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
drsaharhafeez@yahoo.com
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
•2/3rd of Large Intestine
– Cecum,
– Appendix,
– Ascending Colon,
– Transverse Colon (proximal 2/3rd )

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 6th wk and the loop keeps on growing in length with in the umbilical cord
Reduction of physiological hernia:
•By the end of 10th wk, as the abdominal cavity increases in size, the developed loops of Midgut start re-entering the abdominal cavity and by 12th 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 10th 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.