A case of annular pancreas with Wirsung's duct encircling the duodenum: embryological hypothesis based on cholangiopancreatographic and immunohistochemical findings.

We present a resected case of annular pancreas in which Wirsung's duct encircled the duodenum and continued directly to the main pancreatic duct in the body and tail. Furthermore, Wirsung's duct coursed along the right side of the lower bile duct near the major duodenal papilla. Histologically, the islets of Langerhans in the annular pancreas were irregular in shape and were characterized by a striking abundance of pancreatic polypeptide (PP)-positive cells. The PP-rich area that encircled the duodenum was fused with the PP-poor area in the head of the pancreas. The following embryological hypothesis is proposed. The tip of the ventral pancreatic anlage adhered to the duodenal wall and stretched to form a ring during clockwise rotation. The rotation was incomplete, and the pancreatic duct did not cross over the lower bile duct. Since there was adequate ventral anlage in the lower part of the head of the pancreas, fusion between the ducts of the ventral and dorsal anlagen did not occur. The tip of the ventral anlage overgrew and adhered to the dorsal anlage, and the annular duct fused with the main duct of the dorsal anlage.