Session 3: Development of the Midgut and Hindgut Flashcards
What is the primitive gut formed from?
The definitive yolk sac during folding.
What can be found in the primitive gut, cranially and caudally respectively?
Buccopharyngeal membrane cranially Cloacal membrane caudally
What will the midgut form into?
Most of the duodenum (post-junction) Jejunum Ileum Caecum & appendix Ascending colon Proximal 2/3 of the transverse colon
What happens to the midgut at the 5th week at the start of development of the final midgut?
It elongates enormously to form a loop called the primary intestinal loop.
What is the consequence of the rapid growth of the primary intestinal loop during 5th week and beginning of 6th week?
It will run out of space in the abdominal cavity to grow since the liver is also growing at a fast rate at this time. The growth of the abdominal cavity cannot keep up to the pace of the growth of the primary intestinal loop and the liver. This makes the primary intestinal loop to herniate into the umbilical cord.
The primary intestinal loop has something as its axis which it has looped ‘around’. What? What is the primary intestinal loop anchored to?
The superior mesenteric artery. To the yolk sac by the vitelline duct
The superior mesenteric artery becomes an important landmark in the embryo as it divides the primitive intestinal loop into two parts. Which?
A cranial limb superior to the SMA A caudal limb inferior to the SMA
What will the cranial limb give rise to?
The distal duodenum Jejunum And majority of the ileum
What will the caudal limb give rise to?
Very distal part of the ileum Caecum & appendix Ascending colon Proximal 2/3 of the transverse colon
What happens to the primary intestinal loop as it houses inside of the umbilical cord in early 8th week?
It will rotate upon the axis of the SMA 90 degrees counterclockwise.
What is the consequence of this rotation?
This means that the cranial limb will be located to the right of SMA Caudal limb will be located to the left of SMA.
From week 8 to week 10 the primary intestinal loop will continue to elongate in the umbilical cord as well as grow an important part of itself. What?
The caecal bud.
What will the caecal bud eventually form?
The caecum.
What happens to the primary intestinal loop in the 10th week of development?
The abdominal cavity has grown enough to accommodate the primary intestinal loop once again. This means that the primary intestinal loop begins its retraction out of the umbilical cord into the abdominal cavity once again.
What happens during the retraction of the primary intestinal loop?
It will rotate 180 degrees once again counterclockwise upon the axis of the SMA.
What is the consequence of this rotation?
Everything will fall into place and the duodenum will lie posterior to the transverse colon.
Which limb will be the first one to retract into the abdominal cavity? Why?
The cranial limb. Because the caudal limb has the caecal bud which acts like a not preventing it from retracting first.
What is the consequence of the order of retraction of the primary intestinal loop?
That the cranial limb will be located to the left in the abdominal cavity whereas the caudal will be to the left.
The primary intestinal loop has now fully retracted back into the abdominal cavity. Where can the caecal bud and the ascending colon be found?
By the liver in the upper right quadrant.
What happens to the ascending colon and caecal bud during week 11?
The ascending colon starts to elongate and this causes the descension of the caecal bud into the lower right quadrant. The caecal bud will now form the caecum and the appendix.
Everything has now fallen into place, what is the final step to form the final product of the intestines?
The ascending colon and descending colon will have their mesentery fuse with the posterior abdominal wall and become 2nd retroperitoneal.
Give examples of abnormalities of development of the midgut.
Omphalocoele Gastroschisis Meckel’s (Ileal) Diverticulum Vitelline cyst Vitelline fistula Incomplete rotation/Non-rotation Reversed rotation Recanalisation
Explain incomplete rotation of the midgut.
This is when the primary intestinal loop only rotates once and that is during the herniation into the umbilical cord. There is no rotation as the primary intestinal loop retracts into the abdominal cavity once again.
What is the consequence of an incomplete rotation of the midgut?
A left-sided colon
Explain reversed rotation of the midgut.
When the midgut loop makes on 90 degree rotation clockwise instead. (Possibly due to the 90 degree rotation counterclockwise on herniation and then 180 degree clockwise upon retraction)
Consequence of reversed rotation of the midgut.
The transverse colon passes posterior to the duodenum.
Explain the complications of malrotation of the midgut.
Volvulus are likely to form which is when the intestine twists on its own axis causing strangulation and possible ischaemia.
It is likely to happen in non-rotation as the duodenum becomes dilated.
Also has a tendency to happen in reversed rotation as the SMA starts to compress the transverse colon.
Give three types of remnants of the yolk sac.
Vitelline cyst
Vitelline fistula
Meckel’s diverticulum
Explain vitelline cyst.
When the vitelline duct forms a fibrous strand it can also form a cyst. This is usually not harmful.
Explain vitelline fistula.
This is a direct communication between the umbilicus and the intestinal tract. This is potentially harmful and means that faecal matter can leak through the umbilicus.
Explain Meckel’s diverticulum.
When there is a small bulge in the intestinal tract with or without a remnant of the vitelline ligament.
Usually you go by there rules of 2 to explain Meckel’s diverticulum.
2% of the population has it.
It is usually 2 feet from the ileocaecal valve and usually detected in under 2 years of age.
2:1 ratio male:female
Complications of Meckel’s diverticulum.
Can come to contain ectopic gastric or pancreatic tissue and therefore become inflamed.
The reason for this is unknown.
Explain recanalisation as a consequence of the development of the midgut.
Explain its complications.
Since the primitive gut is a simple tube the cell growth can become so rapid that the lumen may become partially or completely obliterated i.e. it fills up.
If this happens it usually happens to the oesophagus, bile duct or small intestines.
In order to sort out a partially or completely obliterated part of the midgut recanalisation will occur to restore the lumen.
If it occurs it might not work at all or only work partially. THis can cause atresia or stenosis of the midgut.
Where in the midgut does atresia/stenosis most commonly occur?
In the duodenum as incomplete canalisation.
Explain pyloric stenosis.
Hypertrophy of the circular muscle in the region of the pyloric sphincter. This is not a recanalisation failure.
This is a common abnormality in the stomach of infants.
Common presentation of pyloric stenosis in infants.
Narrowing exit leads to projectile vomiting.
Will commonly be palpable due to its size.
Briefly explain gastroschisis.
Failure of closure of the abdominal wall during folding of the embryo. This leaves the gut tube and its derivatives outside the body cavity.
The gut content will not be enclosed in any peritoneum at all. Not even visceral peritoneum.
Briefly explain omphalocoele.
When the primitive intestinal loop fails to retract back into the abdominal cavity.
This means that the gut will be covered in peritoneum, however it differs from umbilical hernias as umbilical hernias will also be covered in skin and subcutaneous tissue.
What does the hindgut give rise to?
Distal 1/3 of transverse colon
Descending colon
Rectum
Superior part of anal canal
Epithelium of urinary bladder
The anal canal is divided by an anatomical landmark into a superior and inferior part.
What is dividing it?
The pectinate line.
Why is the pectinate line important?
Because it indicates the differences in arterial supply, venous, lymphatic drainage and innervation.
Blood supply, innervation, type of epithelium and lymphatic drainage of the anal canal above the pectinate line.
Inferior mesenteric artery
Innervated by the S2, S3 and S4 pelvic parasympathetic
Lined with columnar epithelium
Lymph drainage via internal iliac nodes.
Blood supply, innervation, type of epithelium and lymphatic drainage ofthe anal canal inferior to the pectinate line.
Pudendal artery
Innervated by the S2, S3, S4 pudendal nerve
Lined with stratified epithelium
Lymphatic drainage via superficial inguinal nodes
Why is it important to note the differences of the anal canal above and below the pectinate line?
Because the superior part will only have visceral innervation meaning the only sensation will be stretching.
Below the pectinate line there will be somatic innervation and pain, temperature and touch will be sensed.
Give examples of hindgut abnormalities due to its development.
Imperforate anus
Anal and anorectal agenesis
Hindgut fistulae
Structures of the midgut and hindgut which still have their mesenteries.
Jejunum
Ileum
Appendix
Transverse colon
Sigmoid colon
Structures of the midgut and hindgut with fused mesenteries to the posterior abdominal wall.
Duodenum
Ascending colon
Descending colon
Rectum
