Development of the GIT - Midgut & Hindgut Flashcards
Where does the midgut run to and from?
What is it continuous with and what is its blood supply?
- 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
What is formed in week 5 as a result of growth of the midgut?
- 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
What will the cranial and caudal limbs of the primary intestinal loop go on to form?
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
What is meant by physiological herniation of the midgut and why does this occur?
- 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
As herniation of the midgut occurs, how does it rotate?
- 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
What axis does the midgut rotate around?
What is the total amount of rotation performed and what else is occurring during this time?
- 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)
When does the midgut return to the abdomen following herniation?
- 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
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?
- 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
When has the midgut completely returned to the abdomen?
How has it rotated during this time?
- the midgut has completely returned to the abdomen by week 11
- it has undergone 270o anti-clockwise rotation in total
How is the final arrangement of the midgut achieved?
- 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
What is the mesentery of the primary intestinal loop?
How does this change with rotation and coiling of the bowel?
- 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
What structures of the midgut maintain their mesentery?
What is the fate of the transverse mesocolon?
- Structures that maintain their free mesenteries are:
- appendix
- lower end of the caecum
- 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
Where does the mesentery of the jejunoileal loops run to and from?
- 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
Why does the appendix have a variable position?
- 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
What is Meckel’s / ileal diverticulum?
Does it cause symptoms?
- 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
What is meant by the “rule of 2s” for Meckel’s diverticulum?
- 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
Why does omphalocele occur?
What other malformations is it associated with?
- 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
What is meant by gastroschisis?
- 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
Why does gastroschisis tend to have a better prognosis than omphalocele?
What is the major risk associated with this condition?
- 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
What is a major risk factor for gastroschisis?
- it is more common in young mothers (<20) but the reason why is not known
What is meant by non-rotation of the midgut?
Does this present with symptoms?
- 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
What is meant by reversed rotation of the midgut?
- 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
How does volvulus arise from abnormal rotation of the midgut?
What are the risks associated with this?
- 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
What structures are part of the hindgut?
- distal 1/3 of transverse colon
- descending colon
- sigmoid colon
- rectum
- cranial 2/3 of the anal canal
What does the distal end of the hindgut enter into?
What will this structure form?
- 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:
- bladder
- pelvic urethra
- penile urethra
- 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
What happens to the urorectal septum in week 7?
What is formed as a consequence of this?
- 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
What are the 2 origins of the anal canal?
- the upper 2/3 is derived from endoderm of the hindgut
- the lower 1/3 is derived from ectoderm around the proctodeum (anal pit)
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?
- 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
What is the difference between the blood supply to the 2 parts of the anal canal?
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
What is meant by a congenital rectourethral or rectovaginal fistula?
Why does this occur?
- 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)
What is meant by imperforate anus and why does this occur?
- 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
Describe the innervation of the gastrointestinal tract
What are the 2 plexi that are present and what are their functions?
- 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
What happens in Hirschsprung disease / congenital aganglionic megacolon?
- 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
What condition is Hirschsprung disease associated with?
What is the only effective treatment?
- 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
