Specialized EmbryologyGASTROINTESTINAL SYSTEM .The Foregut
The part of the foregut cranial to the side of the origin of the laryngotracheal groove forms part of the oral cavity as well as the pharynx,
while its caudal part to the groove develops into the following parts:Esophagus
Upper half of duodenum
The esophagus develops the foregut just caudal to the opening of the laryngotracheal tube.
It is long and narrow tube ,then proliferation of its epithelial lining leads to transient obliteration of its lumen, which is rapidly becomes re-canalized again after short time.
• Congenital Anomalies• 1.Esophageal stenosis due to incomplete re-canalization.
• 2.Esophageal atrasia due to complete failure of re-canalization.
• 3.Abnormally short esophagus
• 4.A patch of epithelial lining the esophagus may differentiate into gastric mucosa (secrete HCl and leads to ulceration of the lower part of Esophageal mucosa).
• Tracheo-esophageal fistula due to failure of separation.
STOMACHIt develops from the caudal part of the for gut as spindle –shaped dilatation which attached ventrally by ventral mesogastrium and dorsally by dorsal mesogastrium.
The liver will formed inside the ventral mesogastrium ,thus dividing it into :Lesser omentum between porta hepatis of liver and lesser curvature of somach.
Falciform and coronary ligament ,it is ligament that connects the liver to the inner surface of anterior abdominal wall(new umbilicus)and diaphragm.
The spleen develops in the dorsal mesogastrium and divides it into:Gasrtosplenic ligament
Rotation of stomachThe stomach differentiate more and thus fundus ,greater and lesser curvatures become very clear, now lesser curvature faces ventrally ,while ,greater curvature faces dorsally.
Then the stomach rotates 90° clockwise rotation, thus lesser curvature faces to the right while the greater one faces to the left.
As the result of this rotation a small space becomes trapped between the lesser omentum ,posterior surface of stomach and upper part of posterior abdominal wall(stomach bed)this space is known as omental bursa or lesser sac which communicates with the general peritoneal cavity by the epiploic foramen.
Congenital Anomalies of ForegutCongenital hypertrophic pyloric stenosis cause projectile vomiting
Ectopic pancreatic tissue may develops inside the submucosa of pyloric antrum due to invasion of the stomach by the pancreatic bud
Duplication of stomach an d abnormal shaped prepyloric septum
The Mid gut• It grows faster than the vertebral column ,thus it will becomes U-shaped structure with inverted apex.
• This structure has a proximal (cranial) and distal (caudal) limbs with superior mesenteric artery in between the two limbs, while the apex is connected to the umbilicus by vitelline ligament.
The cranial limb will give rise into:Caudal half of duodenum
Most of ileum
The caudal limb will give rise into:Terminal part of ileum
Caecum and appendixAscending colon
Right 2/3of transverse colon
The caecum with the appendix attached to its tip arises as a conical diverticulum from the caudal limb close to the apex of the loop, i.e the point of attachment of the vitello-intestinal duct.
The u-shaped loop projects inside the umbilical cord to form the umbilical hernia of the fetus which is a normal feature between the 6th -12th weeks of intrauterine life.
Such hernia is due to :inadequacy of abdominal cavity which is largely occupied by:
1.The rapidly growing liver.2.The large sized mesonephron on each side of the vertebral column.
By the end of the 12thweek of development ,the loop of the intestine starts to return back into the abdominal cavity because of the following reasons:
1.Regression of mesonephrons.2.No more increase in the size of the liver.
3.An increase in the expansion o the abdominal cavity with an increase in the length of the abdominal wall.
Rotation of the loop and reduction of the herniaJust before the reduction is started ,the loop will rotate 90° anticlockwise, thus the original proximal limb comes to the right while the distal comes to the left.
Once reduction starts the loop will complete its rotation anticlockwise to an angle of 270°, so that the original caudal limb comes to the right and superficial.
The last segment to inter is the caecum which comes to lie superficial to small intestine .
The caecum the descends from just below the liver to reach its adult position (the right iliac fossa).
Congenital Anomalies of Mid gut
• Incomplete reduction results in (omphalocele),where the loop is trapped outside and covers only by amnion of the embryo.
• Incomplete reduction may result congenital umbilical hernia, which is mainly due to defect in the central part of the abd wall.
• The mid gut loop may rotate in opposite direction(clockwise) leading to reverse position of the duodenum and becomes superficial to the colon.
Anticlockwise rotation to 90° only leads to reverse position of abdominal content(situs in versus).
Sub hepatic position of caecum and appendix as it fails to descend to right iliac fossa.
Ligaments: They include:Meckel’s diverticulum ,it measures 2 inches in length, and about 2 feets proximal to ilio-caecal valve and is seen in 2% of population.
Vitello-intestinal fistula connect the intestine with the umbilicus and discharge some of the intestinal contents into the umbilicus.
The whole duct may persists as fibrosed cord extending from the umbilicus to the ileum.
A part of the duct may persists and forms vitelline cyst along the course of the duct.
• Hind gut• It is distal part of gut and closed by cloaca and has connection to allantois and cloacal membrane.
• It gives rise to:
• Left one third of transverse colon
• Descending colon
• Sigmoid colon
• Upper half of the anal canal
• The rectum and anal canal are the most caudal part of the hind gut(caudal to the origin of allantois) dilates and known as endodermal cloaca with the cloacal membrane lying in its ventral wall.
• 1.The endodermal cloaca is divided by mesodermal septum(uro-rectal septum) into:
• Ventral part(urogenital sinus).
• Dorsal part (Rectum and upper half of anal canal).
2.At the mean time the cloacal membrane is divided into:Urogenital membrane(Ventrally).
Anal membrane(Dorsally)3.The mesoderm surround the cloacal membrane is thickened and elevated so the whole membrane comes to lie at the depression of ectodermal cloaca.
4.The anal membrane becomes surrounded by anal tubercles (mesodermal proliferation)that will form the external anal sphincter.
5.Thus the Anal membrane becomes at the bottom of the proctodeum(anal pit)which gives rise to the lower half of anal canal .
6.Rupture of the anal membrane leads to contiuity between two halves of the anal canal and the site of the anal membrane represented in adult by anal valves.
Congenital AnomaliesImperforated Anus due to failure of rupture of the anal membrane.
Congenital recto-vaginal (in female)recto-vesical fistula due to defective urorectal septum.
Rectal atrasia which is obliteration of the lower part of the rectum due to replacement of the caudal part of the anorectal canal by fibrous tissue.
• Liver• Starts as ventral outgrowth (diverticulum)from the ventral aspect of the caudal part of the for gut.
• It grows into the ventral mesogastrium and divided into 2 buds with a stem(stalk).
• These buds are known as pars hepatica (develops into right and left lobes of liver and parscystica which forms the gall bladder, while the stem of the diverticulum forms the bile duct.
• The cells of the liver are arranged in stands which separated from each other by blood sinusoids (from mesoderm of septum transversum).
• The mesoderm is also responsible for the formation of stroma and capsule of the liver ,while kupffer cells from mesenchyme.
Pancrease• It is formed by 2 pancreatic buds(from the endodermal lining of the duodenum),these buds are one dorsal and one ventral.
• The dorsal pancreatic bud grows into the mesoduodenum and forms the tail and the major part of the head of the pancreas.
• the dorsal bud also give the major part of the main pancreatic duct and the accessory pancreatic duct.
• The ventral pancreatic bud accompanies the hepatic diverticulum and forms the remaining part of the head of the pancreas and the uncinate process,
• it also shares in the formation of the main pancreatic duct which drains the rest of the head of the pancreas and the uncinate process.
The endodermal cells of the pancreatic budsform:
The secreting acini that secret the pancreatic enzymes.
Islets of langerhans .
• Pancreatic tissue may invade the pyloric area of stomach.
• Annular Pancreas where a ring of Pancreatic tissue surrounds the whole circumference of duodenum due to abnormal migration of Pancreatic buds
• Cystic fibrosis of the pancreas results in very viscous pancreatic secretion as a result of obstruction of pancreatic duct.
• Develops from the mesoderm in the dorsal mesogastrium which becomes divided into gastro-splenic and lienorenal ligaments.
• It first lobulated but later on the lobed becomes confluent together .The presence of notch in the upper border of the adult spleen is an indication of its earlier lobulation.
Congenital AnomaliesLobulated spleen due to failure of the lobs to become confluent.
• It is cranial half from the foregut , while its caudal part from mid gut ,
• this explained the blood supply from coeliac trunk and superior mesenteric artery.
• It is at first has a dorsal mesentry known as mesoduodenum attaches it to posterior abdominal wall.
• As a result of rotation of the stomach the duodenum comes to the right side and becomes C-shaped structure that loses its mesentry and becomes as retroperitoneal structure except its first one inch.