Embryology - Development of Midgut & Hindgut Flashcards
Where does the midgut run from/to?
2nd half of duodenum to 2/3 along transverse colon
What is the midgut continuous with?
The yolk sac at the vitelline duct
During week 5, the midgut and associated dorsal mesentery undergo rapid elongation. What does this form?
The primary intestinal loop (elongation efficient for SA)
How does the primary intestinal loop communicate with the yolk sac?
Through the vitelline duct
What does the primary intestinal loop have?
Cranial and caudal limbs
What will the cranial limb form?
distal duodenum, jejunum and proximal ilieum
What does the caudal limb form?
distal ileum, caecum, appendix, ascending colon and proximal 2/3 transverse colon
During week 6, what occurs regarding the midgut and the liver?
Rapid elongation of the midgut and growth of the liver
There is not enough room in the abdomen, so what happens to the 1ary intestinal loop?
It herniates into the umbilical cord
As herniation occurs, how does the midgut rotate? What effect does this have on the cranial and caudal limb?
Rotates 90 degrees anti-clockwise
Brings the cranial limb to the right and caudal limb to the left
Jejunoileal loops form
During week 10, what occurs regarding the midgut?
Returns to the abdomen and rotates a further 180 degrees anti-clockwise
What is the effect of the rotation of the midgut 180 degrees anti-clockwise in week 10 on the proximal jejunal loops and the caecum?
Brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver
What does the caecum develop?
A wormlike diverticulum – vermiform appendix
What causes the vitelline duct to be obliterated?
Rotation of midgut a further 180 degrees anti-clockwise as it moves back to the abdomen
By week 11, how much has the midgut rotated?
- The midgut has completely returned to the abdomen
- 270 degrees anticlockwise
Once the midgut has returned to the abdomen, where does the caecum move?
Descends from below the liver to the right iliac fossa
What does this descent of the caecum cause?
Pulls the ascending and transverse colon into place resulting in the final arrangement of the midgut
What causes the final arrangement of the midgut?
Descent of caecum
Once the final arrangement of the midgut has occurred, what regarding the dorsal mesentery of the ascending and descending colon?
The dorsal mesentery of the ascending and descending colons now shortens and degenerates pulling them against the posterior abdominal wall – secondarily retroperitoneal
What does secondarily retroperitoneal mean?
Starts off tethered to mesentery but then gets pushed against posterior wall and loses mesentery
What effect does the descent of the caecum have on the appendix?
Causes the appendix to be located in the retro-caecal position in the majority of individuals
How is the appendix suspended?
Suspended by a mesentery so is relatively mobile (position variable from person to person)
What position is appendix typically in?
Retro-caecal
What is Meckel’s/Ileal Diverticulum?
- A remnant of the vitelline duct that creates an outpocketing of the ileal wall (most common GI malformation)
- Usually asymptomatic
When can Meckel’s/Ileal Diverticulum cause problems?
May contain ectopic pancreatic or gastric tissue (so secretes acid), causing inflammation, ulceration and bleeding
What are the ‘rules of 2’s in Meckel’s/Ileal Diverticulum?
Affects 2% population 2 times more common in males 2 feet (50 cm) from ileocaecal junction 2 inches (3-6 cm) long Symptomatic in 2% cases
What is Omphalocele? What is it caused by?
Infant’s intestines, liver, or other organs stick outside of the belly through the belly button. The organs are covered in a thin, nearly transparent sac that hardly ever is open or broken.
Cause: Problem with herniation as don’t develop enough space (abdominal viscera herniating into base of umbilicus). Failure of midgut to return to return to abdomen in week 10.
What is Omphalocele associated with?
Associated with an increased risk of mortality and other malformation e.g. cardiac or neural tube defects
How can Omphalocele be diagnosed?
Prenatally using ultrasound
What occurs during ‘non-rotation’ of the gut?
Gut undergoes initial 90° anti-clockwise rotation but fails to rotate a further 180° when the gut is retracted
What is result of ‘non-rotation’ of the gut?
Results in small intestine on the right side and large intestine on the left (left sided colon)
Usually asymptomatic
What occurs during ‘reversed rotation’ of the midgut?
- Initial 90° anticlockwise rotation occurs normally, however, gut the rotates 180° clockwise
- Total rotation results in 90° clockwise instead of 270° anti-clockwise
What is the result of ‘reversed rotation’?
- Gut enters abdomen in correct order except duodenum lies ventral (anterior) to transverse colon
What is effect of duodenum lying anterior to transverse colon?
- Doesn’t get pushed against posterior wall so hasn’t lost mesentery (isn’t retroperitoneal but remains suspended by dorsal mesentery)
- Is more mobile
- Can lead to volvulus (twisting) of the midgut
What is effect of volvulus?
- Causes acute obstruction of the bowel and bilious vomiting
- May also constrict arterial supply to the gut causing ischaemia and infarction
What does the hindgut give rise to?
The distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and cranial 2/3 anal canal
What does the distal end of the hindgut enter?
The dorsal part of the cloaca –> anorectal canal
What is the cloaca?
a common cavity at the end of the digestive tract
What is the ventral part of the cloaca called?
Urogenital sinus
What does the ventral part of the cloaca form?
The bladder, pelvic urethra, penile urethra (males) and caudal part of the vagina (females)
During weeks 4-6, how is the urogenital sinus and anorectal canal separated?
A layer of mesoderm extends caudally to separate them –> urorectal septum
What does the urorectal septum appraoch?
Approaches close to the cloacal membrane (ectoderm derivative)
How is anal canal opening created?
In week 7, the cloacal membrane ruptures, creating the anal opening and a ventral opening for the urogenital sinus
What lies between the anal opening and a ventral opening for the urogenital sinus?
The tip of the urorectal septum –> forms the perineal body
What does the perineal body act as?
It acts as a point of attachment for muscle fibres from the pelvic floor and the perineum itself
Where does the inner epithelial lining of the gut tube come from?
Endoderm
What is the upper 2/3 of the anal canal derived from?
The hindgut - endoderm
What is the lower 1/3 of the anal canal derived from?
The proctodeum (anal pit) - ectoderm
When do the upper 2/3 (endoderm) and lower 1/3 (ectoderm) become continuous?
When the cloacal (anal) membrane degenerates
Why is it important that the upper 2/3 and lower 1/3 of the anal canal have different origins?
Different epithelial linings, lymphatic drainage and blood supply
What is the junction between endoderm and ectoderm in the anal canal marked in adults by?
The pectinate line
What are congenital rectourethral / rectovaginal fistulas caused by?
Abnormal cloaca e.g. too small or failure of urorectal septum to extend caudally
What are rectourethral / rectovaginal fistulas?
Abnormal connections between rectum and urethra (men) or rectum and vagina (women)
Opening of hindgut is shifted ventrally to the urethra in males and the vagina in females
What is imperforate anus / anal atresia?
Failure of anal membrane to degenerate (blind ended sac instead)
How can imperforate anus / anal atresia be treated?
Immediate surgery to allow evacuation of faeces
Good long term prognosis
What is the GIT innervated by?
The enteric nervous system (a division of the ANS)
What are the 2 enteric plexi?
- Myenteric (Auerbach’s) plexus
2. Submucosal (Meissner’s) plexus
Where is Myenteric (Auerbach’s) plexus?
Between the circular and longitudinal muscle layers co-ordinates muscle contraction
Where is Submucosal (Meissner’s) plexus?
between the circular muscle and mucosa and regulates secretion
What is the enteric nervous system derived from?
Neural crest cells (ectoderm origin) that migrate from the neural tube to GIT
What is Hirschsprung Disease / Congenital Aganglionic Megacolon?
- Failure of neural crest cells to migrate to bowel
- Absence of enteric ganglia leads to bowel obstruction due to lack of peristalsis (due to lack of nerve supply)
- This causes dilation of the aganglionic part of the bowel (bacteria respires as food builds up) – usually rectum and sigmoid colon
What is Hirschsprung Disease / Congenital Aganglionic Megacolon associated with?
Genetic condition most commonly associated with trisomy 21
What is the effective treatment for Hirschsprung Disease / Congenital Aganglionic Megacolon?
remove bowel and anastomose the remaining healthy bowel with anus
