derivatives of foregut

Derivatives of foregut

Alatise O.I


• The foregut are portion of the fetus that extend from the buccopharyngeal membrane (definitive mouth) to the definitive half of the second part of the duodenum

• It is formed from the endodermal germ layer by cephalo-caudal and lateral folding of the embryo at about three weeks of IU live


• Cranial fold foregut• Caudal fold Hindgut• Lateral fold Midgut• Foregut appeared initial as a tubular

diverticulum cranial to the yolk sac and is located dorsal to the pericardial cavity and septum transversum


• The blind cranial end extremity of the foregut forms the endodermal layer of the buccopharyngeal membrane which separate it from the stomatodeum (ectoderm)

• At about 21days (20 somites) the membrane ruptures allowing communication btw the foregut and the amniotic fluid


• Foregut derivatives are divided into – cranial portion –bounded laterally by the branchial

arches– Caudal portion- bounded by the pericardio-

peritoneal canal which later become the pleural and peritoneal cavity

Cranial portion of the Foregut

• The cranial portion of the foregut forms the pharyngeal pouches and their derivatives

• The first pharyngeal pouch Eustachian tube

• The second pharyngeal pouch Tonsillar fossa

• The third pharyngeal pouch Inferior parathyroid pouch and the thymus

Cranial portion of the Foregut

• The fourth pharyngeal pouch Superior parathyroid gland

• Other derivatives of the cranial portion include – Thyroid gland– Respiratory tree– Pharynx

Respiratory system

• The respiratory tree arises as a median ventral diverticulum called laryngotracheal diverticulum of the foregut.

• The diverticulum appears about the 25 somites (4 weeks IU live) and is in direct continuation with the definitive esophagus

• Two longitudinal ridges which later fuses craniocaudally separate the respiratory system from the esophagus

Respiratory system

• Failure of the fusion of the ridges lead to tracheoesophageal fistula

• The diverticulum elongate in the midline with the caudal end dividing into two lateral outpouching

• The midline cranial portion forms the larynx and tracheal– Note the cartilages and the muscles are from the

mesoderm

Respiratory system

• The lateral buds continue to divides even after birth giving rise to bronchi etc

Caudal portion of the foregut

• Caudal portion of the foregut give rise to he esophagus, stomach, liver, biliary system and pancreas.

• All these structures appear during weeks 4-5.

Esophagus

• The development of the esophagus is complicated by the outpocketing of of the respiratory diverticulum or lung buds from the ventral wall. Essential to normal development is the completion of the esophagotracheal septum which makes the espophagus, the pathway for food, distinct from the trachea, or pathway for air. The esophagus lengthens during the 5th week as the heart descends.

Esophagus

• The wall of the upper esophagus skeletal muscle from the 6th pharyngeal arch

• The wall of the lower esophagus smooth muscle from splanchnic mesoderm

• The dorsal mesenterty of the esophagus, the mesoesophagus becomes incorporated into the diaphragm and is thus, obliterated in the adult.

Stomach

• The stomach appears as a dilation of the foregut caudal to the esophagus during the fourth week of development.

• The stomach evolves by two rotations along a longitudinal and anteroposterior axis.

Stomach

• The longitudinal rotation of the stomach involves a 90° clockwise rotation resulting in the right side of the stomach becoming posteriorly oriented and the left side of the stomach facing anteriorly. This explains why the left vagus nerve innervates the anterior wall of the stomach and the right vagus nerve innervates the posterior wall of the stomach in the adult.

Stomach

• During this rotation one side of the stomach grows faster than the other forming the greater and lesser curvatures of the stomach. The stomach subsequently rocks on its longitudinal axis, causing the pylorus to shift to right and the cardiac orifice to shift to the left.

Stomach

• The stomach, like the rest of the developing gut has a dorsal mesentery (dorsal mesogastrium) and an additional ventral mesentery, (the ventral mesogastrium), formed from the septum transversum.

• The rotation and uneven growth of the stomach pull the dorsal mesogastrium to the left, creating a subdivision of the peritoneal cavity, the omental bursa, posterior to the stomach and between the stomach and the posterior body wall.

Stomach

• The dorsal mesogastrium grows in a caudal direction as the stomach rotates along its anterioposterior axis and forms the greater omentum, a double layered apron over the transverse colon.

• The ventral mesogastrium is separated into parts by the formation of the liver. The portion which attaches to the lesser curvature of the stomach and to the liver is the lesser omentum.

Duodenum

• The duodenum forms at the junction of the foregut and midgut.

• The more cranial (foregut) portion of the duodenum gives rise to the hepatic diverticulum, biliary system and pancreatic diverticula.

• As the stomach rotates the duodenum also rotates and comes to lie against the dorsal body wall.

Duodenum

• The mesoduodenum fuses with the back body wall leaving the main portion of the duodenum secondarily retroperitoneal.

• The superior and descending portions of the duodenum are considered part of the foregut and the horizontal and ascending portions of the duodenum are considered part of the midgut.

Liver and Gallbladder

• The hepatic diverticulum or liver bud penetrates ventrally from the duodenum into the mesodermal septum transversum at 3 weeks.

• The hepatic diverticulum narrows to form the bile duct which gives rise to a gallbladder.

• As the liver grows, it comes to occupy the abdominal cavity and lies suspended by mesentery: anteriorly, the falciform ligament and posterolaterally, the lesser omentum.

Liver and Gallbladder

• The portal vein, hepatic artery, and common bile duct reach the liver through the caudal margin of the lesser omentum.

• Growth of the liver downward from the septum transversum displaces the stomach to the left. Most of the liver becomes covered with peritoneum, but a small portion, the bare area of the adult liver, never completely detaches from the septum transversum. Duplication of the gallbladder can occur.

Pancreas

• Two pancreatic diverticula develop from the foregut in the region of the duodenum, one in the ventral mesentery (ventral mesogastrium) and one in the dorsal mesentery (mesoduodenum).

• Due to rotation of the duodenum, the two buds come to lie together and fuse.

• The pancreas is pushed against the back wall with the duodenum and also becomes secondarily retroperitoneal.

Pancreas

• The duct system of the ventral pancreas and most of the dorsal pancreas persists as the main pancreatic duct.

• A portion of the dorsal pancreatic duct enters the duodenum separately as the accessory pancreatic duct.

• In some cases, the ventral pancreas may enclose the second part of the duodenum, obstructing it. This condition is termed annular pancreas.

Spleen

• Note: The spleen is not a derivative of the foregut . It develops, however, as a mesodermal proliferation between the two leaves of dorsal mesogastrium.

• As the stomach rotates and the liver enlarges, the spleen becomes situated in the left hypochondriac quadrant. It is attached to the back wall by the lienorenal ligament and to the stomach anteriorly by the gastrosplenic ligament. These ligaments form the lateral wall of the omental bursa in the adult.

Blood supply of the foregut

• Branches from the aortic arches and branches from the descending thoracic aorta above the developing diaphragm

• Celiac artery supplies the foregut below the diaphragm

derivatives of foregut

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