Embryology 2 Flashcards by Becky Mladenovic

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)

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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)

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Adult derivatives of cranial limb

Distal duodenum
Jejunum
Proximal ileum

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Adult derivatives of caudal limb

Distal ileum
Caecum
Appendix
Ascending colon
Proximal 2/3 transverse colon

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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

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When does physiological herniation occur

Week 6

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How much does the midgut rotate overall

270 degrees anticlockwise

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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

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When does the second rotation (180 degrees anti-clockwise) of the midgut take place

On its return to the abdominal cavity (week 10)

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What returns to the abdominal cavity first and last

First - cranial limb

Last - cecal bud

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During rotation what occurs to the small and large intestine

Both elongate

Jejunum and ileum also form a number of coiled loops

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Where does the cranial limb move to

Left hand side

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Where does the cecal bud move to

Descends, moving the caecum to the right lower quadrant

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Types of malrotation of the midgut

What do both lead to

Incomplete rotation
Reversed rotation

Both lead to hypermobility of the gut

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Describe incomplete rotation

Midgut only rotates 90 degrees anti-clockwise

Results in a left sided colon (small intestine on the right side)

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Describe reversed rotation

Midgut rotates 90 degrees clockwise

Transverse colon passes posterior to the duodenum so it can be occluded

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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

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Who is more at risk of volvulus

People with hyper mobile guts

i.e. Sufferers of malrotation

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What’s the end of the hindgut called

Cloaca

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How is the cloaca separate from the outside

Cloacal membrane (single layer of endoderm and ectoderm at the proctodeum)

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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)

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Origins of the anal canal

Superior part is derived from the hindgut

Inferior part is derived from endoderm

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What divides the 2 parts of the anal canal

Pectinate line

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Anal canal blood supply above the pectinate line

Inferior mesenteric artery

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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