusoids
- liver cords (endodermal) differentiate into hepatocytes (parenchyma) of the liver and form the lining of biliary ducts
- the mesoderm of the septum transversum gives rise to:
- haematopoietic cells
- Kupffer cells
- connective tissue cells
How does growth of the liver lead to formation of the ventral mesentery?
- the liver continues to rapidly expand and it becomes too large to be contained within the septum transversum
- it bulges caudally into the abdominal cavity
- the mesoderm of the septum transversum between the liver and the foregut becomes membranous and forms the lesser omentum
- the mesoderm of the septum transversum between the liver and ventral abdominal wall becomes membranous and forms the falciform ligament
- the falciform ligament and the lesser omentum together make up the ventral mesentery
What is meant by the bare area of the liver and how is it formed?
- mesoderm on the surface of the liver differentiates into visceral peritoneum, except on its cranial surface
- the cranial surface remains in contact with the rest of the original septum transversum
- this portion of the septum contains densely packed mesoderm and will become the central tendon of the diaphragm
- the surface of the liver that is in contact with the septum is never covered by peritoneum so is the bare area of the liver
What ligaments are formed around the margins of the bare area of the liver?
- around the margins of the bare area, peritoneum reflects to form the coronary ligament
- the coronary ligament ends at the lateral edges of the liver as the right and left triangular ligaments
Why does the liver comprise 10% of total body weight at week 10 of development?
Why does this decrease to 5% after birth?
- the liver of the foetus is large due to its importance in haematopoiesis
- there are large nests of proliferating cells, which produce red and white blood cells, between the hepatic cells and walls of blood vessels
- the activity of these cells gradually ceases and after birth haematopoiesis shifts to the bone marrow
- only small haematopoietic islands remain at birth
When does formation of the gallbladder begin?
When does bile production start and what is the result of this on the first bowel movement of the newborn?
- the gallbladder ad cystic duct form at the end of week 3 as a ventral outgrowth of the bile duct
- the liver starts to produce bile in week 12
- bile enters the gastrointestinal tract as the cystic duct has joined the hepatic duct to form the bile duct
- the first bowel movement of the newborn (meconium) is dark green in colour due to the presence of bile in the GIT
How does the position of the bile duct change during development?
- due to positional changes of the duodenum, the entrance to the bile duct shifts from its original anterior position to a posterior one
- the bile duct passes behind the duodenum
Why does neonatal jaundice occur in 60% of full term infants?
- prior to birth, bilirubin crosses the placenta and is removed by the mother’s circulation
- after birth, the liver conjugates bilirubin which is then excreted into the GIT through the biliary system
- at birth, the immature liver quite often does not have sufficient glucuronosyltransferase to conjugate bilirubin
- this leads to hyperbilirubinaemia and jaundice, which presents with yellow skin and sclera
What is the most severe complication of untreated neonatal jaundice?
How can this be avoided?
kernicterus
- excess unconjugated bilirubin can cross the blood-brain barrier and cause brain damage
- phototherapy oxidises bilirubin to a water soluble form that can be easily excreted by the newborn and does not contribute to kernicterus
- blue light is the most effective
Why does biliary atresia occur?
What condition does it appear similar to and how can they be distinguished?
- hepatic and bile duct epithelium undergoes rapid proliferation and recanalisation during development
- failure of either duct to recanalise leads to biliary atresia
- it causes symptoms indistinguishable from neonatal jaundice, but they do not respond to phototherapy
Why does duplication of the gallbladder occur?
What symptoms is it associated with?
- duplicated gallbladder is a common congenital malformation that is usually asymptomatic
- it is caused by an extra endodermal outpocketing during week 5 and 6
How does the pancreas begin its development?
- the pancreas initially develops as 2 endodermal buds that will fuse together
- they both originat from the endodermal lining of the duodenum
- the dorsal bud appears in week 3 as an outpocketing of duodenum that extends into the dorsal mesentery
- the ventral bud is located close to the bile duct
How does the ventral pancreatic bud change location with rotation of the duodenum?
- with rotation of the stomach, the duodenum rotates 90o to the right and becomes C-shaped
- during this movement, the ventral pancreatic bud moves dorsally in a manner similar to the shifting of the entrance of the bile duct
- the ventral bud finally comes to lie immediately below and behind the dorsal bud
When do the ventral and dorsal pancreatic buds fuse?
What structures arise from each bud?
- the ventral and dorsal pancreatic buds fuse in week 6
- the ventral bud forms the uncinate process and inferior part of the head of the pancreas
- the dorsal bud gives rise to the majority of the head, body and tail
- connective tissue and blood vessels are formed from surrounding mesoderm
What happens to the duct systems when the pancreatic buds fuse?
What forms the main and accessory pancreatic ducts?
- when the buds fuse together, their duct systems become interconnected
- the main pancreatic duct (of Wirsung) is formed by the distal portion of the dorsal pancreatic duct and ALL of the ventral pancreatic duct
- the accessory pancreatic duct (of Santorini) is formed by the proximal portion of the dorsal pancreatic duct
- the accessory pancreatic duct is either obliterated during development or persists as a small channel
Where to the main and accessory pancreatic ducts enter the duodenum?
- the main pancreatic duct + bile duct enter the duodenum at the site of the major duodenal papilla via the Ampulla of Vater
- the accessory pancreatic duct enters at the minor duodenal papilla
What is meant by an annular pancreas and how does it develop?
What are the possible complications of this condition?
- the ventral pancreatic bud may be bilobed
- in this case, one lobe may migrate ventral to the duodenum and one may migrate dorsally to surround the duodenum and produce an annular pancreas
- this can compress the duodenum and cause a gastrointestinal obstruction
What is meant by ectopic pancreatic tissue and where can it be found?
What symptoms is it associated with?
- occurs as a result of inappropriate differentiation of endodermal cells into pancreatic tissue
- can be found in areas ranging from the distal oesophagus to the tip of the primary intestinal loop
- most commonly found in the mucosa of the stomach and Meckel’s diverticulum (duodenum)
- usually asymptomatic but large lesions can cause obstruction, ulceration and haemorrhage
What is different about the origin of the spleen?
- unlike the rest of the abdominal organs, the spleen is not derived from the endoderm
- it is a mesoderm derivative that appears in week 5 as a mesenchymal condensation in the dorsal mesentery
How does rotation of the stomach affect the position of the spleen?
What ligaments are formed as a result of this?
- rotation of the stomach brings the spleen over to the left hand side
- the dorsal mesentery between the stomach and the spleen is now the gastrosplenic ligament
- the dorsal mesentery between the spleen and left kidney is now the lienorenal (splenorenal) ligament
What is an accessory spleen and why does it occur?
What symptoms does it produce?
- additional mesenchymal condensations may occur in the dorsal mesentery and form accessory spleens
- this affects 10% of the population
- they usually form near the hilum of the primary spleen
- they are usually asymptomatic but can affect interpretation of medical images and are an important variant for surgeons to be aware of
