Embryology 2. Development of the Midgut and Hindgut Flashcards
What are the derivatives of the midgut?
Small intestine (including most of the duodenum, post bile duct entry), caecum and appendix, ascending colon, proximal 2/3 of transverse colon.
What causes the primary intestinal loop to form?
The midgut elongates enormously but runs out of space due to the large liver, so the midgut loops.
What does the primary intestinal loop form around?
Superior mesenteric artery.
What is the primary intestinal loop connected to the yolk sac by?
The vitelline duct.
What are the limbs of the midgut primary intestinal loop?
Cranial and caudal limbs.
What are the derivatives of the cranial limb of the primary intestinal loop?
Distal duodenum, jejunum, proximal ileum.
What are the derivatives of the caudal limb of the primary intestinal loop?
Distal ileum, cecum, appendix, ascending colon, proximal 2/3 of transverse colon.
Why is physiological herniation in the 6th week of development necessary?
The abdominal cavity is too small to accommodate both the growing primary loop and the liver, so the intestines herniate into the proximal umbilical cord.
What does the primary loop herniate into in physiological herniation?
The proximal umbilical cord, alongside umbilical vessels.
What is the first rotation of the midgut loop?
It rotates around the axis (superior mesenteric artery) in an anticlockwise direction.
What is the result of the first midgut loop rotation?
The cranial limb moves to the back and the caudal limb to the front.
What is the second rotation of the midgut loop?
It turns 90 degrees anticlockwise twice.
What is the overall rotation of the midgut loop?
270 degrees in an anticlockwise direction.
Where does the cranial limb end up as a result of rotation?
The left hand side.
What happens to the caecum on returning to the abdomen?
It descends into the right lower quadrant.
What is the incomplete rotation?
The midgut only makes one 90 degree rotation so there is a left sided colon.
What is reversed rotation?
The midgut makes one 90 degree clockwise so the transverse colon passes posterior to the duodenum.
What is the result of reversed rotation?
The duodenum can wrap around the transverse colon and occlude it. Hypermobile guts may result.
What is a volvulus?
A bowel obstruction where a loop of bowel has abnormally twisted in on itself.
What is volvulus formation more likely with?
Hypermobile guts.
What can volvulus formation lead to?
Strangulation and ischaemia.
What are the derivatives of the hindgut?
The distal 1/3 of the transverse colon, descending colon, rectum, superior part of the anal canal, and epithelium of the urinary bladder.
What happens at 6 weeks development to the hindgut?
It ends in the cloaca and is separated from the outside by the cloacal membrane.
What is cloacal partitioning?
A wedge of mesoderm grows down into the cloaca, dividing it in to the urogenital sinus anteriorly and the anorectal canal posteriorly.
What are the embryonic derivations of the anal canal?
Superior part from hindgut. Inferior part from the endoderm.
What are the superior and inferior parts of the anal canal separated by?
The pectinate line.
What is the blood supply to the anal canal?
Above pectinate line - inferior mesenteric artery, below - pudendal artery.
What is the innervation of the anal canal?
Above pectinate line - S2/3/4 pelvic parasympathetic, below - S2/3/4 pudendal nerve.
What is the epithelia of the anal canal?
Above Pectinate line - columnar, below - stratified squamous.
What is the lymph drainage of the anal canal?
Above Pectinate line - internal iliac nodes, below - superficial inguinal nodes.
What are the sensations of the anal canal?
Above the Pectinate line - only stretch, below - temperature, touch, pain due to somatic innervation.
What is Meckel’s diverticulum?
The most common GI abnormality. It is a cul-de-sac in the ileum.
What is the rule of 2s for Meckel’s diverticulum?
2% of the population affected, 2 feet from the ileocaecal valve, 2 inches long, detected in under 2s, 2:1 male:female.
What can a complication of Meckel’s diverticulum be?
It can contain ectopic gastric or pancreatic tissue which will secrete enzymes and acids, causing ulceration.
What is a vitelline cycst?
The vitelline duct forms fibrous strands at either end.
What is a vitelline fistula?
A direct communication between the umbilicus and the intestinal tract.
What is the result of a vitelline fistula?
Faecal matter coming out of the umbilicus.
Why is recanalisation of the primitive gut tube needed in some gut structures?
The cell growth becomes so rapid that the lumen is partially or completely obliterated.
What is the aim of recanalisation?
To restore the lumen.
What results from failure to recanalise?
Atresia - complete loss of the lumen, or stenosis - narrowing of the lumen.
Where does most atresia/ stenosis occur in the GI tract?
In the duodenum.
What are the causes of atresias in the duodenum?
Upper duodenum from recanalisation failure, in the lower duodenum from vascular accident.
What is a vascular accident causing atresia?
Loss of blood supply means part of the gut dies.
What is pyloric stenosis?
A common abnormality of the stomach in infants. Narrowing of the exit from the stomach causing projectile vomiting.
What causes pyloric stenosis?
Hypertrophy of the circular muscle in the region of the pyloric sphincter.
What is gastroschisis?
The failure of closure of the abdominal wall during folding of the embryo, which leaves the gut tube and its derivative outside the body cavity.
What is a complication of gastroschisis?
The gut tube and derivatives have no covering as they herniate through the abdominal wall directly into the amniotic cavity.
What is omphalocoele?
The persistence of physiological herniation. Part of the gut tube fails to return to the abdominal cavity following its normal herniation into the umbilical cord.
What is imperforate anus?
Failure of the anal membrane to rupture.
What is anal/anorectal agenesis?
Failure of development of the anus or anus and rectum.
What is a hindgut fistula?
Abnormal connection within the hindgut.
What are the mesenteries retained by?
Jejunum, ileum, appendix, transverse colon, sigmoid colon.
What are the structures of the midgut and hindgut with fused mesenteries?
Duodenum, ascending colon, descending colon, and rectum.
