8) Development of Midgut and Hindgut Flashcards
What is physiological herniation?
Intestines of midgut herniate into umbilical cord to continue development
Describe the primary intestinal loop:
Has cranial and caudal limbs with SMA as its axis
Is connected to yolk sac by vitelline duct
Why is physiological herniation required?
Growth of primary intestinal loop is rapid
Liver is also growing rapidly and abdominal cavity is too small to accommodate both
Describe the midgut rotation:
Rotates in a counter-clockwise direction in a series of 3 ninety degree turns
Describe the first 90 degree rotation:
Brings cranial and caudal limbs to same level, small intestine elongation continued forming coiled loops
What do the further two rotations ensure?
That cranial limb enters the abdominal cavity first
Describe the intestine’s return to the cavity:
Cranial limb moves to the left
Caecal bud returns last and then descends to right lower quadrant
What are the derivatives of the cranial limb?
Distal duodenum, jejunum, proximal ileum
What are the derivatives of the caudal limb?
Distal ileum, caecum, appendix, ascending colon, proximal 2/3rds transverse colon
What is incomplete rotation of the midgut?
Midgut loop makes only one 90 degree rotation
Results in left-sided colon
What is reversed rotation of the midgut and what can it lead to?
Midgut loop makes one 90 degree rotation clockwise
Means that transverse colon passes posterior to duodenum - so can wrap around and occlude it
What is a major complication of midgut defects?
Volvulus - strangulation and ischaemia
What abnormalities can a persistent vitelline duct cause?
Vitelline cyst
Vitelline fistula
Meckel’s diverticulum
What is a vitelline fistula?
Direct communication between umbilicus and intestinal tract
What is Meckel’s diverticulum?
Cul-de-sac in ileum
Can contain ectopic gastric or pancreatic tissue causing ulceration
What structures have their lumens obliterated?
Oesophagus, bile duct, small intestine, duodenum
Why is the lumen obliterated?
Cell growth becomes too rapid
What process restores obliterated lumen?
Recanalisation
What happens if recanalization isn’t successful?
Atresia or stenosis
Where do most of recanalization failures occur?
Duodenum then jejunum and ileum then colon
What causes atresia and stenosis in lower duodenum?
Vascular accident, loss of blood supply so part of gut dies
What can cause a vascular accident?
Malrotation, volvulus, body wall defect
Why is a pyloric stenosis and how does it present?
Hypertrophy of circular muscle in region of pyloric sphincter
Leads to difficulty empting stomach so projective vomiting
What is gastroschisis?
Failure of closure of abdominal wall during folding of embryo leaving gut tube outside of body with no covering
What is omphalocoele (exomphalos)?
Persistence of physiological hernation, covered in amnion
What is the end of the hindgut?
Cloaca
What divides the cloaca and into what?
Wedge of mesoderm (urorectal septum)
Divides into urogenital sinus and anorectal canal
What is the proctodeum?
Anal pit, where ectoderm (inferior) meets endoderm (superior)
What line divides the anal canal?
Pectinate line
What is the epithelium, blood supply and nerve supply above the pectinate line?
Columnar epithelium
IMA
S2,3,4 pelvic parasympathetic
What is the epithelium, blood supply and nerve supply below the pectinate line?
Stratified squamous
Pudendal artery
Pudendal nerve (S2-4)
Describe the difference in sensation above and below the pectinate line:
Above: sensation is only stretch. Pain is dull and poorly localised
Below: sensation is temp, touch and pain sensitive (well localised)
Describe visceral pain:
Poorly localised and pattern reflects development of structure:
Foregut - epigastric
Midgut - Periumbilical
Hindgut - Suprapubic
What is imperforate anus?
Failure of anal membrane to rupture
What other hindgut abnormalities are there?
Anal/anorectal agenesis
Hindgut fistulae
What midgut and hindgut structures maintain their mesentery?
Jejunum, ileum, appendix, transverse and sigmoid colon
What midgut and hindgut structures have fused mesenteries? (think retroperitoneal)
Duodenum, ascending and descending colon, rectum
What does the dorsal mesentery become?
Greater momentum Gastrolineal ligament (stomach to spleen) Lienorenal ligament (spleen to kidney) Mesocolon Mesentery proper
What does the ventral mesentery become?
Lesser omentum
Falciform ligament
What is the blood and nerve supply to midgut?
SMA and SMV
PSNS: vagus
SNS: superior mesenteric plexus
What is the blood and nerve supply to hindgut?
IMA and IMV
PSNS: pelvic (S2-4)
SNS: inferior mesenteric plexus
