Duodenum Development and Structure

Duodenum Development

• The duodenum starts developing early in the 4th week.
• It forms from the caudal part of the foregut and the cranial part of the midgut.
• It rapidly grows into a C-shaped loop projecting ventrally.
• It has a dual arterial supply.

Duodenum Structure

• The duodenum has dual origins, coming from the caudal part of the foregut and cranial part of the midgut.
• It receives a dual blood supply from the coeliac trunk and superior mesenteric artery.
• The opening of the bile duct separates the two parts derived from the foregut and midgut.
• It rapidly grows into a C-shaped loop.

Duodenum Position and Development

• The stomach rotates, moving the duodenum to the right and bringing it into a retroperitoneal position during weeks 5-6.
• The luminal opening during this process closes as the epithelial cells proliferate.
• Recanalization of the duodenum is complete by the end of the embryonic period.
• The ventral mesentery of the duodenum disappears.

Duodenal Defects

• Incomplete recanalization can cause stenosis (narrowing) of the 3rd and 4th parts of the duodenum.
• Complete occlusion of the lumen in the 2nd and 3rd parts of the duodenum can lead to atresia.
• Infants with these defects typically exhibit bile-containing vomiting.
• An X-ray or ultrasound might show a "double bubble" sign.

Canalization Process

• During development, the lumen of the duodenum is initially blocked by an epithelial plug.
• This plug is gradually broken down allowing for canalization.

Liver Development

• The liver appears in the 4th week as a ventral hepatic diverticulum arising from the caudal foregut.

Liver Growth and Size

• The liver rapidly grows during weeks 5-10, occupying much of the abdominal cavity.
• By the 9th week, the liver accounts for about 10% of the total body weight.
• The right and left hepatic lobes are initially of similar size, but the right lobe usually becomes larger later during development.

Liver Cell Development

• Proliferating endodermal cells form hepatic cords, which then anastomose around endothelium-lined spaces called hepatic sinusoids.

Liver Structure and Derivatives

• Hepatic cords and the epithelial lining of the intrahepatic bile system derive from endoderm.
• The fibrous tissue, hematopoietic tissue, and Kupffer cells originate from mesenchyme of the septum transversum.
• Hepatic sinusoids arise from vitelline veins.
• Blood cell formation (hematopoiesis) typically initiates in the liver during the 6th week of development.
• Bile formation begins in the 12th week.

Biliary Apparatus

• The small caudal part of the hepatic diverticulum evolves into the gallbladder.
• The stalk of the diverticulum becomes the cystic duct.
• The stalk connecting the hepatic and cystic ducts forms the bile duct, opening into the duodenum.

Biliary Process

• Bile duct opening is initially obstructed but canalizes later.
• Bile formation starts around the 12th week.
• Bile staining the intestinal contents (meconium) is usually noticeable by the 13th week.

Pancreas Development

• The pancreas develops as two buds (dorsal and ventral) from the caudal foregut.
• The dorsal bud is typically larger and develops first, anterior to the ventral pancreas.

Pancreas Position and Fusion

• During rotation, the ventral and the bile duct moves the ventral bud dorsally.
• The ventral bud is situated posterior to the dorsal bud and eventually fuses with the dorsal bud.
• Their ducts merge to form the main pancreatic duct, which typically empties into the duodenum.

Pancreas Duct Development

• The proximal part of the dorsal duct can persist as a separate accessory pancreatic duct emptying into the duodenum via the minor duodenal papilla.
• The pancreas is retroperitoneal in final position.

Insulin and Glucagon Secretion

• Insulin production begins around the 10th week.
• Glucagon production begins around the 15th week.

Spleen Development

• The spleen develops from mesenchyme within the dorsal mesogastrium and appears in the fifth week of development.
• It develops lobes early in fetal life.
• These lobes normally disappear by birth.
• The spleen plays a critical role in hematopoiesis (blood cell formation) up until late in fetal life, and this ability is maintained through adulthood.

Mesentery Derivatives

• The mesenteries that connect organs to the body wall are important for understanding their development and anatomy.
• The dorsal mesoesophagus becomes the crura of the diaphragm.
• The ventral mesoesophagus disappears.
• The dorsal mesogastrium becomes the greater omentum, lienorenal ligament, and gastrosplenic ligament.
• The ventral mesogastrium becomes the lesser omentum, the visceral peritoneum of the liver, and the falciform ligament.

Anomalies

• Variations in hepatic ducts, cystic ducts, and bile ducts are common but typically not clinically significant.
• Extrahepatic biliary atresia (a blockage of the bile ducts outside the liver) is a serious congenital anomaly that can be life-threatening.
• Accessory or ectopic pancreatic tissue and annular pancreas are other possible congenital abnormalities.

Duodenal/Pancreatic Mesenteries

• The dorsal mesentery of the duodenum persists in the proximal part as the hepatoduodenal ligament.
• The ventral mesentery of the duodenum mostly disappears.