13 - GI Embryology Flashcards
Primitive Gut
Forms during 4th week of development
Extends from Buccopharyngeal Membrane (rostral) to Cloacal Membrane (caudal)
Why does the gut fold?
Dorsal surface grows faster than the ventral surface.
This causes the Buccopharyngeal Membrane and Cloacal Membrane to move towards each other.
Cardiogenic Mesenchyme and Septum Transversum
Originally rostral
Folding brings it caudally, ending up caudal to the buccopharyngeal membrane
At this point, the primitive gut is sort of recognizable.
After folding,
A portion of the yolk sac is incorporated into the embryo as bowel, but the midgut remains open
Cephalocaudal and Lateral folding
Occur simultaneously.
Meeting and fusion of cranial, lateral and caudal edges of the embryo create the primordial foregut and hindgut.
Midgut remains open until
Week 6.
It connects to the yolk sac via vitelline duct.
Buccopharyngeal membrane opens at
4 weeks
Cloacal membrane opens at
7 weeks
What delimits the bowel?
Flexion of the embryo
After the gut forms - Attached to the body wall how?
Via dorsal and ventral mesentaries. Ventral mesentary is lost except in the region of the liver.
Vitelline duct remains in the umbilical cord.
Septum Transversum
Partially separates thoracic and abdominal cavities
Septum Transversum - Superior Portion
Primitive pericardial cavity
Septum Transversum - Inferior Portion
Future peritoneal cavity
Communication between pericardial and peritoneal cavities
Pericardioperitoneal canals
Pericardioperitoneal canals are closed by
Formation of the pleuroperitoneal membranes
Pleuroperitoneal membranes
Close pericardioperitoneal canals
Contribute muscle to the definitive diaphragm
Definitive Diaphragm
Composite Structure: Septum Transversum Pleuroperitoneal Membranes Paraxial Mesoderm Esophageal Mesenchyme
Dorsal mesentary
Thins to allow the cut to be flexibly suspended
Endoderm
Lining of the gut
Specified (via a series of regionally specific transcription factors) before gut tube is complete
Boundaries between regions
Plastic
Depend on interactions between endoderm & mesoderm
Language: Paracrine secretion of growth factors
Boundaries of GI Regions
Begin with Sonic HedgeHog expression in posterior endoderm, which spreads to the whole gut.
Induces a series of Hox genes in the mesoderm
Mesoderm then influences epithelial differentiation.
Wnt Signaling - Intestinal Epithelium
WNT = Intestine
No WNT = Stomach
Mesenchyme of stomach
Expresses Barx1
Secretes WNT inhibitors (sFRP1, 2)
Mesenchyme of intestine
Secretes BMP4
Induces mesenchyme anterior to it to express SOX9 + NKX-2
Becomes pyloric sphincter
Foregut
Part of the bowel from the stomach to the biliary apparatus, all are supplied by the celiac artery.
Foregut derivatives
Pharynx and its derivatives Lower respiratory tract Esophagus Stomach Duodenum proximal to the ampulla of Vater Liver Biliary Apparatus Pancreas
Esophagus - Development
Elongates rapidly
Grows faster at the cranial end
Epithelium obliterates lumen
Week 8 - Esophagus recanalized by apoptosis.
Failure at this step causes polyhydramnios, esophageal atresia or tracheo-esophageal fistula
Stomach - Development
Does not descend. Arises from region just caudal to septum transversum.
Then stomach enlarges and rotates.
Polyhydramnios
Pregnant woman’s abdoman extends
Heart sounds faint
Clue to esophagus not being recanalised
Does not prevent development. Baby looks normal.
Baby aspirates upon first feeding. Lipid pneumonia. BAD NEWS
Stomach rotates
90 degrees Clockwise
Creates the lesser sac
Facilitated by vacuolization and apoptosis
Greater curvature of the stomach
Previously dorsal, then becomes right (false?). Grows faster than lesser curvature.
Lesser Sac
Dorsal mesograstrium moves to the left.
Ventral mesogastrium attaches to liver and body wall.
Inferior recess forms the greater omentum.
Layers fuse to obliterate the lesser sac.
From the duodenum arises
Liver
Biliary System
Pancreas
Ventral pancreatic bud
Rotates around and joins the dorsal pancreatic bud
They fuse to form the pancreas
Hepatic diverticulum
Grows from the duodenum into the ventral mesentery (Week 4)
Divides into cranial and caudal buds
Cranial bud grows faster (becomes hepatic parenchyma)
Hematopoietic colonists arrive ~ week 6
Caudal bud gives rise to the biliary system.
Bare Area of the Liver
Liver presses against septum transversum, eliminating ventral mesentary on that part.
Ligaments attached to the liver
Falciform ligament
Hepatogastric ligament
Hepatoduodenal ligament
Ventral mesogastrium
Supports liver and stomach
Pancreas is shaped by
Rotation of the stomach
Cardiogenic mesenchyme induces ventral pancreatic bud (home of the main duct) to form.
Notochord induces dorsal pancreatic bud (most of the pancreas) to form.
Rotation combines the two.
Aberrant rotation can lead to
Annular pancreas
Annular pancreas
Ring around the duodenum
Not a problem as a fetus
As the duodenum grows, the pancreas gets cut off!!
It’s like pyloric stenosis but PLUS DIGESTING YOURSELF!!!!!!
Midgut
All are supplied by the superior mesenteric artery
Grows rapidly
Herniates into the umbilical cord
Rotates around an axis of the SMA 90 degrees
Herniation comes back in
Rotates around the SMA 180 degrees!
Derivatives of the midgut
Small intestine (except proximal duodenum)
Cecum
Appendix
Ascending colon
Right 1/2 to 2/3 of the proximal transverse colon.
Rotation of the midgut
Cranial and caudal loop form
Cranial growth»_space;> caudal growth
Apex of the loop is the vitelline duct
Cranial loop moves to the right, caudal loop moves to the left (90 degrees counterclockwise)
Reduction of midgut hernia
180 degrees further rotation
Brings cecum to the right, moves down, becomes secondarily retroperitoneal.
Loops of bowel
Fuse with the body wall
Become secondarily retroperitoneal
Retroperitoneal viscera
Thoracic esophagus
Rectum
Secondarily retroperitoneal viscera
Ascending colon Descending colon Pancreas Duodenum Part of the transverse colon?
Volvulus
Serious complication of excessive flexibility.
Twists around itself, cuts off blood, infarcts.
Meckel’s Diverticulum
Bad news
Diverticulum near vitelline duct
Pluripotent cells. Can lead to inappropriately-located tissue. DIGEST YOURSELF GURL
Can lead to omphallomesenteric fistulas, cysts or ligaments.
Vitelline Duct
MUST be obliterated.
Hindgut
Supplied by the inferior mesenteric artery
Originally a cloaca
Partitioned to form rectum and urogenital sinus (forming bladder, ureters & urethra)
Derivatives of the hindgut
Left 1/3 to 1/2 of the distal transverse colon
Descending colon
Sigmoid colon
Rectum
Superior part of the anal canal
Epithelium of urinary bladder and most of urethra.
Urorectal septum
Divides cloaca into rectum and urogenital sinus.
Anal Pit
Recanalization of cloacal membrane
Pectinate line
Where anal pit used to be.
Proctodeum
Forms lower 1/3 of the rectum
The upper 2/3 are formed by the hindgut.
Anorectal malformations
Fistula between rectum and scrotum
Rectal atresia
Fistula between rectum and urethra
Fistula between rectum and vagina
