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
Anal canal blood supply below the pectinate line
Pudendal artery
Anal canal innervation above the pectinate line
Parasympathetic - pelvic nerve (S2-4)
Innervation blood supply below the pectinate line
Pudendal nerve (S2-4)
Epithelia type in the anal canal above the pectinate line
Columnar
Epithelia type in the anal canal below the pectinate line
Non keratinised stratified squamous
Lymphatic drainage of the anal canal above the pectinate line
Internal iliac nodes
Lymphatic drainage of the anal canal below the pectinate line
Superficial inguinal nodes
What sensations are possible above the pectinate line
Only stretch
What sensations are possible below the pectinate line
Temperature
Touch
Pain
What is Meckel’s diverticulum
A ‘cul-de-sac’ in the ileum
What complication is seen in Meckel’s diverticulum
Ulceration
Ectopic gastric/pancreatic tissue in the diverticulum secretes enzymes and acids
Why is Meckel’s diverticulum said to follow a rule of 2’s
2% population affected 2 feet from the iliocecal valve 2 inches long Affects males twice as often as females Usually detected in under 2's
What is a vitelline cyst
Vitelline duct that forms fibrous strands at either end
What is vitelline fistula
Direct communication between the umbilicus and intestine
What is patent urachus
The urachus (fibrous remnant of allantois) fails to close so there is a direct communication between the umbilicus and the bladder
What is omphalocoele
Persistence of a physiological herniation so part of the gut fails to return to the abdominal cavity
Since the umbilical cord is covered by a reflection of the amnion, an epithelial layer covers the defect
What is gastroschisis
Failure of the abdominal wall to close during embryonic folding leaving the gut tube outside the body cavity
Unlike omphalocoele, there is no covering over the gut tube
What are the hindgut abnormalities
Imperforate anus
Anal agenesis
Hindgut fistulae
What is imperforate anus
Failure of the anal membrane to rupture
Also called anal atresia
What is anal agenesis
Failure of development of the anal canal
What is hindgut fistulae
Abnormal connection within the hindgut
Why is recanalisation necessary in some structures of the gut tube
Cell growth becomes so rapid that the lumen is partially or completely occluded
What occurs in failure of recanalisation
Atresia (complete failure)
Stenosis (partial failure)
What’s the order of incidence of atresia/stenosis in the gut
Duodenum
Jejunum and ileum
Colon
What’s the most common cause of atresia in the upper duodenum
Recanalisation failure
What’s the most common cause of atresia in the lower duodenum
Vascular accident - there is a loss of blood supply and part of the gut dies
Caused by malrotation, volvulus and body wall defects
What is pyloric stenosis
Narrowing of the exit from the stomach causing projectile vomiting
Causes of pyloric stenosis
Hypertrophy of the circular muscle in the region of the pyloric sphincter
Which structures of the midgut retain mesenteries
Jejunum
Ileum
Appendix
Transverse colon
Which structures of the midgut have fused mesenteries
Duodenum
Ascending colon
Which structures of the hindgut retain mesenteries
Transverse colon
Sigmoid colon
Which structures of the hindgut have fused mesenteries
Descending colon
Rectum
When is cloacal partitioning complete
End of Week 7
When does cloacal partitioning begin
Week 6
When does a patent urachus present
At birth
In men it can present later in life - high pressure caused by obstruction of bladder outflow (by benign prostatic hypertrophy) can lead to opening of urachus
How do you distinguish a vitelline fistula and patent urachus
Inject contrast into cyst and see whether dye goes into bladder or intestines
