The pancreas develops from ventral and dorsal buds (anlage) each with its own duct originating from the primitive intestine, which at a later stage develops into the second, or descending, portion of the duodenum.1
The dorsal bud, forming part of the head, the body and the tail of the pancreas, grows toward the spleen. The ventral bud is divided into right and left parts. While the left part atrophies, the right one rotates dorsally and forms the remainder of the head of the pancreas.
Failure of rotation of this ventral pancreatic bud together with rotation of the duodenum, results in the encirclement of the second part of the duodenum in its upper two-thirds. Usually the encirclement is complete, but sometimes there is a gap anteriorly.
The annular pancreatic tissue usually drains through a duct which passes around the entire duodenum before entering the main pancreatic duct, or it opens
VAN der HORST LFVP. Annular Pancreas. Arch Surg. 1961;83(2):249-252. doi:10.1001/archsurg.1961.01300140091017