Development of the gastrointestinal tract: Midgut and hindgut Flashcards
What are the boundaries of the midgut?
Second half of the duodenum to 2/3 of transverse colon.
Where is the midgut continuous with the yolk sac?
The vitelline duct.
What happens to the midgut in week 5?
The midgut and associated dorsal mesentery undergo rapid elongation to form the primary intestinal loop, which communicates with the yolk sac through the vitelline duct.
What structures develop from the cranial limb of the primary intetinal loop?
Distal duodenum, jejunum, and proximal ileum.
What structures develop from the caudal limb of the primary intestinal loop?
Distal ileum, caecum, appendix, ascending colon and proximal 2/3 transverse colon.
Aside from the gut tube, what other GI organ starts to grow during week 6?
Liver.
Into what structure must the primary intestinal loop herniate and why?
Umbilical cord - not enough space in the abdo cavity.
What also happens to the midgut as is herniates into the umbilical cord.
Rotates 90 degrees anticlockwise bringing the cranial limb to the right and the caudal limb to the left. Jejunal loops also form.
Describe the rotation and position of the midgut in week 10.
Returns to the abdomenand rotates a further 180 degrees anti-clockwise. Brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver.
What is the name of the diverticulum formed by the caecum?
Vermiform appendix
In what week is the vitelline duct obliterated and what does it form?
Week 10 - Meckel’s diverticulum.
What is the extent of the anti-clockwise rotation of the midgut by week 11?
270 degrees
To where does the caecum decend following the return of the midgut to the abdomen?
Right iliac fossa
How do the ascending and descending parts of the colon become secodarily retroperitoneal?
Dorsal mesentery of ascending and descending colon shortens and degenerates pulling them against the posterior abdominal wall - secondarily retroperitoneal.
How is the appendix position in the majority of individuals?
Retrocaecally - 64%.
Does the appendix have its own mesentery?
Yes - makes it fairly mobile.
What other positions may the appendix be found in?
May project inferiorly towards the pelvic brim.
How common is Meckel’s diverticulum?
Occurs in 2% of the population. 2x more common in males.
Where in the GI tract does Meckel’s diverticulum occur?
2 feet from the ileocaecal junction
How long is Meckel’s diverticulum?
2 inches
What is the rule of 2s?
Meckels diverticulum: 2% of people, 2x more common in males, 2 feet from ileocaecal junction, 2 inches long.
What is omphalocele?
Failure of the midgut to return to the abdomen in week 10. Associated with increased risk of mortality and other malformation eg cardiac or neural tube defect.
What is the aetiology of non-rotation of the midgut?
Undergoes initial 90 degree rotation but fails to rotate further 180 when gut it retracted. Results in small intestine on right side and large on the left. Usually asymptomatic.
What happens to cause reverse rotation of the midgut?
Initial 90 anti-clockwise rotation occurs but gut then rotates 180 degrees clockwise. Results in a total 90 degree clockwise rotation instead of 270 degrees. Colon in right place but duodenum lie ventral to transverse colon.
How can abnormal rotation of the midgut lead to volvulus?
Can lead to parts that would normally be retroperitoneal to be suspended by dorsal mesentery. This can lead to twisting of the midgut and volvulus.
What are the symptoms of volvulus?
Bowel obstruction and bilious vomiting. May also constrict arterial supply to the gut causing ischaemia and infarction.
What structures does the hindgut give rise to?
Distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and cranial 2/3 of the anal canal.
What happens to the distal part of the hindgut?
The anorectal canal entering the dorsal part of the cloaca.
What happens to the ventral part of the cloaca?
Becomes the urogenital sinus - forms the bladder, pelvic urethra, penile urethra (males) and caudal part of the vagina.
How does the anorectal canal become seperate from the urogenital sinus?
During weeks 4-6 a layer of mesoderm extends caudally seperate the two - uro-rectal septum.
What happens as the urorectal septum approaches the cloacal membrane (ectoderm derivative)?
In week 7 the cloacal membrane ruptures creating the anal opening and ventral opening and a ventral opening for the urogenital sinus.
What forms the perineal body?
The tip of the urorectal septum.
From what is the upper 2/3 of the anal canal formed from?
Hindgut (endoderm)
What is the lower 1/3 of the anal canal derived from?
Protodeum (anal pit) - ectoderm
When do the two parts of the anal canal become continuous?
When the cloacal membrane degenerates.
What marks the junction between endoderm and ectoderm in the anal canal of an adult?
Pectinate line
How common are congenital rectourethral/rectovaginal fistulas?
1 in 5,000 births
What is the aetiology of rectourethral/rectovaginal fistulas?
Caused by abnormal cloaca e.g. too small or failure of urorectal septum to extend caudally. Opening of hindgut is shifted ventrally to the urethra in males and the vagina in females.
What is an imperforate anus?
Failure of anal membrane to degenerate. Usually requires immediate surgery. Good long-term prognosis.
What is the name of the division of the autonomic nervous system innervating the GI tract?
The enteric nervous system.
Name the two enteric plexi?
Myenteric (Auerbach’s): between the circular and longitudinal muscle layers. Coordinates muscle contraction.
Submucosal (Meisser’s): between the circular muscle and mucosa and regulates secretion.
From what is the enteric nervous system derived?
Neural crest cells
What is Hirschprung disease/congenital aganglionic megacolon?
Failure of neural crest cells to migrate to bowel. Abscence of enteric ganglia leads to bowel obstruction due to lack of peristalsis. This causes dilation of the aganglionic of the bowel - usually rectum and sigmoid colon.
Genetic condition most commonly associated with trisomy 21. Only effective treatment is to remove the affected bowel and anastamose the remaining healthy bowel with anus.
