Development of the Midgut and Hindgut Flashcards
Where does the midgut run to and from?
It runs from the 2nd part of the duodenum to 2/3 along the transverse colon
What is the midgut continuous with?
The yolk sac at the vitelline duct
this is around the level of the umbilicus
Where does the hindgut run to and from?
It runs from the final 1/3 of the transverse colon to the superior 2/3 of the rectum
What happens to the midgut during week 5 of development?
The midgut and associated dorsal mesentery undergo rapid elongation
This forms the primary intestinal loop
How does the primary intestinal loop communicate with the yolk sac?
Through the vitelline duct
Both the yolk sac and the vitelline duct are encompassed in the umbilical cord
What will the primary intestinal loop go on to form?
It grows very quickly to form cranial and caudal limbs
What will the cranial limb of the primary intestinal loop go on to form?
- distal part of duodenum
- jejunum
- proximal ileum
What will the caudal limb of the primary intestinal loop go on to form?
- distal ileum
- caecum
- appendix
- ascending colon
- proximal 2/3 of transverse colon
What happens to the midgut during week 6 of development?
What is the consequence of this?
There is rapid elongation of the midgut and growth of the liver
There is not enough room in the abdomen for the rapidly extending intestinal loop
What happens due to there not being enough space in the abdomen for the primary intestinal loop?
The primary intestinal loop herniates into the umbilical cord
What happens as the primary intestinal loop herniates into the umbilical cord?
The midgut rotates around its axis and rotates 90o anti-clockwise
Jejunoileal loops form
What is the result of the midgut rotating 90o anti-clockwise?
the cranial limb is brought to the right and the caudal limb is brought to the left
During which week does the midgut return to the abdominal cavity?
For 4 weeks it develops outside the abdominal cavity
It returns to the abdominal cavity in week 10, when there is enough space for it
What happens in week 10 when the midgut returns to the abdomen?
It rotates a further 180o anti-clockwise
In total, the midgut has rotated 270o anti-clockwise
What is the result of the midgut rotating 180o anti-clockwise in week 10?
The proximal jejunal loops are brought to the left side
The caecum ends up sitting just inferior to the liver
What wormlike diverticulum develops from the caecum?
the vermiform appendix
What happens to the vitelline duct as the midgut returns to the abdomen in week 10?
Why does this happen?
The vitelline duct is closed off and obliterated
The primary function of the yolk sac was to get nutrients, but once the gut tube has developed this is no longer needed
What is the position of the midgut by week 11 of development?
The midgut has completely returned to the abdomen and has undergone 270o anti-clockwise rotation in total
What happens to the caecum once the midgut has returned to the abdomen?
It descends from below the liver to the right iliac fossa
As the caecum descends, which structures does it bring with it?
It pulls the rest of the gut tube with it
It will pull the ascending and transverse colon into their adult anatomical positions
What happens to the ascending and descending colons as the caecum pulls them into position?
the dorsal mesentery of the ascending and descending colons shortens and degenerates
This pulls them against the posterior abdominal wall
Are the ascending and descending colons intraperitoneal or retroperitoneal?
Secondarily retroperitoneal
Why does the dorsal mesentery shorten as it moves?
What happens to the gut tube during this process?
This is due to elongation of the lumbar region
As it grows longer, it pulls the gut tube closer to the posterior wall
The dorsal mesentery decreases in length until it is lost
Why is the position of the appendix variable?
The position of the appendix is dictated by movement of caecum
What is the position of the appendix in the majority (64%) of individuals?
The appendix is in the retrocaecal position
This is where the appendix is located just behind the caecum
Why is the appendix relatively mobile?
It is suspended by a mesentery
Mesenteries dictate how mobile an organ is, and organs with longer mesenteries tend to be more mobile
Why are the symptoms and site of pain in appendicitis different in different people?
It is due to the position of the appendix being variable
What Meckel’s/Ileal diverticulum?
This is where a remnant of the vitelline duct creates an outpocketing of the ileal wall
This is due to the vitelline duct not being obliterated entirely
What are the usual symptoms of Meckel’s diverticulum?
It is usually asymptomatic unless there is heterotopic tissue present
(usually ectopic pancreatic or gastric tissue)
What is ectopic tissue refer to?
Certain types of tissue being located in areas where they are not usually found
What does ectopic tissue usually cause?
Inflammation, ulceration and bleeding
What are the ‘rule of 2s’ that characterise Meckel’s diverticulum?
- affects 2% of the population
- 2 times more common in males
- it is located 2 feet (50 cm) from the ileocaecal junction
- it is 2 inches (3-6 cm) long
- it is symptomatic in 2% of cases
What causes omphalocele?
Failure of the midgut to return to the abdomen in week 10 of development
The intestines do not fully develop in the abdominal cavity
What is omphalocele associated with?
An increased risk of mortality and other malformations
e.g. association with cardiac and neural tube defects
How can omphalocele be diagnosed prenatally?
ultrasound
How does the severity of omphalocele vary?
this depends on whether other organs have also failed to return to the abdomen
e.g. liver may also herniate through due to confined abdominal space
What is the difference between omphalocele and gastroschisis?
Gastroschisis - intestines develop outside intestinal cavity so have no amnion around them
Omphalocele - intestines herniate into umbilical cord first so are covered with amnion
What happens in non-rotation of the midgut?
It undergoes the initial 90o anti-clockwise rotation
It fails to rotate a further 180o when the gut is retracted back into the abdomen
What is non-rotation of the midgut usually referred to as and why?
It results in the small intestine on the right side and the large intestine on the left side
It is referred to as left-sided colon
What are the symptoms of left-sided colon?
It is usually asymptomatic
What happens in reversed rotation of the midgut?
The initial 90o anti-clockwise rotation occurs
When the gut retracts into the abdomen, it rotates 180o clockwise
The overall rotation is 90o clockwise
How does the arrangement of the gut change in reversed rotation of the midgut?
The majority of the gut enters the abdomen in the correct order, except for the duodenum
How does the position and properties of the duodenum change in reversed rotation of the midgut?
It is usually posterior to the colon and becomes retroperitoneal as it is pushed against the body wall
In reversed rotation, it lies anterior to the transverse colon and is intraperitoneal
What is caused by abnormal rotation of the midgut?
Parts of the GI tract that would normally be retroperitoneal become intraperitoneal
This is because they remain suspended by the dorsal mesentery
What is twisting of the midgut known as?
Why does this happen after abnormal rotation of the midgut?
Twisting of the midgut is volvulus
It happens due to presence of the dorsal mesentery meaning the organs are moveable and can twist on themselves
What symptoms and consequences can volvulus cause?
- acute obstruction of the bowel
- bilious vomiting
- constriction of the arterial supply to the gut
What is bilious vomiting and how is it brought about?
It occurs when bile starts to make its way back up the GI tract
Bile usually enters the 2nd part of the duodenum, but due to the volvulus, it can’t
What is the result of constricting the arterial supply to the gut?
Tissue ischaemia and infarction
What structures does the hindgut give rise to?
- the distal 1/3 of the transverse colon
- descending colon
- sigmoid colon
- rectum and cranial 2/3 of the anal canal
What does the distal end of the hindgut enter?
What forms here?
It enters the dorsal part of the cloaca
This is where the anorectal canal will form
What is the cloaca?
The region where excretions from the future bladder and gut tube empty in to
What is the ventral part of the cloaca?
Urogenital sinus
This goes on to form the bladder, pelvic urethra, penile urethra/vagina
What forms relating to the hindgut during weeks 4-6?
Urorectal septum
a layer of mesoderm extends caudally to separate the urogenital sinus and the anorectal canal
What happens relating to the hindgut in week 7 of development?
the urorectal septum approaches close to the cloacal membrane
In week 7, the cloacal membrane ruptures
What is the result of the cloacal membrane rupturing?
It creates the anal opening and a ventral opening for the urogenital sinus
The tip of the urogenital septum lies between them and forms the perineal body
What is the upper 2/3 of the anal canal derived from?
cranial part
The hindgut
It is derived from the endoderm
What is the lower 1/3 of the anal canal?
What is is derived from?
(caudal part)
Anal pit
It is derived from proctodaeum which is derived from the ectoderm
What separates the cranial and caudal parts of the anus?
The cloacal (anal) membrane
When the membrane degenerates, they become continuous
What is the result of the cranial and caudal parts of the anal canal being derived from different cells?
They have different epithelial linings, nerve and blood supply and lymphatic drainage
In an adult, what is the junction between endoderm and ectoderm derivatives marked by?
It is marked by the pectinate line
This is the region where the cloacal membrane ruptures
Why is the pectinate line important clinically?
It delineates whether haemorrhoids are internal or external
What happens if septation of the cloaca goes slightly wrong?
It results in abnormal connections (fistulas)
These may be rectourethral or rectovaginal
Why might an abnormal cloaca lead to fistulas?
The cloaca may be too small so there is not enough space for 2 structures to form
There may be failure of the urorectal septum to extend caudally
What happens in males due to abnormal septation of the cloaca?
The opening of the hindgut is shifted ventrally to the urethra
The connection between the anus and the urethra is a urorectal fistula
What happens in females due to abnormal septation of the cloaca?
The opening of the hindgut is shifted ventrally to the vagina
The connection between the rectum/anus and the vagina is a rectovaginal fistula
What causes an imperforate anus?
The anal membrane fails to degenerate
This means there is no way of evacuating the anus
What is the long-term prognosis in majority of cases of imperforate anus?
There is a good long term prognosis as long as immediate surgery is performed to allow evacuation of faeces
What is the nervous supply of the GI tract?
The enteric nervous system
This is a division of the autonomic nervous system
What are the 2 different enteric plexi?
- myenteric plexus
2. submucosal plexus
Where is the myenteric plexus found and what does it do?
It is between the circular and longitudinal muscle layers
It coordinates muscle contraction
What is the other name for the myenteric plexus?
Aucherbach’s plexus
Where is the submucosal plexus found and what does it do?
It is between the circular muscle and the mucosa
It regulates secretion
What is the other name of the submucosal plexus?
Meissner’s plexus
What is the enteric nervous system derived from?
neural crest cells that migrate from the neural tube to the GI tract
The neural crest cells are of ectoderm origin
What is the other name for Hirschsprung disease and what is it caused by?
Congenital aganglionic megacolon
It is caused by a failure of the neural crest cells to migrate to the bowel
Why does absence of enteric ganglia lead to bowel obstruction?
There is a lack of peristalsis
There is no nervous system so intestinal contents cannot be squeezed through the gut
What is the major consequence of Hirschsprung disease?
Dilation of the aganglionic part of the bowel
This is usually the rectum or sigmoid colon
How often does someone with Hirschsprung disease empty their bowels?
What is the consequence of this?
They can go weeks/months without emptying their bowels
There is a build-up of bacteria which produce gases
This leads to a distended abdomen
what is the only effective treatment for Hirschsprung disease?
removing the affected bowel which does not have a nervous system
the remaining healthy bowel is anastomosed with the anus
What does the severity of Hirschsprung disease depend on?
How much of the gut tube lacks enteric ganglia
