Embryology of GI Tract

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Embryology of GI Tract
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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)

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General Outline of GI organ development
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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

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Congenital defects in esophageal development

• Problem may arise is when the tracheoesophageal
  septum doesnt 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

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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

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Development of the stomach
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Stages of the development of the stomach
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Congenital defects of the stomach

• Pyloric stenosis narrowing of the pyloric antrum
  due to smooth muscle hypertrophy of the pyloric
  sphincter

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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)

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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)

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Development of the liver
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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

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Development of the pancreas
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Stages of the development of the pancreas
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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.

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Development of the Midgut
Primary intestinal loop
Physiological umbilical hernia
Reduction of the hernia
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Congenital defects in Midgut development

1. Omphalocele when primary intestinal loop of
   midgut doesnt come back into the abdominal
   cavity baby is born with guts hanging out it is
   a congenital hernia of the umbilicus.
2. Meckels 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.
3. 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.

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Congenital defects in Midgut development

• 4- abnormal intestinal rotation
• Volvulus abnormal twisting of the mistentry
• Malrotation
• Non-rotation

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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
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Development of the hindgut
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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
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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.

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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.