Embryology of the Midgut and Hindgut

Question 1

endoderm gives rise to

Answer 1

• epithelium of the bowel
• hepatocytes of the liver
• endo and exocrine cells of the pancreas

Question 2

visceral mesoderm gives rise to

Answer 2

• muscle wall
• connective tissue - for the wall, for the pancreas and liver
• visceral peritoneum

Question 3

3 developmental parts of the gut

Answer 3

• Foregut; starting at the lower end of the hypopharynx
• Midgut; starting at the third part of the duodenum
• Hindgut starting two third of the way along the transverse colon

Question 4

arteries and nerves

Answer 4

• Foregut; supplied by the coeliac trunk and innervated by the greater splanchnic nerve (T6-T9)
• Midgut; supplied by the superior mesenteric artery and innervated by the lesser splanchnic nerve (T10-T11)
• Hindgut supplied by the inferior mesenteric artery and innervated by the least splanchnic nerve (T12 (L1))

Question 5

referred pain

Answer 5

• Painful stimuli stimulate sensory nerves in the sympathetic splanchnic nerves.
• These nerve fibers enter the central nervous system through the spinal nerve roots
– T1-T5 for the heart and lungs
– T6-T9 for the foregut
– T10-T11 for the midgut
– T12 (L1) for the hind gut
Pain is felt in the skin that is supplied by the same nerve roots
– T1-T5 for the heart and lungs
– T6-T9 for the foregut
– T10-T11 for the midgut
– T12 (L1) for the hind gut
Usually, the pain is felt at the front rather than the whole dermatome

Question 6

development of the tube

Answer 6

• Stage 1, elongation
• Stage 2, physiological herniation
• Stage 3, rotation
• Stage 4, retraction
• Stage 5, fixation

Question 7

stage 2-physiological herniation

Answer 7

• During the 6th week of development, the loop of midgut elongates rapidly, particularly at its cephalic limb.
• This rapid growth, along with the enlargement of the liver and other abdominal viscera, makes the abdominal cavity too small to contain the primary intestinal loop, thus the midgut is pushed out into extraembryonic cavity within the umbilical cord.
(blue circle is where umbilical cord is attached to embryo)
Herniation as goes in to umbilical cord then comes back out, elongation taking place outside embryo in umbilical cord
Some babies are born without pulling the bowel back in to abdomen - exomphalos

Question 8

stage 3-rotation

Answer 8

• The elongated loop of midgut rotates 270° in an anticlockwise direction as it is viewed from the front, the rotation is around an axis formed by the superior mesenteric artery. The midgut loop rotates 90° during herniation, and 180° during the return of that loop into the abdominal cavity
• Elongation of the small intestine limb (cephalic limb) continues during the process of rotation, thus the jejunum and ileum form a number of coiled loops.
• The large intestine limb also elongates considerably, but it does not form coils.

Question 9

Stage 4-retraction

Answer 9

• During the 10th week, the herniated midgut starts to return back into the abdominal cavity, the cause for this return could be due to the regression of the mesonephric kidney, reduced growth of the liver and expansion of the abdominal cavity.
• The Jejunum is the first part that returns back to the abdomen and settles in its left side, the following returning intestinal coiled loops gradually located more and more to the right.
• The caecal bud which appears as a small dilatation in the distal limb of the intestinal loop, is the last part of the midgut that returns back to the abdomen. It settles down at the right upper part of the abdomen below the Rt. Lobe of the liver.
• Fixation of the hepatic flexure of the colon and elongation of the ascending colon brings the caecum gradually to the Rt. Iliac fossa, and placement of the ascending colon and hepatic flexure in the Rt. Side of the abdomen.

Question 10

Stage 5 – fixation

Answer 10

• During this stage, some of the gut mesenteries come to lie against the back of the abdomen and fuse into position.
• These parts of the bowel are then fixed to the posterior abdominal wall with an anterior single layer of peritoneum.
• They are now in a retroperitoneal position

Question 11

fixed or mobile?

Answer 11

•	Fixed;
–	Duodenum (except the first 1cm)
–	Ascending colon
–	Descending colon
–	Rectum
•	Mobile;
–	Stomach
–	Jejunum and ileum
–	Appendix (caecum)
–	Transverse colon
–	Sigmoid colon