Development of pancreas and annular pancreas
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Development of the Pancreas and Annular Pancreas
1. Overview
The pancreas develops from posterior foregut endoderm and arises as two distinct outgrowths - a dorsal bud and a ventral bud - that later fuse into a single organ. This process occurs between the 5th and 8th weeks of embryonic life.
2. The Two Pancreatic Buds
Dorsal Pancreatic Bud
- Appears first, slightly cranial to the ventral bud
- Develops within the dorsal mesentery (dorsal mesogastrium)
- Forms the tail, body, and superior portion of the head of the pancreas
- Contains the dorsal duct, which becomes the distal portion of the main pancreatic duct (of Wirsung) and the entire accessory duct (of Santorini)
- Molecular induction: FGF2 and Activin (a TGF-β family member), produced by the notochord and dorsal aortic endothelium, repress SHH expression in the gut endoderm destined to become the dorsal bud, thereby allowing pancreatic specification
Ventral Pancreatic Bud
- Arises as an outgrowth of the hepatic diverticulum, near the entry of the bile duct into the duodenum
- Has a bilobed (bifid) origin - the left ventral bud normally regresses; only the right portion completes rotation
- Grows within the ventral mesentery
- Forms the uncinate process and inferior part of the head
- Contains the ventral duct, which becomes the proximal portion of the main pancreatic duct (of Wirsung)
- Molecular induction: the ventral bud is induced by visceral mesoderm
Both processes lead to upregulation of PDX-1 (Pancreatic and Duodenal Homeobox 1 gene), the master regulator of pancreatic development.
3. Rotation and Fusion (5th-8th weeks)
E-G: Rotation of the ventral pancreatic bud (with the bile duct) around the duodenum, followed by fusion with the dorsal bud and formation of the main and accessory pancreatic ducts - The Developing Human, Fig. 11.10
As the duodenum rotates to the right and becomes C-shaped, the ventral pancreatic bud (carried along with the bile duct) swings posteriorly and to the left around days 37-42 of gestation. It comes to lie posterior to the dorsal bud, and the two buds then fuse.
C & D: Fusion complete - the pancreas now lies along the dorsal abdominal wall in retroperitoneal position - The Developing Human, Fig. 11.10
4. Duct System After Fusion
After the buds fuse, their ducts anastomose:
| Duct | Formed From | Opens Into |
|---|---|---|
| Main pancreatic duct (of Wirsung) | Entire ventral duct + distal part of dorsal duct | Major duodenal papilla (with bile duct) |
| Accessory pancreatic duct (of Santorini) | Proximal part of dorsal duct | Minor duodenal papilla (~2 cm cranial to main duct) |
Fusion result: the dorsal pancreas (red) and ventral pancreas (blue) with their duct anatomy - Langman's Medical Embryology
5. Cellular Differentiation
After bud fusion, all three parenchymal cell types - acinar, ductal, and islet cells - differentiate from a common pancreatic progenitor lineage.
- Pancreatic islets of Langerhans develop from parenchymal pancreatic tissue in the 3rd month; insulin secretion begins at ~5th month
- Islet cell specification is controlled by paired homeobox genes:
- PAX4 + PAX6 co-expression → β cells (insulin), δ cells (somatostatin), γ cells (pancreatic polypeptide)
- PAX6 alone → α cells (glucagon)
- Visceral mesoderm surrounding the pancreatic buds forms the pancreatic connective tissue and stroma
Islet Cell Proportions (mature pancreas)
| Cell Type | Proportion | Hormone |
|---|---|---|
| Beta (β) | 50-80% | Insulin, amylin |
| PP / F cells | 10-35% | Pancreatic polypeptide |
| Alpha (α) | 5-20% | Glucagon |
| Delta (δ) | ~5% | Somatostatin |
| Epsilon (ε) | Rare | Ghrelin |
6. Final Position
As the stomach, duodenum, and ventral mesentery rotate, the fused pancreas comes to lie along the dorsal abdominal wall in a retroperitoneal position.
7. Annular Pancreas
Definition
A congenital anomaly in which a thin band of pancreatic tissue forms a ring (annulus) around the descending (second) part of the duodenum, at the preampullary level. Incidence: approximately 1 in 1,000 (imaging studies) to 3 in 20,000 (autopsy studies).
Embryologic Mechanism
A: Bifid ventral bud. B: The two segments rotate in opposite directions encircling the duodenum. C: Resulting annular pancreas with site of obstruction - The Developing Human, Fig. 11.11
The defect arises by one of two mechanisms:
- The ventral bud splits (becomes bifid) - the right portion migrates normally (posteriorly), but the left portion migrates in the opposite direction (anteriorly). The two segments then encircle the duodenum and fuse with the dorsal bud, forming a ring.
- Failure of the left ventral bud to regress - persistence of the left ventral lobe contributes to annular tissue.
Both result in the same anatomic consequence: pancreatic tissue encircling and potentially compressing the duodenum.
Associations and Genetics
- Trisomy 21 (Down syndrome) - most common association
- Cardiac defects
- Intestinal malrotation
- Duodenal atresia
- Genitourinary anomalies
- Tracheoesophageal fistula
- Females affected more frequently than males
- Familial cases reported (siblings, identical twins); microduplication on chromosome 6q24.2 (utrophin gene) identified in familial cases
- Shh and Ihh gene defects show 42-85% incidence of annular pancreas in mouse models
Clinical Presentation
Two peaks of presentation:
| Age Group | Symptoms |
|---|---|
| Neonates / Infants | Non-bilious vomiting, feeding intolerance, failure to thrive; complete or partial duodenal obstruction |
| Adults (4th-7th decades) | Abdominal pain, pancreatitis, biliary obstruction, nausea, vomiting, bloating |
- Prenatal: duodenal obstruction prevents the fetus from swallowing amniotic fluid → polyhydramnios; may be detected on prenatal ultrasound
- Inflammation (pancreatitis) in the annular tissue can worsen or precipitate obstruction
Investigations
- Children: abdominal X-ray (classic "double bubble" sign), ultrasound, upper GI series
- Adults: CT scan, MRCP, or ERCP; diagnosis sometimes made at laparotomy
- Classic radiological sign: mid-duodenal stricture with proximal dilatation on barium contrast upper GI series
Treatment
- Duodenoduodenostomy - surgical bypass of the obstructed segment; treatment of choice in children and many adults
- The annular tissue must never be divided (contains functional pancreatic tissue and the pancreatic duct may run through the annulus)
- Complex pancreatic surgery may be required in adults
- Long-term risks: increased risk of acute/recurrent pancreatitis and pancreaticobiliary neoplasia (including duodenal carcinoma); ongoing surveillance recommended
8. Other Developmental Anomalies of the Pancreas
| Anomaly | Mechanism | Clinical Significance |
|---|---|---|
| Pancreas divisum | Failure of dorsal and ventral duct systems to fuse (up to 10% of population) | Accessory duct becomes main conduit; possible recurrent pancreatitis |
| Ectopic pancreatic tissue | Separate nests of pancreatic tissue | Found in gastric mucosa, proximal duodenum, jejunum, pyloric antrum, Meckel's diverticulum; usually asymptomatic |
| Pancreatic agenesis | Failure of bud formation; PDX-1 mutations | Rare; associated with neonatal diabetes |
Sources:
- The Developing Human: Clinically Oriented Embryology (Moore et al.), Ch. 11 - pancreatic buds and annular pancreas
- Langman's Medical Embryology, Ch. 15 - molecular regulation, PAX4/PAX6 signaling, annular pancreas mechanism
- Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Ch. 55 - embryonic/fetal development, islet cell proportions, annular pancreas clinical details
- Gray's Anatomy for Students, Ch. 4 - bifid ventral bud mechanism, polyhydramnios
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