Embryology 2 - Midgut and Hindgut Flashcards
What are the derivatives of the cranial limb of the midgut loop?
Distal duodenum, jejunum, proximal ileum
Everything else in the midgut comes from the caudal limb
Explain what is meant by “physiological herniation”?
The intestines herniate out into the umbilical cord as during growth they become too large to fit inside the abdominal cavity. It is a normal physiological process and resolves later in development.
What happens to the midgut during physiological herniation?
Rotates 90* anti-clockwise 3 times:
1st rotation puts them on horizontal plane
2nd rotation puts caudal limb on top of cranial
3rd rotation puts small bowel posterior to large bowel
How is physiological herniation resolved?
After 3rd rotation, loop of intestine moves back into the abdomen and the caecal bud begins to descend to form the ascending colon.
What error during rotation would result in a left-sided colon?
One 90* turn only
What error during rotation would result in the transverse colon sitting posteriorly to duodenum rather than anteriorly?
Reversed rotation - i.e 1x 90* rotation clockwise instead of 3 anti-clockwise
What happens if the caecal bud fails to descend?
Absence of ascending colon
List 3 abnormalities that result from persistence of the Vitelline duct
Vitelline cyst
Vitelline fistula
Meckel’s diverticulum
What is a Vitelline cyst?
Vitelline duct forms fibrous strands either side of a pouch of fluid
What is a Vitelline fistula?
Direct communication between the umbilicus and intestinal tract - faecal matter passes out of umbilicus
What is a Meckel’s diverticulum?
Out-pouching of the ileal wall as a result of incomplete obliteration of the Vitelline duct
Explain the rule of 2s with regard to Meckel’s diverticulum
2 feet from ileocaecal junction 2 inches long 2% of population 2:1 male:female ratio Usually presents in under 2s
Explain the need for recanalisation during development
In some gut structures, cell growth becomes so rapid that the lumen becomes obliterated –> must then be recanalised
Give 3 examples of structures that must be recanalised
Oesophagus
Bile duct
Small intestine
What happens if recanalisation is unsuccessful?
Atresia - no orifice –> blind-ended tube
Stenosis if only partially unsuccessful
Apart from recanalisation failure, why else might you get atresia?
Vascular problems, usually caused by malrotation/volvulus
Where is atresia most likely to be due to vascular problems?
Lower duodenum onwards
What is gastroschisis?
What does it result in clinically?
Failure of the abdominal wall to close during folding of the embryo
Results in gut tube and derivatives outside of body
What is omphalocoele (aka exomphalos)?
Persistence of physiological herniation
How does omphalocoele differ from an umbilical hernia?
In an umbilical hernia, the protrusion is covered with skin and subcutaneous tissues, whereas in omphalocoele, the protrusion is covered with amnion only.
Furthermore, umbilical hernias occurred after physiological herniation has successfully resolved.
What divides the anal canal into superior and inferior?
The pectinate line
Why is there a clear distinction between the regions of the anal canal above and below the pectinate line?
The regions are derived from different embryonic tissues
How is the cloaca divided into the urogenital sinus and anorectal canal?
A wedge of mesoderm grows down into it, forming a separation –> becomes the urorectal septum
Where is the perineal body found?
At the point at which the urorectal septum meets the cloacal membrane
What must occur in order for the anal canal to fully form?
The cloacal membrane must rupture at the proctodeum
What abnormality results if the cloacal membrane fails to rupture?
Imperforate anus
Which midgut/hindgut structures retain their mesenteries, i.e. are intraperitoneal?
Jejunum/ileum
Appendix
Transverse colon
Sigmoid colon
Which midgut/hindgut structures are secondarily retroperitoneal as they lose their mesentery?
Duodenum
Ascending and descending colons
Rectum
What are the 2 derivatives of the ventral mesentery?
Lesser omentum
Falciform ligament
What is the innervation of the midgut vs the hindgut?
Midgut - parasympathetic = vagus nerve
- sympathetic = superior mesenteric ganglion + plexus
Hindgut - parasympathetic = pelvic nerves
- sympathetic = inferior mesenteric ganglion + plexus
Briefly describe what happens as the midgut elongates
Elongates enormously - runs out of space
Therefore forms a loop with 2 limbs - cranial and caudal
SMA points down axis of loop
Loop connected to yolk sac via Vitelline duct
During resolution of physiological herniation, what re-enters the abdominal cavity first?
The jejunum
Apart from recanalisation failure and vascular accident, why else may there be oesophageal atresia?
Tracheo-oesophageal septum may be posteriorly displaced by chance
Septum may be anteriorly displaced due to pressure on foregut dorsal wall
What sign may be observed during pregnancy that indicates oesophageal atresia?
Accumulation of amniotic fluid
