Embryology 2 Flashcards
What is the primary intestinal loop
A loop of the midgut that forms because the midgut elongates massively and runs out of space (due to large size of developing liver)
Features of the primary intestinal loop
Has the superior mesenteric artery at its axis
Connected to the yolk sac by the vitelline duct
Has a cranial limb (superior to SMA and vitelline duct) and a caudal limb (inferior to SMA and vitelline duct)
Adult derivatives of cranial limb
Distal duodenum
Jejunum
Proximal ileum
Adult derivatives of caudal limb
Distal ileum Caecum Appendix Ascending colon Proximal 2/3 transverse colon
What is physiological herniation
The intestines herniate into the proximal umbilical cord alongside the umbilical vessels as the abdominal cavity is too small to accommodate the primary intestinal loop and the liver
When does physiological herniation occur
Week 6
How much does the midgut rotate overall
270 degrees anticlockwise
Where does the first rotation (90 degrees anti-clockwise) of the midgut take place
In the umbilical cord around the axis formed by the SMA
When does the second rotation (180 degrees anti-clockwise) of the midgut take place
On its return to the abdominal cavity (week 10)
What returns to the abdominal cavity first and last
First - cranial limb
Last - cecal bud
During rotation what occurs to the small and large intestine
Both elongate
Jejunum and ileum also form a number of coiled loops
Where does the cranial limb move to
Left hand side
Where does the cecal bud move to
Descends, moving the caecum to the right lower quadrant
Types of malrotation of the midgut
What do both lead to
Incomplete rotation
Reversed rotation
Both lead to hypermobility of the gut
Describe incomplete rotation
Midgut only rotates 90 degrees anti-clockwise
Results in a left sided colon (small intestine on the right side)
Describe reversed rotation
Midgut rotates 90 degrees clockwise
Transverse colon passes posterior to the duodenum so it can be occluded
What is volvulus
A bowel obstruction where a loop of the bowel has abnormally twisted in on itself
It can lead to strangulation and ischaemia
Who is more at risk of volvulus
People with hyper mobile guts
i.e. Sufferers of malrotation
What’s the end of the hindgut called
Cloaca
How is the cloaca separate from the outside
Cloacal membrane (single layer of endoderm and ectoderm at the proctodeum)
Describe cloacal partitioning
Anteroposterior division of the cloaca
A wedge of mesoderm called the urorectal septum grows down into the cloaca resulting in:
- urogenital sinus anteriorly
- anorectal canal posteriorly
- perineal body where the urorectal septum fuses with the cloacal membrane (outer surface)
Origins of the anal canal
Superior part is derived from the hindgut
Inferior part is derived from endoderm
What divides the 2 parts of the anal canal
Pectinate line
Anal canal blood supply above the pectinate line
Inferior mesenteric artery