What does the midgut give rise to?
- Small intestine, including most of the duodenum post bile duct entry
- Caecum and appendix
- Ascending colon
- Proximal 2/3 or transverse colon
Draw a diagram illustrating the position of the midgut in an embryo
What causes the midgut to make a loop?
It elongates enormously, and quickly runs out of space due to the large size of the developing liver
What does the loop made by the midgut have at its axis?
The superior mesenteric artery
What is the loop made by the midgut connected to?
The yolk sac
How is the midgut loop connected to the yolk sac?
By the vitelline duct
What are the limbs of the midgut loop called?
Cranial and caudal limbs
Label this diagram
- A - Cranial limb
- B - Superior mesenteric artery at axis
- C - Caudal limb
- D - Vitelline duct to yolk sac
What are the derivatives of the cranial limb?
- Distal duodenum
- Proximal ileum
What are the derivates of the caudal limb?
- Distal ileum
- Ascending colon
- Proximal 2/3 of transverse colon
What happens to the primary loop during the 6th week of development?
It elongates very rapidly
What happens to the liver during the 6th week of development?
It grows very rapidly
What is the problem with the rapid growth of both the liver and the primary loop?
The abdominal cavity is too small to accomodate both
What is the result of the abdominal cavity being too small too accomodate the primary loop and the liver?
What is physiological herniation?
Where the intestines herniate into the proximal umbilical cord, alongside the umbilical vessels
What are the stages in rotation of the midgut loop?
- First rotation
- Second rotation
By how much is the first rotation of the midgut loop?
By how much is the second rotation of the midgut loop?
What happens in the first rotation of the midgut loop?
During herniation into the umbilical cord, the midgut rotates around the axis formed by the SMA in a counter clockwise direction (cranial limb moves to back, caudal to the front)
Draw a diagram illustrating what happens in the first rotation of the midgut loop?
Does elongation of the small intestinal lumen continue during rotation?
What is the result of the elongation of the small intestinal loop continuing during rotation?
The jejunum and ileum form a number of coiled loops
What happens to the large intestine during rotation?
It lengthens, but does not participate in the coiling phenomenon
When does the second rotation of the midgut loop occur?
When it returns into the abdominal cavity, around week 10
In what direction does the second rotation of the midgut loop occur?
Turns 90 degrees counter-clockwise twice
Draw a diagram illustrating the second rotation of the midgut loop
In total, by how much does the midgut loop rotate?
270 degrees counter-clockwise
Which limb returns to the abdomen first?
The cranial limb
In what direction does the cranial limb move on return to the abdomen?
To the left hand side
What returns to the abdomen last following rotation?
The cecal bud
What happens once the cecal bud has returned to the abdomen?
It descends, moving the ceacum to the right lower quadrant
What does the rotation of the midgut loop account for?
The positions of the small and large intestines, and the twisted apperances of the mesentery of the small intestine
Are abnormalities of rotation common?
What do abnormalities of rotation cause?
Abnormal positioning of the midgut derivatives, e.g. the appendix on the left
What happens in incomplete rotation?
The midgut only makes one 90 degree rotation
What is the consequence of incomplete rotation?
Left sided colon
Draw a diagram showing the consequence of incomplete rotation
What happens in reversed rotation?
The midgut makes one 90 degree rotation clockwise
What is the consequence of reversed rotation?
The transverse colon passes posterior to the duodenum, and can wrap arround and occlude
Draw a diagram showing the consequences of reversed rotation
What does incomplete or reversed rotation lead to?
What is a volvulus?
A bowel obstruction where a loop of bowel has abnormally twisted in on itself
What makes a volvulus more likely?
What can a volvulus lead to?
What does the hindgut give rise to?
- The distal 1/3 transverse colon
- Descending colon
- Superior part of anal canal
- Epithelium of the urinary bladder
Draw a diagram illustrating the position of the hindgut
What happens to the hindgut at 6 weeks?
It ends in the cloaca
What separates the cloaca from the outside?
The cloacal membrane
What does the cloaca undergo after formation?
An anteroposterior division
What happens in cloacal partitioning?
A wedge of mesoderm grows down into the cloaca
What is the cloaca divided into in partitioning?
- The urogenial sinus anteriorly
- The anorectal canal posteriorly
Draw a labelled diagram illustrating the progression of cloacal partitioning
What is the anal canal derived from?
- The superior part is derived from the hindgut
- The inferior part is derived from the endoderm
What is the pectinate line?
The line at which the two parts fo the anal canal meet
Draw a diagram illustrating the formation of the anal canal
Label this diagram
- A - Rectum
- B - Pectinate line
- C - 'White' line
- D - Anal pecten
- E - Anal aperture
What is the blood supply for the anal canal above the pectinate line?
Inferior mesenteric artery
What is the blood supply for the anal canal below the pectinate line?
What is the innervation of the anal canal above the pectinate line?
S2/3/4 pelvic parasympathetic
What is the epithelia in the anal canal above the pectinate line?
What is the lymph drainage of the anal canal above the pectinate line?
Internal iliac nodes
What is the blood supply of the anal canal below the pectinate line?
What is the innervation of the anal canal below the pectinate line?
S2/3/4 Pudendal N.
What is the epithelia of the anal canal below the pectinate line?
Stratified squamous (non-keratinised)
What is the lymph drainage of the anal canal below the pectinate line?
Superficial inguinal nodes
Why do the two different parts of the anal canal vary in several ways?
Because they have different origins
What sensation is possible above the pectinate line?
What sensation is possible below the pectinate line?
Why is the tissue below the pectinate line sensitive to temperature, touch, and pain?
Due to its somatic innervation by the Pudendal nerve (S2/3/4)
What are some common congital defects of the GI tract?
- Vitelline duct
- Meckel's Diverticlum
- Vitelline cyst
- Vitelline fistula
- Recanalisation problems
- Pyloric stenosis
What is the most common GI abnormality?
What is the problem with a vitelline duct?
It can persist, resulting in a number of different abnormalities
What is Meckel's Diverticulum also known as?
What does Meckel's Diverticulum follow?
A rule of 2's
What is the rule of 2's followed by Meckel's diverticulum?
- 2% of population affected
- 2 feet from ileocecal valve
- 2 inches long
- Usually detected in under 2's
- 2:1 male:female
Can Meckel's Diverticulum be asymptomatic?
What does Meckel's Diverticulum cause?
Why does Meckel's Diverticulum cause ulceration?
Because the diverticulum can contain ectopic gastric or pancreatic tissue. The ectopic tissue will secrete enzymes and acids into the tissue not protected from them, causing ulceration
Draw a diagram illustrating Meckel's diverticulum
What is a vitelline cyst?
When the vitelline duct forms fibrous strands at either end
Draw a diagram illustrating a vitelline cyst?
What is a vitelline fistula?
When there is direct communication between the umbilicus and the intestinal tract
What is the result of a vitelline fistula?
Faecal matter coming out of the umbilicus
Draw a diagram illustrating a vitelline fistula
Why is recanalisation required in the embryo?
The primitive gut tube is a simple tube. In some gut stuctures, cell growth becomes so rapid that the lumen is partially or completely obliterated. Recanalisation occurs to restore the lumen
What structures can cause the obliteration of the lumen in the primitive gut tube?
- Bile duct
- Small intestine
What happens if recanalisation is wholly or partially unsuccessful?
Atresia or stenosis of the stucture can occur
When does atresia of the gut tube structure occur?
When there is a complete loss of the lumen
When does stenosis of the gut tube structures occur?
When there is a narrowing of the lumen
Where does most atresia/stenosis occur?
In the duodenum
What is the most likely cause of atresia/stenosis in the duodenum?
Incomplete canalisation, but 'vascular accidents' can also contribute
What is meant by a 'vascular accident'?
Where there is a loss of blood supply and that part of the gut dies
Where does atresia occur more, the duodenum or the jejenum?
Where does atresia/stenosis occur more, the jejenum or the ileum?
Where does atresia/stenesis occur more, the ileum or the colon?
What are atresias most often due to in the upper duodenum?
What are atresias most commonly due to in the lower duodenum?
A vascular accident
What can cause a vascular accident in the lower duodenum?
- Body wall defects
What is pyloric stenosis?
A narrowing of the exit from the stomach
Where is pyloric stenosis common?
What does pyloric stenosis result in?
Characteristic projectile vomiting
What causes pyloric stenosis?
Hypertrophy of the circular muscle in the region of the pyloric sphincter
What are some defects of the abdominal wall?
What is gastroschisis?
The failure of closure of the abdominal wall during folding of the embryo, leaving the gut tube and its derivatives outside the body cavity
What happensto the gut tube/derivates in gastroschisis?
There is no covering over them as they herniate through the abdominal wall directly into the amniotic cavity
What is omphalocoele?
The persistance of physiological herniation
What physiological herniation persists in omphalocoele?
A part of the gut tube fails to return to the abdominal cavity following normal herniation into the umbilical cord
Is there a covering in omphalocoele?
Why is there a covering in omphalocoele?
Since the umbilical cord is covered by a reflection of the amnion, this epithelial layer covers the defect
What are some hindgut abnormalities?
- Imperforate anus
- Anal/anorectal agenesis
- Hindgut fistulae
What is an imperforate anus?
Failure of the anal membrane to rupture
Draw a diagram of an imperforate anus
What is anal/anorectal agenesis?
Failure of development
Draw a diagram illustrating anal/anorectal agenesis?
What is a hindgut fistulae?
An abnormal connection within the hindgut
Draw a diagram illustrating hindgut fistulae
What are the mesenteries retained by?
- Transverse colon
What structures of the midgut/hindgut are fused with mesenteries?
- Ascending colon
- Descending colon
- Rectum (no peritoneal covering in distal 1/3)
What happens, regarding gut development, in week 3?
Tubular gut begins to form
What happens, regarding gut development, in week 4?
- Primordia of liver, pancreas, and trachea
- Buccopharyngeal membrane ruptures
What happens, regarding gut development, in week 5?
- Expansion and early rotation of the stomach
- First intestinal loop appears
- Caecum and bile duct develop
What happens, regarding gut development, in week 6?
- Increased liver growth
- Herniation of intestinal loop
- Appendix develops
- Cloacal partioning begins - urorectal septum appears
What happens, regarding gut development, in week 7?
- Pancreatic buds fuse
- Clocal partioning complete
- Rupture of clocal membrane
What happens, regarding gut development, in week 8?
- Counterclockwise rotation of herniated loop
What happens, regarding gut development, in week 10?