Embryology of GI Tract

General Outline of GI organ development GI organs develop mainly from: Foregut ( which is supplied by celiac artery) Midgut (which is supplied by superior mesenteric artery) Hingut ( which is supplied by inferior mesenteric artery)

General Outline of GI organ development Foregut Midgut Hindgut ·         Lungs ·         2nd part of duodenum ·         Last 1/3 of transverse colon ·         Stomach ·         Jejunum ·         Descending colon ·         Part of duodenum ·         Ileum ·         Rectum ·         Gall bladder ·         Ascending Colon   ·         Liver ·         2/3 of transverse colon ·         Pancreas

Development of the esophagus Diverticulum coming off the foregut is called the lung bud, which forms trachea and tree of bronchial lungs Epithelial lining of lungs is from endoderm Smooth muscle and connective tissue is derived from visceral mesoderm Tracheoesophageal septum is derived from splanchnic mesoderm and divides the trachea and esophagus

Congenital defects in esophageal development Problem may arise is when the tracheoesophageal septum doesn’t divide the esophagus from the trachea completely (tracheoesophageal fistula, esophageal stenosis)  the baby looks normal but vomits when food or drink is given  this is because there is a dead end  surgery can fix this problem nowadays

Development of the stomach Stomach is made from left & right sides which turns so that: Left side becomes anterior and the left vagus nerve can be found here Right side becomes posterior and the right vagus nerve can be found here Left side of stomach grows faster than the right side and this results in greater curvature being formed on left side Right side gets the lesser curvature because it grows slower than the left

Development of the stomach 1 2 3 Stages of the development of the stomach

Congenital defects of the stomach Pyloric stenosis: narrowing of the pyloric antrum due to smooth muscle hypertrophy of the pyloric sphincter

Development of the liver liver diverticulum comes off right below the stomach Liver starts to grow into the lower part of the septum transversum  this part becomes thin and becomes the ventral mesentery Components of Liver: Endothelial cells (form hepatocytes = liver cells) Connective tissue from the splanchnic mesoderm which surrounds liver cells Septum transversum contributes to the visceral peritoneum (which is present all over surface of liver), Kupffer cells (macrophages that help break down stuff in liver), and hemopoietic cells (cells that form blood in the fetus in the first seven months; this function is later taken over by the bone marrow)

Development of the liver-cont. Ventral mesentery divides into: Falciform ligament (derived from septum transversum) Lesser omentum (derived from septum transversum) Hepatic duct – connection between liver cells and gut Hepatic duct grows a bit more to form the cystic duct Gall bladder connects cystic duct to bile duct So, bile flows from the hepatic duct  gall bladder  secreted into bile duct Gall bladder: formed from endothelial lining and splanchnic mesoderm covering – NO SEPTUM TRANSVERSUM (sept. transversum only for liver)

Development of the liver

Development of the pancreas Dorsal and ventral pancreatic buds present (both have a duct) Ventral pancreatic bud rotates towards dorsal bud Dorsal pancreatic duct disappears and ventral pancreatic duct remains Ventral pancreatic duct gives rise to head of pancreas and uncinate process Dorsal pancreatic duct gives rise to body and tail of pancreas So, there are two openings into the duodenum from the pancreas

Development of the pancreas 1 2 3 Stages of the development of the pancreas

1 2 3 4 5 6 7 8 9

Development of the Midgut Primitive gut starts elongating First intestinal loop formed – this is the primary intestinal loop That part of the primary intestinal loop that grows faster gives rise to the jejunum and ileum; this is where the vitelline duct connects the yolk sac to the midgut Primary intestinal loop herniates out into the umbilical cord. As it does this, it rotates 90 about the axis of the superior mesenteric artery (loops are lengthening as this happens). At 70 days, the loop is pulled back into the abdominal cavity. As this happens, the loop rotates 180 again around the superior mesenteric artery. So, the primary intestinal rotates as total of 270 around the superior mesenteric artery. This results in the cecum ending up right under the liver. The cecum now starts to drop toward the right iliac fossa and pulls the large intestine with it. The jejunum and ileum both end up on the left side.

Development of the Midgut Primary intestinal loop Physiological umbilical hernia Reduction of the hernia

Congenital defects in Midgut development Omphalocele: when primary intestinal loop of midgut doesn’t come back into the abdominal cavity; baby is born with guts hanging out; it is a congenital hernia of the umbilicus. Meckel’s Diverticulum: outpocket where the vitelline duct used to be. It is a congenital sac or blind pouch sometimes found in the lower portion of the ileum. It represents the persistent proximal end of the yolk stalk. Sometimes it is continued to the umbilicus as a cord or as a tube forming a fistulous opening at the umbilicus. Strangulation may cause intestinal obstruction. The vitelline duct ends up remaining between the yolk sac and the ileum. Baby starts defecating through the umbilical cord. Double Intestine: two openings with the septum in-between. This may make the region susceptible to bacterial growth (infections) or the formation of a cyst.

Congenital defects in Midgut development 4- abnormal intestinal rotation: Volvulus: abnormal twisting of the mistentry Malrotation Non-rotation

                                     

Development of the hindgut 26 days: After formation of the tail fold, the allantois and hind gut open into a common chamber the cloca. The cloacal membrane separates cloaca from the proctodaeum. The allantois appears at about 16 days as a small diverticulum projecting from the caudal end of the yolk sac into the connecting stalk The urorectal septum separates the hindgut from the allantois. It grows towards the cloacal membrane. It is derived from mesoderm at the junction between the connecting stalk and yolk sac. allantois

Development of the hindgut

Development of hindgut During the 7th week the cloacal membrane disappears, exposing a ventral urogenital sinus opening and a dorsal anal opening. The tip of the urorectal septum, separating the two openings forms the perineal body. The urorectal septum grows towards the cloacal membrane but does not fuse with it. It is derived from mesoderm at the junction between the connecting stalk and yolk sac. Urogenital sinus Anal opening urorectal septum

Development of the hindgut The Anal Canal •At the end of the 8th week, after rupture of the cloacal membrane,proliferation of ectoderm occludes the anal opening. •During the 9th week the opening isrecanalized. •Thus the terminal part of the anal canal is ectodermal in origin andsupplied by the inferior rectal artery. •The junction between ectoderm and endoderm is the pectinate line.

Development of the hindgut Aganglionic Megacolon – Hirschsprung Disease Due to congenital absence of parasympathetic ganglia in the colon. This is a neural crest migration defect. It may be due to a genetic mutation of the RETgene, a tyrosine kinase receptor involved in neral crest cell migration. It varies in extent – 80% involve sigmoid colon and rectum; 3% involve the whole colon.