EMBRYOLOGY OF PANCREAS & SMALL INTESTINE Prof. Mujahid Khan اسم ورقم المقرر – Course Name and No. 5/2/2019

Primordial Gut At the beginning of the 4th week the primordial gut is closed at its cranial end by the oropharyngeal membrane Primordial gut forms during the 4th week as the head, tail and lateral folds incorporate the dorsal part of the yolk sac into the embryo

Primordial Gut The endoderm of primordial gut gives rise to most of the epithelium and glands of the digestive tract The epithelium at the cranial (upper) and caudal (lower) ends of the tract is derived from ectoderm of the stomodeum and proctodeum

Foregut The derivatives of foregut are: The primordial pharynx and its derivatives The lower respiratory system The esophagus and stomach The duodenum, proximal to the opening of the bile duct The liver, biliary apparatus and pancreas

Stomach During middle of the 4th week a slight dilation indicates the site of the stomach primordium It first appears as a fusiform enlargement of the caudal (lower) part of the foregut It initially oriented in the midian plane It soon enlarges and broadens ventrodorsally During the next 2 weeks its dorsal border grows faster than the ventral border This demarcates the greater curvature of the stomach

Rotation of Stomach As the stomach enlarges and acquires the adult shape it slowly rotates 90 degrees in a clockwise direction around its longitudinal axis

Effects of Rotation The ventral border (lesser curvature) moves to the right The dorsal border (greater curvature) moves to the left The original left side becomes the ventral surface The original right side becomes the dorsal surface

Effects of Rotation Before rotation the cranial and caudal ends of the stomach are in median plane After rotation the stomach assumes its final position with its long axis almost transverse to the long axis of the body Rotation explains why the left vagus nerve supplies the anterior wall of the adult stomach and right vagus nerve innervates its posterior wall

Duodenum Early in the 4th week the duodenum begins to develop from the caudal (lower) part of the foregut and cranial (upper) part of the midgut The junction of the two parts of the duodenum is just distal to the origin of the bile duct As the stomach rotates, the duodenal loop rotates to the right and comes to lie retroperitoneally

Duodenum Because it is derived from the foregut and midgut, it is supplied by the branches of the celiac and superior mesenteric arteries During the 5th and 6th weeks, the lumen of the duodenum is temporarily obliterated by proliferation of its epithelial cells It normally recanalized by the end of the embryonic period

Ventral Mesentery This thin double layered membrane gives rise to: The lesser omentum, passing from the liver to the lesser curvature of the stomach called hepatogastric ligament From the liver to the duodenum the hepatoduodenal ligament Falciform ligament extending from the liver to the ventral abdominal wall

Pancreas It develops between the layers of the mesentery from dorsal and ventral pancreatic buds of endodermal cells These buds arise from the caudal (lower) part of the foregut Most of the pancreas is derived from the dorsal pancreatic bud

Pancreas As the duodenum rotates to the right and becomes C-shaped, the ventral pancreatic bud is carried dorsally with the bile duct It soon lies posterior to the dorsal pancreatic bud and later fuses with it The ventral pancreatic bud forms the uncinate process and part of the head of the pancreas As the stomach, duodenum and ventral mesentery rotate, the pancreas comes to lie along the dorsal abdominal wall

اسم ورقم المقرر – Course Name and No. 5/2/2019

Pancreas The main pancreatic duct forms from the duct of the ventral bud and the distal part of the duct of the dorsal bud The proximal part of the duct of the dorsal bud often persists as an accessory pancreatic duct The two ducts often communicate with each other In about 9% of people, the pancreatic ducts fail to fuse

Midgut The derivatives of the midgut are: The small intestine including most of the duodenum The cecum, appendix, ascending colon and the right half to two-thirds of the transverse colon

Midgut Midgut derivatives are supplied by the superior mesenteric artery The midgut loop is suspended from the dorsal abdominal wall by an elongated mesentery As the midgut elongates, it forms a ventral U-shaped loop of gut, the midgut loop It projects into the proximal part of the umbilical cord

Midgut This movement of the intestine is a physiological umbilical herniation It occurs at the beginning of the 6th week The midgut loop communicates with the yolk sac through the narrow yolk stalk until the 10th week

Midgut Umbilical herniation occurs because there is not enough room in the abdomen for the rapidly growing midgut The shortage of space is caused mainly by the relatively large liver and two sets of kidneys during this stage of development The midgut loop has a cranial limb and a caudal limb Yolk stalk is attached to the apex of the midgut loop where the two limbs join

Midgut The cranial limb grows rapidly and forms small intestinal loops The caudal limb undergoes very little change except for development of cecal diverticulum which is a primordium of the cecum and appendix

Rotation of Midgut Loop While it is in the umbilical cord the midgut loop rotates 90º counterclockwise around the axis of superior mesenteric artery This brings cranial limb to the right and the caudal limb to the left During rotation the midgut elongates and forms intestinal loops e.g. Jejunum and ileum

Return of Midgut Loop to Abdomen During the 10th week the intestines return to the abdomen What causes the return of the intestine is not clearly known The decrease in the size of the liver and kidneys and the enlargement of the abdominal cavity are important factors This process is called reduction of the physiological midgut hernia

Return of Midgut Loop to Abdomen The small intestine formed from cranial limb returns first It passes posterior to the superior mesenteric artery and occupies the central part of the abdomen As the large intestine returns, it undergoes further 180º counterclockwise rotation Later, large intestine comes to occupy the right side of the abdomen

Umbilical Hernia When the intestines return back to the abdominal cavity during the 10th week and then herniate through an imperfectly closed umbilicus, an umbilical hernia forms Hernia reaches its maximum size at the end of the first month after birth

Umbilical Hernia It usually ranges from 1 to 5 cm The defect through which the hernia occurs is the linea alba Hernia protrudes during crying, straining, or coughing It can easily be reduced through the fibrous ring at the umbilicus Surgery is not usually performed until it persists to the age of 3 to 5 years

Meckel Diverticulum This outpouching is one of the most common anomalies of the digestive tract This congenital ileal diverticulum occurs in 2 to 4% of people 3 to 5 times more prevalent in males than females It sometimes becomes inflamed and causes symptoms that mimic appendicitis

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