Development of GI System Flashcards
When do we start to see the primordia for the GI tract?
During 4th week embryonic folding pulls the endoderm of yolk sac into cranial and caudal ends of embryo forming the foregut, midgut, and hindgut with narrow opening to yolk sac forming vitelline duct
What closes off both end of the gut tube?
buccopharyngeal (cranial) cloacal membranes (caudal) endoderm and ectoderm in direct contact
What does endoderm contribute to?
epithelial lining and associated glands of GI tract
What does splanchnic mesoderm contribute to?
CT, vasculature, and smooth muscle walls
What does ectoderm contribute to?
enteric ganglia, nerves, glia of GI tract, epithelium of lower 1/3 of anus
When does the GI gastroepithelium mature?
8-10 weeks, early peristaltic contractions week 10
How is the dorsal mesentery formed?
End of 4th week gut tube caudal to developing diaphragm suspended from post abdominal wall by dorsal mesentery which extends from lower esophagus to caudal end of hindgut
What are adult derivatives of the dorsal mesentery?
greater omentum (gastrosplenic, gastrocolic, and splenorenal ligaments), small intestinal mesentery, transverse mesocolon, and sigmoid mesocolon
How is the ventral mesentery formed?
develops between ventral body wall and foregut running from septum transversum and stomach to umbilicus
What are adult derivatives of the ventral mesentery?
less omentum (hepatoduodenal and hepatogastric ligaments), falciform ligament of liver, coronary ligament, triangular ligament
What makes something a retroperitoneal organ?
organs excluded from peritoneal cavity, initially suspended within cavity but later become fused with body wall ex. ascending and descending colon, duo, most of pancreas
What happens in the first 90 degree rotation of the stomach?
Ventral border moves to the right and dorsal border moves to left
Left stomach surface becomes ventral and right dorsal
What is the dorsal mesogastrium?
Portion of dorsal mesentery between body wall and stomach
What happens to the dorsal mesogastrium as the stomach enlarges?
enlarges forming a large draping double fold of mesentery –> greater omentum
Rotation separates abdominal cavity into lesser sac behind the stomach and greater sac ant and continuous with greater abdominal cavity
What is hypertrophic pyloric stenosis? Causes? Symptoms?
Narrowing of pyloric lumen obstructing food passage
Inability of sphincter to relax due to faulty NCC migration so ganglion cells of enteric not properly populated -> hypertrophy of muscularis externa
Projectile non-bilious vom after feeding, fewer and smaller stools, failure to gain weight
What does the liver begin as?
Diverticulum from gut endoderm that grows into septum transversum
What does the endoderm differentiate into for the liver?
hepatocytes, bile ducts, and hepatic ducts
What does the splanchnic mesoderm differentiate into for the liver?
stromal cells, Kupffer, and stellate cells
What develops into the exocrine and endocrine portions of the pancreas?
pancreatic buds
What happens during the 5th week in pancreatic development? What do the dorsal and ventral portions become?
Ventral pancreas migrates around post and fuses with dorsal pancreas
Dorsal= head, body, tail
Ventral= uncinate process
Dorsal pancreatic duct connection to duo lost as it reconnects to ventral pancreatic duct (can be retained as accessory)
What are anomalies in pancreatic development?
pancreatic duct, pancreas divisum (two totally separate ducts, prone to pancreatitis) annular pancreas (duo obstruction or stenosis, bilious vomiting if inf to bile duct)
Where is the bile ducts and gall bladder formed from?
formed by outgrowths from cystic endoderm
How does the bile duct open?
Canalization
What is biliary atresia?
fetal and perinatal forms, obliteration of extrahepatic or intrahepatic ducts –> ducts replaced by fibrotic tissue –> progressive neonatal jaundice with onset after birth, white stools, and dark urine, treat with liver transplant or else 12-19 mth
When does the spleen develop and how?
4th week
mesenchymal condensation develops in dorsal mesogastrum, derived from mesoderm and eventually populated by myeloid lymphoid cells
What are the 3 rotations that happen during midgut development?
- 90 degree counterclockwise rotation (day 50) bringing cecum and future ascending colon back into left side of abdominal cavity
- 180 counterclockwise rotation (day 70) of gut loop as it continues reentering abdomen bringing cecum to URQ and ascending colon ant to duo
- Last rotation day 73 bringing midgut into cavity and day 77 cecum descends to LRQ carrying ascending colon with it so that it is along the right side of abdomen and transverse colon overlies duo and pancreas
What is omphalocele? How does this happen?
herniation of gut into umbilicus remains and still covered by peritoneum, increased risk with trisomy 13 and 18
Lateral body folding and fusion is abnormal creating a wall weakness allowing bowel to herniate
Connective tissue of skin and hypaxial musculature of body wall do not form normally creating wall weakness
What is gastroschsis?
herniation of abdominal contents through abdominal wall and into exterior, no peritoneal covering
What is Meckel’s diverticulum?
failure of yolk stalk(vitelline duct) connection to midgut to regress so midgut connected to umbilicus abdominal wall swelling, intestinal obstruction, bowel sepsis, and GI bleeding
What is the rules of 2’s in terms of Meckel’s diverticulum?
2% incidence, 2xs more common in males, 2% have medical symptoms, 2 ft proximal to terminal ileum, 2 inches long, 2 yrs old
What is volvulus?
rolled up, twisted intestines –> acute abdominal pain, vomiting, and GI bleeding, increased risk with gut rotation anomalies
What happens when there is non-rotation of the midgut loop?
complete first 90 degree counterclockwise rotation but does not do 180 rotation so left sided colon and right sided SI
What happens when there is reverse gut rotation?
completes initial 90 degree but then rotates 180 clockwise instead of counter transvers colon post to duo risk of ischemia and stenosis of colon
What is intestinal stenosis and atresia?
during 5-6th month lumen of intestines become nearly obliterated due to endodermal proliferation and villi formation
Duodenal stenosis= partial occlusion due to ineffective recanalization usually involves horizontal and or ascending parts
Duodenal atresia= complete occlusion of the lumen remains
What does the urorectal septum partition the cloaca into?
dorsal anorectal canal and ventral urogenital sinus
Septum develops fork-like extensions of lateral cloacal walls that grow toward one another dividing the cloaca into urogenital sinus and anal canal
What opens the urogenital sinus and anal canal to the exterior?
Rupture of the cloacal membrane
How does the anal rectal lumen?
Apoptotic death of epithelial anal plug
What does the mesenchyme surrounding the anal canal proliferate into?
anal pit with ectodermal lined walls
What germ layer makes up the superior 2/3’s of the anal canal? Inferior?
Sup= endodermal Inf= ectoderrmal
What does the epithelium of the superior 2/3’s of the anal canal? Inferior?
Sup= BV supply and innervation to hindgut
Inf= BV supply and innervation of anal pit
What does the pectineal line divide?
Origin of the hindgut and anal pit
How does an imperforated anus form?
persistent anal membrane, types based on relative location to levator ani and pelvic landmarks
What is Hirschbrung’s disease?
Congenital aganglionic megacolon
Absence of ganglionic plexus dues to absence of NCC
Lack of peristalsis (colon fails to relax)
Increase wall thickens due to hypertrophy in intestines proximal to agnaglionic segment
Abnormal colonic dilation or distension (megacolon)
What are some urorectal septum developmental defects?
persistent anal membrane anal atresia anoperitenel fistula rectovaginal fistula rectrourehtral fistula rectovesical fistula
