Lecture 11: Development of the Digestive Sytem Flashcards
The epithelium of the digestive system is made up of what kind of ‘derm; what are the exceptions?
Endoderm
*except mouth (1st arch portion) and anal canal = ECTODERM*
The smooth muscle, vasculature, CT, and any cells arising from monocytes are made up of what kind of ‘derm?
Mesoderm: splanchnic mesoderm
The autonomic ganglion, post-ganglionic neurons, in the digestive tract arise from which cell type?
Neural Crest
What are the boundaries of the foregut, midgut, and hindgut?
- If above the yolk sac, then it’s foregut
- Same level as yolksac, is migut
- Below yolksac, is hindgut
What are the derivatives of the Foregut; arterial suppky?
- Pharynx
- Respiratory system
- Esophagus
- Stomach
- Liver and Pancreas
- Biliary apparatus
- Proximal duodenum
*CELIAC TRUNK*
What are the derivatives of the Midgut; arterial supply?
- Small intestine (duodenum)
- Cecum and veriform appendix
- Ascending colon
- Right 1/2 of transverse colon
*SUPERIOR MESENTERIC ARTERY*
What are the derivative of the hindgut; arterial supply?
- Left 1/2 of transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Superior anal canal
- Epithelium of urinary bladder/urethra
*INFERIOR MESENTERIC ARTERY*
What’s a Tracheoesophageal (TE) Fistula; which week does this occur?
- Abnormal migration of the tracheoesophageal folds and formation of the septum = abnormal passage between trachea and esophagus
- Commonly associated w/ esophageal atresia.
- Occurs during WEEK 5
What are the presenting signs and symptoms of TE fistula?
- Prenatally: polyhydramnios
- Birth: coughing, gagging, cyanosis, vomiting, voluminous oral secretions, possible respiratory distress
What is shown in this picture, and what can this tell you?
- This image shows coiling of a NG tube within the esophagus.
- This indicates an infant has a TE Fistula
What week does the stomach begin to rotate and explain what occurs.
- Week 4
- At the start the ventral border is to the right and dorsal border is to the left
- Dorsal wall grows much faster and forms the greater curvature, while the ventral wall forms the lesser curvature
- Stomach will rotate 90° clockwise about a longitduinal axis, the left side is now ventral, while the right is now dorsal.
What occurs to the left and right vagal nerves during developmental stomach rotation?
- Left vagal nerve becomes anterior
- Right vagal nerve becomes posterior
When the stomach does 90° of clockwise rotation about the longitudinal axis, what space forms?
The lesser sac (omental bursa)
What is Hypertrophic Pyloric Stenosis; common presenting sx?
- Narrowing of pyloric lumen obstructing food passage due tp hypertrophy of the muscularis externa in this region.
- Immediate post-feeding vomiting that is nonbilous and forceful (projectile vomiting), fewer and smaller stools, failure to gain weight or weight loss
Explain the formation of the liver and biliary system.
- All form from a single diverticulum of gut endoderm
- Endoderm differentitates into hepatocytes, bile ducts, and hepatic ducts
What happens as the gallbladder begins to elongate; what forms?
Forms the cystic duct and where the cystic and hepatic ducts merge forms the common bile duct.
Explain formation of the pancreas
- Develops from 2 distinct buds, dorsal and ventral primordia
- Dorsal forms the body and tail
- Ventral forms the head and uncinate process
- As duodenum rotates to the right, ventral pancreatic bud is carried dorsally
Where do the 2 pancreatic ducts arise from?
- Main pancreatic duct arises from ventral primordia
- Towards tail pancreatic duct comes from the dorsal primordia
What is Annular pancreas; when would bilious vomit be seen?
- Bifed ventral pancreatic bud will grow out and fuse around the duodenum, causing an obstruction (narrowing).
- If annulus develops inferior to bile duct, child will have sx’s similar to pyloric stenosis, EXCEPT, the vomit will be bilious
- If obstruction is superior to bile duct, vomit will be non-bilious
Any accessory pancreatic ducts arise from which part?
Dorsal pancreatic bud
Explain the rotation of the midgut; how much rotation occurs?
- Due to limited size of the abdomen, lengthening midgut herniates out into umbilicus at 6 weeks, rotating 90°.
- Comes back in during week 10 and rotates 180°
- Total of 270° rotation
What do the cranial and caudal ends of the gut form at the end of midgut rotation?
- Cranial forms the small intestine
- Caudal forms the large intestine
What is Omphalocele; occurs when?
- Weeks 6-10
- Herniated bowel does not fully retract back into the abdomen, will be covered w/ an amnion and peritineum membrane
What is Gastroschisis; occurs when?
- Week 4
- Herniation of abdominal contents due to muscular defect in anterior abdominal wall.
What is embryologic mechanism of Hirschsprung Disease (aka megacolon)?
- Most common neonatal obstruction
- Failure of NC to migrate
- Lack autonomic ganglion cells (from NC) distal to dilated segment: area cannot relax is always constricted
- Enlarged section is normal
What is nonrotation of the midgut?
- Completes first 90° CCW rotation but does not do remaining 180° CCW rotation
- Ends up with left-sided colon and rightsided small intestines
- Pt’s may have obstructions
What is reversed rotation of the midgut?
- Completes initial 90° CCW but then does a 180° CW rotation
- Transverse colon ends up posterior to the duodenum, normally the duodenum is posterior to the transverse colon
Explain formation of the definitive gut lumen
- Start with hollow gut tube (week 6),
- Gut tube is then occluded by endodermal proliferation
- Apoptosis (recanalization) occurs and by week 8 you have now formed the definitive hollow gut.
What 2 defects can occur during formation of the definitive gut lumen?
1) Duplication - pocket forms in wall of the gut tube
2) Incomplete recanalization = stenosis typically seen in 3rd part of duodenum
What is Meckel’s Diverticulum, can lead to?
- Abnormality of the vitelline duct (normally regresses)
- Projection from the ilium to the abdominal wall
- Can lead to intestinal obstruction, GI bleeding, bowel sepsis
What is the rule of 2’s for Meckel’s Diverticulum?
- 2% of population
- Found 2 feet from ileocecal junction
- Usually about 2 inches long
- 2 types of ectopic tissues: gastric and pancreatic
- 2:1 ratio of male to female
What is a Omphalomesenteric fistula?
- Persistent attachment of vitelline duct to umbilicu, fistula causes poop to leave out of bellybutton
What ‘derm layers are the cloacal membrane and urorectal septum composed of?
Cloacal membrane = Endoderm and Ectoderm
Urorectal septum = mesoderm
What does the urorectal septum do; what happens to the cloacal membrane?
- Migrates down and partitions cloaca into a dorsal anorectal canal and a ventral urogenital sinus.
- Cloacal membrane ruptures thereby opening both the UG sinus and anal canal to the exterior
How do parts of the anal canal differ; what separates the anal canal?
- The anal canal is seprated into 2 distinct parts, one from the hindgut endoderm and the anal pit from ectoderm, separated by the pectineal line
- Each part has a separate blood and nerve supply
What is the neurovasculature of the hindgut endoderm in anal canal?
- Nerves: Autonomic
- Blood vessels: Inferior mesenteric/superior rectal a.
- Lymphatics: Inferior mesenteric lymph nodes
What is the neurovasculature of the hindgut ectoderm of the anal canal?
- Nerves: Somatic nerve –> Pudendal. Painful hemorrhoids
- Blood vessels: Iliac –> middle and inferior rectal
- Lymphatics: Meets at pectineal line. Superior inguinal lymph nodes
What is underlying cause of these Anorectal Malformations in figures A and B?
A) Proctodeum did not form correctly
B) Proctodeum did not migrate at all
What is the underlying cause of the Anorectal Malformations seen in figures D and H?
Urorectal septum abnormally divided the tract
Differentiate between the pleuropericardial membrane, the pleuroperitoneal membrane and the septum transversum.
- Pleuropericardial folds migrate to midline and are separated from pleural canals by the pleuropericardial folds
- Septum transversum meets with the pleuroperitoneal membranes and fuses.
- Tissue from the body wall is then pulled into the diaphragm
*left fuses more slowly*
How does the diaphragm form?
- Septum transversum with the central tendon
- Dorsal mesentary becomes the right and left crus of the diaphragm
- Peripheral musculature gets pulled off body wall
What is shown here and how does this occur, what side does it usually occur on?
- Congenital diaphragmatic hernia
- Defective formation and/or fusion of the pleuroperitoneal membrane, large opening in posterolateral diaphragm (foramen of Bochdalek)
- 85-90% occur on the left side
What is Mesentery?
- Double layer of serous membrane from splanchinc mesoderm
- Allow BV’s, nerves, and lymphatics to get to an organ and then back to the body wall.
- Has a parietal and visceral layer
In the Abdominal cavity and liver what does the: dorsal mesogastrium, ventral mesogastrium, and ventral mesentery form?
Dorsal mesogastrium - greater omentum (around stomach)
Ventral mesogastrium - lesser omentum (stomach to liver)
Ventral mesentery - falciform ligament (liver to abdominal wall)
What are the mesenteries of the mid/hindgut; are they from ventral or dorsal?
- Mesoduodenum - mesentery around duodenum
- Mesentery proper - containing jejunum and ileum
- Mesocolon - around transverse colon and sigmoid colon
- Mesorectum - mesentery around the rectum
*ALL from DORSAL*
What is the axis of rotation of the midgut?
Superior Mesenteric Artery!!!
What are the divisions of the Dorsal mesogastrium?
- Splenorenal (leinorenal) ligament
- Gastrolienal ligament
- Greater omentum
How is the greater omentum formed?
- As the stomach migrates and rotates, the dorsal mesogastrium is pulled down and rotated
- You now have fusion of 2 double layers (4 total) of splanchic mesoderm, creating the greater omentum
What does fusion of mesenteries with body walls result in?
Secondary Retroperitoneal structures
Which structures become secondary retropeitoneal?
- Mesogastrium dorsal to spleen (part holding pancreas)
- Mesoduodeum
- Ascending mesocolon
- Descending mesocolon
What does it mean for a structure to be intraperitoneal; what are the intraperitoneal structures?
- Anything surrounded by mesentery
- Stomach
- Tail of pancrea
- 1st part of duodenum
- Jejunum, ileum
- Transverse Colon
- Sigmoid Colon
We have a mneomic for retroperitoneal structures, using it, indicate which structures are primary and secondarily retroperitoneal?
SADPUCKER
S - suprarenal glands (primary)
A - aorta and IVC (primary)
D - duodenum 2nd-4th part (secondary)
P - pancreas, except tail (primary)
U - ureters (primary)
C - colon, ascending/descending (secondary)
K - kidneys (primary)
E - esophagus (primary)
R - rectum (primary)
