Development of the GIT - Midgut & Hindgut Flashcards

1
Q

Where does the midgut run to and from?

What is it continuous with and what is its blood supply?

A
  • it runs from the distal 1/2 of the duodenum (immediately distal to the entrance of the bile duct) to 2/3 along the transverse colon
  • the midgut is continuous with the yolk sac at the vitelline duct
  • it is supplied by the superior mesenteric artery throughout its entire length
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2
Q
A
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3
Q

What is formed in week 5 as a result of growth of the midgut?

A
  • the midgut and associated dorsal mesentery undergo rapid elongation to form the primary intestinal loop
  • the primary intestinal loop communicates with the yolk sac via the vitelline duct
  • it is divided into cranial (cephalic) and caudal limbs
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4
Q

What will the cranial and caudal limbs of the primary intestinal loop go on to form?

A

Cranial limb:

  • distal part of the duodenum
  • jejunum
  • proximal ileum

Caudal limb:

  • distal ileum
  • caecum
  • appendix
  • ascending colon
  • proximal 2/3 of the transverse colon
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5
Q

What is meant by physiological herniation of the midgut and why does this occur?

A
  • during week 6, there is rapid elongation of the midgut and growth of the liver
  • the abdominal cavity temporarily becomes too small to contain all of the intestinal loops
  • the primary intestinal loop herniates into the umbilical cord
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6
Q

As herniation of the midgut occurs, how does it rotate?

A
  • as herniation occurs, the midgut rotates 90o anti-clockwise
  • this brings the cranial limb to the right and the caudal limb to the left
  • jejunoileal loops form
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7
Q

What axis does the midgut rotate around?

What is the total amount of rotation performed and what else is occurring during this time?

A
  • the midgut rotates around an axis formed by the superior mesenteric artery
  • the overall rotation is 270o anti-clockwise
  • during rotation, elongation of the jejunum and ileum to form coiled loops (jejunoileal loops) is occurring
  • rotation occurs during physiological herniation (90o) and during return of the intestinal loops into the abdominal cavity (180o)
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8
Q

When does the midgut return to the abdomen following herniation?

A
  • the herniated intestinal loops begin to return to the abdomen in week 10
  • as this is occurring, the midgut rotates a further 180o anti-clockwise
  • this brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver
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9
Q

What is the first part of the intestine to return to the abdomen following herniation?

What forms from the caecum during this process and what is obliterated?

A
  • the proximal portion of the jejunum is the first to re-enter the abdominal cavity and this comes to lie on the left side
  • later returning loops will settle more and more to the right
  • the last part of the gut to re-enter the abdomen is the caecal bud
    • this is a small dilation of the caudal limb of the primary intestinal loop
  • the caecal bud lies in the RUQ directly below the right lobe of the liver, before descending into the right iliac fossa
  • during the descent, the distal end of the caecal bud forms a narrow diverticulum - the vermiform appendix
  • the vitelline duct is obliterated during return of the midgut to the abdomen
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10
Q

When has the midgut completely returned to the abdomen?

How has it rotated during this time?

A
  • the midgut has completely returned to the abdomen by week 11
  • it has undergone 270o anti-clockwise rotation in total
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11
Q

How is the final arrangement of the midgut achieved?

A
  • once the midgut has returned to the abdomen, the caecum descends from below the liver to the right iliac fossa
  • this pulls the ascending and transverse colon into place on the right side of the abdomen, resulting in the final position of the midgut
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12
Q

What is the mesentery of the primary intestinal loop?

How does this change with rotation and coiling of the bowel?

A
  • the mesentery proper undergoes profound changes with rotation and coiling of the bowel
  • when the caudal limb of the loop moves to the right side of the abdomen, the dorsal mesentery twists around the origin of the superior mesenteric artery
  • when the ascending and descending colon obtain their definitive positions, their mesenteries press against the posterior abdominal wall

this anchors the ascending and descending colon in a (secondarily) retroperitoneal position

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13
Q

What structures of the midgut maintain their mesentery?

What is the fate of the transverse mesocolon?

A
  • Structures that maintain their free mesenteries are:
  1. appendix
  2. lower end of the caecum
  3. sigmoid colon
  • the transverse mesocolon fuses with the posterior wall of the greater omentum, but maintains its mobility
  • its line of attachment extends from the hepatic flexure of the ascending colon to the splenic flexure of the descending colon
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14
Q

Where does the mesentery of the jejunoileal loops run to and from?

A
  • it is initially continuous with the mesentery of the ascending colon
  • when the mesentery of the ascending mesocolon fuses with the posterior abdominal wall, the mesentery of the jejunoileal loops obtains a new line of attachment
  • the new line of attachment runs from the area where the duodenum becomes intraperitoneal to the ileocaecal junction
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15
Q

Why does the appendix have a variable position?

A
  • the descent of the caecum causes the appendix to be located in the retrocaecal position in the majority of individuals (posterior to the caecum or colon)
  • the appendix is suspended by a mesentery and is relatively mobile
  • it can project inferiorly towards the pelvic brim, affecting the symptoms and site of pain in appendicitis
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16
Q

What is Meckel’s / ileal diverticulum?

Does it cause symptoms?

A
  • a remnant of the vitelline duct that creates an outpocketing of the ileal wall
  • it is usually asymptomatic but can contain ectopic pancreatic or gastric tissue, which may cause inflammation, ulceration and bleeding
17
Q

What is meant by the “rule of 2s” for Meckel’s diverticulum?

A
  • affects 2% of the population (most common gastrointestinal malformation)
  • 2 times more common in males
  • occurs 2 feet (50cm) from the ileocaecal junction
  • 2 inches (3-6cm) long
  • causes symptoms in 2% of cases
18
Q

Why does omphalocele occur?

What other malformations is it associated with?

A
  • it involves herniation of abdominal viscera through an enlarged umbilical ring
  • the defect originates from the failure of the midgut to return to the abdomen in week 10 following its physiological herniation (week 6)
  • it varies in size and can involve only the intestines, but also the liver, spleen, stomach and gallbladder
  • unlike gastroschisis, the viscera are covered by amnion
  • it is associated with increased risk of mortality and other malformations inlcuding cardiac anomalies and NTDs
19
Q

What is meant by gastroschisis?

A
  • this refers to protrusion of abdominal contents through the body wall directly into the amniotic cavity
  • it occurs lateral to the umbilicus, usually on the right side
  • it occurs due to abnormal closure of the body wall around the connecting stalk
  • viscera are NOT covered by amnion, so bowel may be damaged by exposure to amniotic fluid
20
Q

Why does gastroschisis tend to have a better prognosis than omphalocele?

What is the major risk associated with this condition?

A
  • unlike omphalocele, it is not associated with chromosome abnormalities or other severe defects
  • volvulus (rotation of the bowel) resulting in a compromised blood supply may occur and kill large regions of intestine, resulting in foetal death
21
Q

What is a major risk factor for gastroschisis?

A
  • it is more common in young mothers (<20) but the reason why is not known
22
Q

What is meant by non-rotation of the midgut?

Does this present with symptoms?

A
  • this occurs when the gut undergoes an initial 90o anti-clockwise rotation, but fails to rotate a further 180o when the gut is retracted
  • the colon and caecum are the first structures to return to the abdomen, and they settle on the left side
  • this results in the small intestine on the right side and large intestine on the left
  • also known as “left-sided colon”
  • it is usually asymptomatic
23
Q

What is meant by reversed rotation of the midgut?

A
  • the midgut undergoes the initial rotation of 90o clockwise
  • the midgut then rotates 180o clockwise when returning to the abdomen, resulting in a total rotation of 90o clockwise
  • the gut enters the abdomen in the correct order except that the transverse colon passes behind the duodenum and lies behind the SMA
24
Q

How does volvulus arise from abnormal rotation of the midgut?

What are the risks associated with this?

A
  • abnormal rotation of the midgut can cause parts that would normally be retroperitoneal (e.g. duodenum) to remain suspended by a dorsal mesentery
  • this can lead to twisting of the midgut about the mesentery
  • it causes acute obstruction of the bowel and bilious vomiting
  • it can also constrict the arterial supply to the gut, resulting in ischaemia and infarction
25
Q

What structures are part of the hindgut?

A
  1. distal 1/3 of transverse colon
  2. descending colon
  3. sigmoid colon
  4. rectum
  5. cranial 2/3 of the anal canal
26
Q

What does the distal end of the hindgut enter into?

What will this structure form?

A
  • the distal end of the hindgut enters into the posterior region of the cloaca - the primitive anorectal canal
  • the allantois enters into the anterior region - the primitive urogenital sinus
  • the urogenital sinus will form the:
  1. bladder
  2. pelvic urethra
  3. penile urethra
  4. caudal part of the vagina
  • the urorectal septum forms from mesoderm during weeks 4-6 and extends caudally to separate the urogenital sinus and anorectal canal
27
Q

What happens to the urorectal septum in week 7?

What is formed as a consequence of this?

A
  • as the embryo grows and caudal folding continues, the urorectal septum comes to lie close to the cloacal membrane
  • in week 7, the cloacal membrane ruptures
  • this creates the anal opening for the hindgut and a ventral opening for the urogenital sinus
  • between these 2 openings, the tip of the urorectal septum forms the perineal body
28
Q

What are the 2 origins of the anal canal?

A
  • the upper 2/3 is derived from endoderm of the hindgut
  • the lower 1/3 is derived from ectoderm around the proctodeum (anal pit)
29
Q

What line marks the boundary between the upper 2/3 and lower 1/3 of the anal canal?

When and why do they become continous?

A
  • ectoderm in the region of the proctodeum proliferates and invaginates to create the anal pit
  • degeneration of the cloacal membrane (anal membrane) establishes continuinty between the upper 2/3 and lower 1/3
  • the junction between endoderm and ectoderm derivatives is marked by the pectinate line
  • at this line the epithelium changes from columnar to stratified squamous epithelium
30
Q

What is the difference between the blood supply to the 2 parts of the anal canal?

A

Upper 2/3:

  • originates from endoderm and is supplied by the superior rectal artery
  • the superior rectal artery is a continuation of the inferior mesenteric artery

Lower 1/3:

  • originates from ectoderm and is supplied by the inferior rectal arteries
  • these are branches of the internal pudendal arteries
31
Q

What is meant by a congenital rectourethral or rectovaginal fistula?

Why does this occur?

A
  • caused by abnormalities in formation of the cloaca and/or the urorectal septum
  • e.g. if the cloaca is too small, or the urorectal septum does not extend far enough caudally
  • the opening of the hindgut shifts anteriorly, leading to an opening of the hindgut into the urethra (males) or vagina (females)
32
Q

What is meant by imperforate anus and why does this occur?

A
  • this occurs when there is failure of the anal membrane (cloacal membrane) to degenerate
  • it requires immediate surgery to allow evacuation of faeces but has good long term prognosis
33
Q

Describe the innervation of the gastrointestinal tract

What are the 2 plexi that are present and what are their functions?

A
  • the GIT is innervated by the enteric NS (division of autonomic NS)
  • there are 2 enteric plexi:

Myenteric (Aucherbach’s) plexus:

  • exists between the circular and longitudinal muscle layers and coordinates muscle contraction

Submucosal (Meissner’s) plexus:

  • exists between the circular muscle and mucosa and regulates secretion
  • the enteric NS is derived from neural crest cells (ectoderm) that migrate from the neural tube
34
Q

What happens in Hirschsprung disease / congenital aganglionic megacolon?

A
  • there is failure of neural crest cells to migrate to the bowel
  • absence of enteric ganglia leads to bowel obstruction due to lack of peristalsis
  • this causes dilation of the aganglionic part of the bowel - usually the rectum or sigmoid colon
35
Q

What condition is Hirschsprung disease associated with?

What is the only effective treatment?

A
  • it is a genetic condition most commonly associated with trisomy 21
  • the only effective treatment is to remove the affected bowel and anastomose the remaining healthy bowel with the anus
36
Q
A