Pancreas Anatomy and Development

• The pancreas originates from two separate buds that rotate and fuse during development.
• The ventral bud forms the posterior/inferior head and uncinate process.
• The dorsal bud forms the body, tail, and anterior head.
• Abnormalities in pancreatic development can lead to conditions such as annular pancreas and heterotopic pancreas.
• The pancreas is located in the retroperitoneum, behind the stomach and transverse colon, and in front of the aorta and vena cava.
• It typically weighs between 40-180g and is 15-20cm long, lacking a distinct capsule.

Exocrine Component

• The exocrine component consists of lobular units of acini, making up 80% or more of the gland.
• Acini drain into increasingly larger ducts, merging into the main pancreatic duct (Wirsung) and accessory duct (Santorini).
• The duct of Wirsung and common bile duct usually end together in the papilla of Vater.
• The duct of Santorini ends separately at the minor duodenal papilla.
• Normally, the Wirsung and Santorini ducts are connected; if not, pancreas divisum results.
• Acinar cells are large, pyramidal, and polarized.
• The luminal border has microvilli, the apical cytoplasm has eosinophilic zymogen granules, and the basilar cytoplasm is basophilic.
• Centroacinar cells are located in the acinus' central portion, with paler cytoplasm and oval nuclei.
• Intercalated ducts drain the acinus and merge to form intralobular ducts, which are lined by cuboidal cells with pale cytoplasm.
• Interlobular ducts are larger, lined by mucin-producing columnar cells.
• Wirsung and Santorini ducts' microscopic composition is similar to interlobular ducts, but with more goblet cells.

Endocrine Component

• The endocrine component is mainly the Langerhans islets.
• Islets make up 1%-2% of the adult pancreas, but a larger proportion (~20%) at birth.
• Islets can be compact (90%) or diffuse, with diffuse islets in the ventral bud-derived portion of the head.
• Islets are not encapsulated and have a rich capillary network.
• The average islet diameter is 225 μm; over 400 μm is considered abnormal.

Islet Cell Types

• B cells: Insulin-secreting cells, constituting two-thirds to three-fourths of the islet, located in the islet center.
• B cells also secrete islet amyloid polypeptide (IAPP).
• A cells: Glucagon-secreting cells, one-fifth to one-fourth of islet cells, mainly at the periphery.
• D cells: Somatostatin-secreting cells, less than 10% of islet cells, scattered throughout the islets.
• PP cells: Pancreatic polypeptide-secreting cells, scarce and usually in diffuse islets.
• Pancreatic endocrine cells are of endodermal origin.
• Islet cells stain with synaptophysin, chromogranin, and CD56, and are typically pancytokeratin negative.

Other Endocrine Cells

• Endocrine cells can be found in connection with ducts and acini.
• Most of these cells are Kulchitsky (serotonin-producing) and PP type.
• G (gastrin-producing) cells are not normally present in the adult pancreas.

Nesidioblastosis

• Nesidioblastosis is pancreatic islet hypertrophy in the context of hyperinsulinemic hypoglycemia.
• It is seen in infants with persistent neonatal hyperinsulinemic hypoglycemia, infants of diabetic mothers, and in other conditions.
• Morphologically, islets are associated with ducts, forming ductuloinsular complexes; islet cell hypertrophy and beta cells with pleomorphic nuclei can be seen.
• Focal and diffuse forms of nesidioblastosis exist.
• Morphologic changes can be seen in normal infants.
• These changes have occasionally been reported in adult patients with persistent hyperinsulinemic hypoglycemia and after bariatric surgery.