Paired Ventral Pancreatic Anlage and Annular Pancreas

An annular pancreas is a rare malformation and its pathogenesis is still controversial. In the normal course of development between the 8- and 12-mm stages (sixth week), the common duct and the right portion of the ventral prim-ordium are carried dorsally around the circumference of the duodenum to lie adjacent to the dorsal pancreas. This rotation is the result of duodenal growth, during which all enlargement is from the ventral side only. The duct of the longer, dorsal pancreas anastomoses with that of the ventral pancreas to form the main pancreatic duct (duct of Wirsung), which opens into the common duct. If the proximal portion of the dorsal primordium duct persists, it forms an accessory duct (duct of Santorini). How this normal pattern is altered to produce an annular pancreas is not clear, and a number of explanations have been proposed. Tieken's theory suggests that hypertrophy of both lobes occurs, and that these eventually coalesce to form a ring; Lecco's theory proposes adhesion of the distal tip of the ventral primordium to the duodenal wall prior to its migration; Baldwin's theory is based on persistence of a hypothetical left lobe assuming that the ventral lobe is originally a paired structure; while Erimoglu's theory involves the formation of a ring by fusion of aberrant pancreatic tissue from the duodenum [1-4]. It is now generally accepted that the ring formation originates from the ventral pancreas, as suggested by Lecco (fig. 3) [2]. However, on the basis of clinicopathological analyses of pancreata with pancre-aticobiliary maljunctions, many gastroenterologists and pathologists have come to believe that the ventral pancreatic anlage is initially paired, and that the left lobe normally disappears over time, as shown by Odgers [5, 9, 10].

Annular Infantile
Fig. 3. Sections of annular pancreata, as suggested by Lecco. An infantile (a) and an adult annular pancreas (b). HE.

With improvements in imaging techniques such as computed tomography, ERCP and magnetic resonance cholangiopancreatography (MRCP), annular pancreata are being recognized with increasing frequency. Most cases have been diagnosed by ERCP and/or MRCP, although some have been discovered incidentally during surgery or autopsy [11-13]. At present, if patients with an annular pancreas have no symptoms or related complications such as weight loss due to pyloric stenosis, severe abdominal pain, obstructive jaundice or a pancreaticobiliary maljunction, etc., they are followed up conservatively. Therefore, annular pancreas case reports with a histological analysis are still rare. As most resected and/or autopsied annular pancreata that have been investigated histopathologically support Lecco's hypothesis [14, 15], there is a discrepancy between histopathological analyses of the annular pancreata and clinicopathological analyses of pancreata with a pancreaticobiliary maljunction, i.e. the former support Lecco's hypothesis of a single ventral pancreas, while the latter support Baldwin's hypothesis of paired ventral pancreata [3, 6, 14, 15]. Whether the ventral pancreatic anlage is single or paired is the most basic and important embryological point in understanding the pathogenesis of annular pancreas [16-20], because many researchers have come to believe that if the left lobe of the ventral pancreatic anlage does not disappear a pancreaticobiliary maljunction occurs [21, 22]. However, Nobukawa and colleagues and Muraoka and colleagues reported a case with an annular pancreas with a

Fig. 4. a Low-power view of a pancreatic polypeptide-stained section. The normal main pancreatic duct (arrow), the common bile duct, and an unusually large pancreatic duct (asterisk) from the annular pancreas were found. b High-power view of an HE-stained section. The unusually large pancreatic duct (asterisk) from the annular pancreatic tissue on the opposite side of the normal pancreatic head flows into the major papilla.

Fig. 4. a Low-power view of a pancreatic polypeptide-stained section. The normal main pancreatic duct (arrow), the common bile duct, and an unusually large pancreatic duct (asterisk) from the annular pancreas were found. b High-power view of an HE-stained section. The unusually large pancreatic duct (asterisk) from the annular pancreatic tissue on the opposite side of the normal pancreatic head flows into the major papilla.

persistence of the left lobe of the ventral pancreatic anlage which did not cause a pancreatobiliary maljunction [6, 23, 24]. The histogenesis of the ventral pancreatic anlage has not yet been clarified, and not even in the most recent textbooks of embryology [25-27].

Our evaluations revealed that the ring formation originated from the left lobe of paired ventral pancreata (fig. 4), thus supporting Baldwin's hypothesis. It was proved that persistence of the left lobe of paired ventral pancreata was not associated with occurrence of a pancreatobiliary maljunction.

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