Embryology Flashcards
Where does the foregut end and start?
Distal oesophagus to halfway along duodenum
Where does the midgut end and start?
Halfway along duodenum to the 2/3 along way of transverse colon
Where does the hindgut start and end?
Distal 1/3 of transverse colon to the upper anal canal
What supplies the foregut?
Coeliac trunk
Symp = greater splanchnic nerve (T5/6-T9)
Para = vagus
Visceral pain felt in epigastric region
What supplies the midgut?
Superior mesenteric artery
Symp = lesser splanchnic nerve (T10-11)
Para = vagus
Visceral pain is felt in umbilical region
What supplies the hindgut?
Inferior mesenteric artery
Symp = Least splanchnic nerve (T12-L1) and lumbar splanchnic nerves
Para = pelvic splanchnic nerves
Visceral pain is felt in suprapubic region
Stages of midgut development
Elongation
Physiological herniation (protusion of abdomen into umbilical cord) - week 6
Rotation (around axis of SMA)
Retraction (back into abdomen) - week 10
Fixation
Elongation
Elongation of yolk sac forms primary intestinal loop.
The upper part of loop (cephalic limb) goes on to make the distal part of the duodenum, the jejunum and part of the ileum)
The lower part of the loop (caudal limb) goes on to make the distal part of the ileum, caecum, appendix, ascending colon and proximal 2/3 of transverse colon
Physiological herniation
intestinal loops herniate (moves) into umbilical cord in 6th week.
This is because abdominal cavity is too small for the gut loops and liver.
Gut loop starts to rotate 90 degrees anticlockwise around the SMA
Rotation
Anticlockwise, First 90 degrees occurs as gut loop herniates. brings caudal limb more cranially.
Continued elongation
small intestine develops coils, large intestine also elongates but doesn’t coil
Retraction
Week 10, gut loop returns to abdomen. rotates further 180 degrees anticlockwise. Total roation = 270 degrees anticlockwise.
Retraction causes position of gut
Jejunum returns first to left side. Then ileum settles towards the right.
Caecum returns last to upper right quadrant.
This video shows it well: https://www.youtube.com/watch?v=AscKR_cQExY
Fixation
Some mesenteries come into close contact with posterior abdominal wall and fuse to posterior wall
Considered ‘retroperitoneal’
Fascial layer - Toldt fascia develops between the parietal peritoneum on posterior body wall and the visceral peritoneum on the organ.
Which components are fixed?
Duodenum, ascending colon, descending colon and rectum
Caecal bud and appendix
McBurneys point. Appendix can be in different positions. If tip is inflamed and makes contact with parietal peritoneum could be atypical presentation if appendix is ticked in unusual place.
Initially dull central pain in umbilical region. Visceral sensory nerves along sympathetic nerves to T10 region of spinal cord, perceived as same as wall.
When irritates parietal peritoneum then pain becomes well localised to overlying skin of anterior abdominal wall. Somatic nerves now involved.
Clockwise rotation would result in…
The duodenum lying anteriorly to the colon. Should be posterior.
Meckel’s diverticulum
Vitelline duct normally degenerates. But can persist causing out-pouching from the ilium - a ‘diverticulum’. It can ulcerate and bleed, inflammation can mimic appendicitis.
Omphalocoele
Midgut loops doesn’t return into the abdomen in the 10th week but remains in umbilical cord. Gut covered in layer of amnion. High mortality as often have other congenital malformations and chromosomal anomalies.
