Development of midgut and pancreas

In this presentation the development of Small intestine and Pancreas has been discussed. The viewer would be able to understand the concept of physiological herniation and rotation of the Primary intestinal loop with in the connecting stalk.

1. DEVELOPMENT OF MIDGUT
(Small Intestine) AND PANCREAS
Dr.Sahar Hafeez
[email protected]
2014

2. Learning Objectives
The students should be able to;

•Enlist the derivatives of Midgut.

•Explain the dual origin of Duodenum

•Explain the process of physiological herniation of the Midgut loop and the
reduction of hernia.

•Describe the final positioning of the intestinal loops within the abdominal
cavity

•Enlist the common malformations of Midgut

•Explain the normal development of Pancreas

•Enlist the common malformation related to the development of Pancreas

3. Derivatives of Midgut:

•Distal half of the Duodenum
•Entire Small Intestine
– Jejunum,
– Ileum
•2/3
rd
of Large Intestine
– Cecum,
– Appendix,
– Ascending Colon,
– Transverse Colon
(proximal 2/3
rd
)

4. Dual origin of Duodenum: As the Midgut starts from the point of Ampulla of
Vater, therefore, the upper half of Duodenum is a part of Foregut, while, the lower
half of Duodenum is a part of Midgut)
EXTENT OF MIDGUT
Starts from below the Major duodenal papilla and terminates
at the junction of the proximal two-third of the Transverse
Colon with the distal one-third.

5. The Midgut;
•dorsally is suspended in the
midline by a double-layered
‘Dorsal mesentery’
•ventrally communicates with the
yolk sac by way of a wider
Vitelline duct/Yolk stalk

6. Development of Primary intestinal loop
•The middle wider part of the primitive gut tube which is in communication with
the yolk sac elongates and forms a u-shaped loop, the Primary intestinal
loop/Midgut loop.

•The loop is suspended from dorsal abdominal wall by an elongated mesentery.

Physiological Herniation of gut:
•There is a rapid growth of the loop & as there is very little space available with
in the abdomen (because of massive Liver & a pair of Kidneys), the midgut loop
herniates into the connecting stalk/Umbilical cord.
•Herniation starts from the 6
th
wk and the loop keeps on growing in length with
in the umbilical cord

Reduction of physiological hernia:
•By the end of 10
th
wk, as the abdominal cavity increases in size, the developed
loops of Midgut start re-entering the abdominal cavity and by 12
th
wk all of the
Midgut re-enters & settled in the abdominal cavity
The tip of the loop is connected with the yolk sac by a
Vitelline duct & this connection persists till the end of 10
th

wk.

7. Rotation of Midgut with in the Umbilical cord
•When the Midgut loop enters the cord, it has a ‘Cranial limb’ & a ‘Caudal limb’
and the “superior mesenteric artery”(SMA) is the axis of the loop.

•The caudal limb soon develops a swelling at its end, the ‘Cecal bud’. This bud
acts like a marker throughout the rotation.

•Immediately after entering the cord, the loop rotates 90⁰ counterclockwise on
the axis of artery. Therefore, the cranial limb comes to lie on the right & caudal
to the left.

•The original cranial limb shows a rapid growth in its length as compare to the
caudal limb.

•Then another 90⁰ counterclockwise rotation happens and as a result, the
original cranial limb will become caudal and the original caudal limb will
become cranial
Total 180⁰ counterclockwise rotation along
the axis of SMA takes place with in the cord

9. A:
Midgut loop with SMA. A
cranial limb & a caudal limb
with Cecal bud

10. B:
Counterclockwise 90⁰
Rotation. Cranial limb
becomes right & Caudal
becomes left.
C:
Another Counterclockwise
90⁰Rotation. Original Cranial
limb becomes caudal and
original Caudal limb becomes
Cranial.

11. D:
Rapidly growing coils of
Jejunum & Ileum in the
original cranial limb
E:
Re-entry of the coils of small
intestine. Note that original
cranial limb is entering first
and the last to enter is the
Cecum with Appendix

13. Blood supply of Midgut

•The Superior Mesenteric Artery, a branch of Dorsal Aorta will supply all
the structures of the Midgut.

14. The first structure to enter the
abdominal cavity are the coils of
Jejunum & they will occupy the left
upper corner of the cavity below
the stomach

Then the coils of Ileum will enter
and will occupy the center of the
abdominal cavity (umbilical region)

The last structure to re-enter will
be the Cecum with Appendix (it will
be pushed down by the structures
already present in the right
quadrant and further pulled down
by the growing coils of Ileum.
Ultimately it settles down in the
right Iliac fossa.

Final Positioning of the structures of Small intestine

15. Malformations of the Midgut
Meckel’s Diverticulum:
Present in 2-4% of people, a small portion of Vitelline duct persists. It is located about
40 – 60 cm away from the ‘Ileocecal valve’ on the anti-mesenteric border of the Ileum.

Vitelline Cyst:
In this case both ends of the duct are transformed into fibrous cords, while the middle
portion forms a large cyst.

Vitelline Fistula:
Sometimes the Vitelline duct remains patent over its entire length, thus forming a
direct communication b/w the umbilicus & intestinal tract.

16. Omphalocele:
Sometimes the intestinal loops fail to return to the abdominal
cavity. The loops remain in the umbilical cord and at birth
present as a large swelling in the umbilical cord covered only by
amnion.

17. Abnormal rotation of Intestinal loop
A.Left-sided Colon: If the intestinal loop just rotates 90⁰
counterclockwise in total, then the Colon and Cecum are the first parts of
the gut to re-enter the abdomen. They settle in the left side of the cavity
instead of right side.
B.Reverse Rotation: In some cases the primary intestinal rotates 90⁰
clockwise instead of counterclockwise. As a result, the transverse colon
passes behind the duodenum while entering the abdomen.

18. Development of Pancreas


•Pancreas is formed by two buds originating from the endodermal lining of Duodenum

•The Dorsal pancreatic bud is located in the dorsal mesentery
•The Ventral pancreatic bud is closely related to the Bile duct.

•With the rotation of Duodenum, the ventral pancreatic bud migrates dorsally along
with the Bile duct & finally comes to lie immediately below & behind the Dorsal
pancreatic bud.

•The parenchyma & the duct systems of the dorsal & ventral buds also fuse.

•Ventral bud forms only the Uncinate process & inferior part of head of Pancreas. The
rest of the Pancreas is formed by the Dorsal bud

•Main Pancreatic duct (of Wirsung) is formed by the distal part of dorsal pancreatic
duct and the entire ventral pancreatic duct.

•The proximal part of Dorsal pancreatic duct persists as the Accessory pancreatic duct
(of Santorini)

19. Development of Pancreas

20. Congenital Malformations of Pancreas
Annular Pancreas:
•Sometimes the ventral bud develops as 2
lobes
•The right lobe of the bud migrates along
its normal route but the left lobe migrates
in the opposite direction.
•As a result, the duodenum is surrounded
by pancreatic tissue which forms a ring
around the duodenum (Annular Pancreas)
•Sometimes it constricts the duodenum
and causes complete obstruction.

Heterotopic Pancreatic tissue:
•Most commonly found in the mucosa of
Stomach and in Meckel’s diverticulum.