Embryology Flashcards
How does embryonic folding help development of the GI tract?
Creating a ventral body wall
Creating a tube within a cavity
In which week does the embryo fold?
4th
When the GI tube first develops, what does it look like?
A simple tube with 2 blind ends
1 opening at the umbilicus (midgut continuous with the yolk sac)
Where does the primitive gut tube run from and to?
Stomatodeum (future mouth) to the proctodeum (future anus)
What is the internal lining of the gut tube derived from?
Endoderm
What is the external lining of the gut tube derived from?
Splanchnic mesoderm
Which parts of the GI tract are derivatives of the foregut?
Oesophagus Stomach Pancreas Liver Gall bladder Duodenum (to duodenal papilla)
Which parts of the GI tract are derivates of the midgut?
Duodenum (after duodenal papilla) Jejunum Ileum Caecum Ascending colon Proximal 2/3 of the transverse colon
Which parts of the GI tract are derivatives of the hindgut?
Distal 1/3 of the transverse colon Descending colon Sigmoid colon Rectum Upper anal canal (Internal lining of the bladder and urethra)
What is the blood supply to each embryonic division?
Foregut = coeliac trunk Midgut = superior mesenteric artery Hindgut = inferior mesenteric artery
Describe the blood supply to the duodenum
Proximal to duodenal papilla = gastroduodenal and superior pancreaticoduodenal (from the coeliac trunk)
Distal to duodenal papilla = inferior pancreaticoduodenal (from SMA)
Describe the blood supply to the head of the pancreas
Superior pancreaticoduodenal (coeliac trunk) Inferior pancreaticoduodenal (SMA)
Describe the intraembryonic coelom
Begins as one large cavity
Later subdivided by the future diaphragm into the abdominal and thoracic cavities
Peritoneal membrane lines the abdominal cavity
What is a mesentery?
A double layer of peritoneum suspending the gut tube from the abdominal wall
Allows a conduit for nerve and blood supply
Allow mobility where needed
How is the mesoderm formed?
Condensation of splanchnic mesoderm
Describe the greater omentum
Formed by the dorsal mesentery
Suspended by the greater curve of the stomach
First structure seen when the abdominal cavity is opened
Describe the lesser omentum
Formed by the ventral mesentery
Free edge of the portal triad
The rotation of the stomach contributes to the formation of the …
Greater and lesser sacs
Omenta
How is the lesser sac formed?
The right hand sac pushed posterior to the stomach due to the enlargement of the liver
(Left and right sac formed due to having a dorsal and ventral mesentery at the foregut)
Describe the rotation of the stomach
Around the longitudinal axis and then around the anterior-posterior axis
Puts the vagus nerve anterior and posterior to the stomach
Shifts cardia and pylorus from midline
Contributes to moving lesser sac behind the stomach
Creates the greater omentum
Describe secondary retroperitoneal
Began development invested by peritoneum
Mesentery is lost through fusion at the posterior abdominal wall
When and where does the respiratory diverticulum form?
4th week
Forms in the ventral wall of the foregut at the junction with the gut where the pharyngeal will form
(Lungs and bronchial tree derived from the foregut)
How do the respiratory primordium and the oesophagus separate?
Tracheoesophageal septum grows
How often do we get abnormal positions of the tracheoesophageal septum?
1 in 3000 live births
Describe the development of the stomach
Week 4 a slight dilation of the distal foregut occurs (stomach primordium)
Faster growth at the dorsal border creates the greater curve
Rotation - changes position of mesenteries
What structures are formed in the ventral mesentery?
Liver
Biliary system
Part of the pancreas (uncinate process and inferior head)
What structures are formed in the dorsal mesentery?
Superior head, neck, body and tail of the pancreas
Where is the lesser omentum?
Attaches the liver to the lesser curve of the stomach
What happens in the duodenum in the 5th-6th weeks?
Lumen is obliterated due to the rapid growth of the length and lining
Recanalised by the end of the embryonic period
Why does the duodenum become retroperitoneal?
Rotation of the stomach pushes the duodenum to the right and then against the posterior abdominal wall
(Secondary retroperitoneal)
Which structures are secondary retroperitoneal?
Duodenum (except cap)
Pancreas
What is another name for the lesser sac?
Omental bursa
What is the sign, during embryonic life, of atresia of the oesophagus?
Polyhydramnios
Prevents normal passage of amniotic fluid in the intestinal tract resulting in accumulation of excess fluid in the amniotic sac
How is the spleen suspended?
Splenorenal ligament - connecting spleen to posterior abdominal wall in the area of the left kidney
Gastrosplenic ligament - connects the spleen to the stomach
What forms the roof of the epiploic foramen of Winslow?
Free edge (lesser omentum)
What is the gubernaculum?
Connects the developing testicle to the developing scrotum
Gets shorter and guides the testicle into place
What is the primitive gut a derivative of?
Definitive yolk sac (during folding)
What does the midgut make a loop around?
Superior mesenteric artery (axis)
Why does the midgut need to herniate?
Midgut elongates enormously
Runs out of space because the abdominal cavity doesn’t grow at the same rate
Liver is also growing rapidly
Not enough space
What attaches the midgut to the yolk sac?
Vitelline duct
How are the cranial and caudal limbs defined?
Above and below where the SMA enters
When does physiological herniation take place?
6th week
Where do the intestines herniate?
Into the umbilical cord
Describe midgut rotation
A series of 3 90 degree turns anticlockwise
Starts with cranial limb (above SMA)
1st turn occurs during herniation
2nd and 3rd turns occur on returning to the abdominal cavity
Where do the limbs of the midgut come to lie?
Cranial limb = left side of SMA
Caudal limb = right side of SMA
What happens during the second rotation?
Get the crossing of the 2 limbs
Caudal limb lies in front of the cranial limb
What happens during the third rotation?
Cranial limb derivatives are pushed towards the left
Descent of caecal bud to create the ascending colon
What are the cranial limb derivatives?
Distal duodenum
Jejunum
Proximal ileum
What are the caudal limb derivatives?
Distal ileum Caecum Appendix Ascending colon Proximal 2/3 transverse colon
Describe incomplete rotation of the midgut
Midgut loop makes only one 90 rotation
Left sided colon
Limbs don’t twist on each other - cranial limbs not pushed back in first
Describe reversed rotation
Midgut makes one 90 clockwise rotation
Less crossing
Caudal limb is not anterior to cranial limb
Transverse colon passes posterior to the duodenum
When should the vitelline duct close?
After herniation resolves
Name some consequences of a persistent vitelline duct
Vitelline cyst
Vitelline fistula
Meckel’s diverticulum
Describe a Meckel’s diverticulum
Most common GI anomaly
Small diverticulum of small intestine where the vitelline duct was
May be free or attached to the abdominal wall
Rule of 2s: affects 2% population, 2 feet from ileocaecal, 2 inches long, under 2s, 2:1 male:female
Which types of ectopic tissue can a Meckel’s diverticulum contain?
Gastric
Pancreatic
Where in the GI tract does recanalisation have to occur?
In the oesophagus, bile duct and small intestine
What is the difference between atresia and stenosis?
Atresia = no lumen Stenosis = lumen narrowed
Stenosis in the duodenum is most likely to be a result of …
Incomplete recanalisation
What is pyloric stenosis?
Due to hypertrophy of the smooth muscle (circular) in the region of the pyloric sphincter
Not a recanalisation failure
Common abnormality in infants - projectile vomiting
Where are atresia and stenoses most likely to occur?
Duodenum
What divides the anal canal?
Pectinate line (superior and inferior)
The urorectal septum divides the cloaca into the …
Urogenital sinus
Anorectal canal
What is the proctodeum?
Anal pit
Small depression of ectodermal tissue
Comes up into the body to meet the endoderm tube
What is the perineal body?
Pyramidal fibromuscular mass in the midline of the perineum
At the junction between the urogenital triangle and the anal triangle
Describe the blood supply, innervation and lymphatic drainage of superior anal canal
Inferior mesenteric artery
S2,S3,S4 - pelvic PNS
Lymphatic drainage - internal iliac nodes
Describe the blood supply, lymphatic drainage and innervation of inferior anal canal
Pudendal artery
S2, S3, S4 - pudendal nerve
Lymphatic drainage = superficial inguinal nodes
Describe the epithelia difference between superior and inferior anal canal
Superior = columnar Inferior = stratified squamous
What is the sensation possible above the pectinate line?
Stretch
Poorly localised pain
What sensations can be felt below the pectinate line?
Temperature
Touch
Pain (localised)
Which type of innervation does the parietal peritoneum receive?
Somatic innervation
What is an imperforate anus?
Failure of anal membrane to rupture
What is anal/anorectal agenesis?
Failure of normal formation of anal canal
Absence of primordial tissue
What are mesenteries retained by?
Jejunum Ileum Appendix Transverse colon Sigmoid colon
Where do fused mesenteries occur?
Duodenum
Ascending and descending colon
Rectum
What structures are formed from the dorsal mesentery?
Greater omentum Gastrosplenic ligament Splenorenal ligament Mesocolon Mesentery proper
What does the ventral mesentery become?
Lesser omentum
Falciform ligament
What is the innervation to the midgut?
PNS - vagus nerve
SNS - superior mesenteric ganglion and plexus
What is the innervation to the hindgut?
PNS - pelvic (S2,3,4)
SNS - inferior mesenteric ganglion and plexus
