Distribution after Fusion of Ventral and Dorsal Anlagen Branch Duct Fusion of the Pancreatic Duct and Annular Pancreas

Identification of the Originating Primordium

After fusion, the 'ventral' and 'dorsal' pancreata can be distinguished [16] by the distribution of the PP islets [17, 18], which are distributed selectively in

Fig. 5. Schematic drawing of various junctions/sites between the terminal bile duct and ventral pancreatic duct system in patients with PBM. From [15] with permission.
Fig. 6. Macroscopic appearences of coronary sections of ventral (VEN) and dorsal pancreas (DOR).

the ventral pancreas. In some cases both pancreata can be identified macro-scopically (fig. 6) There are two further distinct characteristic differences. One is the shape of the islets: those in the ventral pancreas, which include abundant PP cells, are irregular, in contrast to the neatly round or oval islets found in the dorsal pancreas (fig. 7). The other is the distribution of fatty infiltration in the pancreas: there is more fat in the dorsal pancreas than in the ventral pancreas [16]. The ventral primordium forms the posterior part of the head of the pancreas, completely or partially surrounding the CBD (fig. 8) and the uncinate

Uncinate Pancreas

Fig. 7. Irregularly shaped and round or oval islets. a An irregularly shaped islet and oval islet were positively stained by the Grimelius silver method. b An irregularly shaped islet included abundant PP cells immunohistochemically, whereas an oval islet contained few PP cells, The border between both islets was identified as a fusion line of the ventral and dorsal pancreata. From [18] with permission.

Fig. 7. Irregularly shaped and round or oval islets. a An irregularly shaped islet and oval islet were positively stained by the Grimelius silver method. b An irregularly shaped islet included abundant PP cells immunohistochemically, whereas an oval islet contained few PP cells, The border between both islets was identified as a fusion line of the ventral and dorsal pancreata. From [18] with permission.

Fig. 8. Distribution of ventral and dorsal pancreas after fusion. CBD = Common bile duct; DOR = Dorsal pancreas; MPD = main pancreatic duct; SMV = superior mesenteric vein; VEN = ventral pancreas.

process. However, the dorsal bud forms the remaining ventral parts of the head, the isthmus, the body, and the tail of the pancreas.

The fusion line between both pancreata has no defined border, but it is the so-called 'locus minoris resistantiae' and it is the easiest 'pathway' for a duodenal diverticulum to penetrate the pancreas [19], as shown in figure 9.

Pancreas Divisum

The term pancreas divisum originally signified a very rare congenital anomaly in which the parenchyma of the ventral and dorsal pancreas are separated as a double pancreas. Recently, however, the term has been widely used to describe

Fig. 9. Duodenal diverticulum (arrow) penetrating into a fusion line, signified by the dotted line, of the ventral (VEN) and dorsal (DOR) pancreata. From [19] with permission.

two ductal systems that do not unite or communicate and separately drain to the two duodenal papillae [20, 21]. In this condition, pancreatic juice from the dominant dorsal moiety flows out only through the minor papilla, in which the outlet is notably small in most cases. This raises the question of whether this variation plays a role in the development of pancreatic pain or pancreatitis. The clinical relevance of pancreas divisum has been argued repeatedly [20]. Figure 10 shows an example of isolated dorsal pancreatitis associated with pancreas divisum. This condition strongly suggests inadequate drainage from the minor papilla.

Fig. 10. So-called dorsal pancreatitis in patients with pancreas divisum. Tissue of the ventral pancreas did not show any abnormal findings (a), whereas the dorsal pancreas tissue demonstrated marked atrophy or disappearance of acinar cells and inter- and intralobular fibrosis (b). HE stain. X100 (a), X100 (b).

Fig. 10. So-called dorsal pancreatitis in patients with pancreas divisum. Tissue of the ventral pancreas did not show any abnormal findings (a), whereas the dorsal pancreas tissue demonstrated marked atrophy or disappearance of acinar cells and inter- and intralobular fibrosis (b). HE stain. X100 (a), X100 (b).

Branch Duct Fusion of the Ventral and Dorsal Pancreatic Duct

A case of fusion via two so-called inferior branches between the ventral and dorsal pancreatic ducts was studied both macroscopically and immunohistochem-ically, based on the organogenesis of the pancreas [22], as shown in figure 11. Radiologically, branch fusion seemed to be composed of an inferior branch of the ventral pancreatic duct and an inferior branch of the dorsal pancreatic duct. By mapping PP islets in the material obtained by pancreatoduodenectomy, however, the branch was identified as a branch of the dorsal pancreatic duct. Thus, fusion between two inferior branches was not established, but was found to consist of an inferior branch of the dorsal pancreatic duct connected with the ventral pancreatic duct. We therefore challenge the concept of the ansa pancreatica [23].

Annular Pancreas

An annular pancreas consists of a collar or ring of pancreatic tissue surrounding the second part of the duodenum and continuing into the head of the pancreas on either side. The gut lumen is usually narrowed.

According to Suda [24], the anomalous phenomenon of annular pancreas can be explained clearly as the result of two fusions between the PP-rich and PP-poor areas (fig. 12). The posterior fusion is considered to be part of the normal developmental process because of the arrangement of the duct system. The fusion of the anterior portion is thought to be anomalous.

Annular pancreatic tissue was thus demonstrated to arise from the ventral primordium, which supports Lecco's theory [25], the most reliable one, that the free end of the ventral anlage is fixed.

Nobukawa et al. [26] describe an annular pancreas originating from paired ventral pancreata, which supported Baldwin's hypothesis [27], and attempted to

Accessory Duct Pancreas

Fig. 11. Branch duct fusion of the ventral and dorsal pancreatic duct. ERCP. a The ventral (VPD) and dorsal (DPD) pancreatic ducts fuse via the 'two inferior branches'. b From the distribution of the PP islets, the branch fusion is shown to consist of a side-to-end fusion between the ventral pancreatic duct and the inferior branch (arrow) of the dorsal pancreatic duct. AP = Accessory papilla; CBD = common bile duct; PV = papilla of Vater. From [22] with permission.

Fig. 11. Branch duct fusion of the ventral and dorsal pancreatic duct. ERCP. a The ventral (VPD) and dorsal (DPD) pancreatic ducts fuse via the 'two inferior branches'. b From the distribution of the PP islets, the branch fusion is shown to consist of a side-to-end fusion between the ventral pancreatic duct and the inferior branch (arrow) of the dorsal pancreatic duct. AP = Accessory papilla; CBD = common bile duct; PV = papilla of Vater. From [22] with permission.

clarify the pathogenesis. The patient was a 1-day-old Japanese male newborn, born after 32 weeks of pregnancy. He died the next day from respiratory failure due to esophageal atresia. Autopsy incidentally demonstrated an annular pancreas that was examined histologically. An unusually large pancreatic duct encircled by pancreatic tissue passed around the duodenum, and the duct was confirmed to connect with the major papilla after joining with the common channel (fig. 13), as indicated later. The islets of the encircling pancreas were positive for pancreatic polypeptide. The normal main and accessory pancreatic duct were also identified. The former and the CBD formed the common channel. Histologic and immunohistochemical evaluation demonstrated that the ring formation originated from the left lobe of the paired ventral pancreata.

Absence of Pancreatic Body and Tail

Congenital aplasia of the body and tail of the pancreas is an extremely rare anomaly. Ghon and Roman [28] reported the case of a 14-year-old boy in whom the head of the pancreas was flat and disk shaped, but neither the body nor tail

Fig. 12. Annular pancreas derived according to Lecco's theory. a Endoscopic pancreatogram in a case of annular pancreas (a 64-year-old woman). An additional ring-like pancreatic duct (arrows) is observed surrounding the second portion of the duodenum. From [31] with permission. b Macroscopic appearance of the serial cut-surfaces of the resected specimen. The patient had carcinoma in the middle part of the extrahepatic bile duct, and pancreaticoduodenectomy was performed. The dotted line signifies the fusion line between the ventral and dorsal pancreata. From [24] with permission. c A scheme of the pancreatic duct of the case in (b). The ventral (VEN) and dorsal pancreas (DOR) were determined by the procedure demonstrated in figure 7. A pancreatic duct (arrows) started from the anterior portion of the annular tissue to the lateral and posterior portions, finally connecting to the MPD. From [24] with permission. AP = Accessory papilla; CBD = common bile duct; MPD = main pancreatic duct; PP = pancreatic polypeptide rich area; PV = papilla of Vater.

Annular Duct Pancr

Fig. 13. Annular pancreas, supporting Baldwin's hypothesis. Ring formation originated from the left lobe (L-VEN) of the paired ventral pancreata. AP = Accessory papilla; CBD = common bile duct; DOR = dorsal pancreas; MPD = main pancreatic duct; PV = papilla of Vater; R-VEN = right lobe of ventral pancreas.

Fig. 13. Annular pancreas, supporting Baldwin's hypothesis. Ring formation originated from the left lobe (L-VEN) of the paired ventral pancreata. AP = Accessory papilla; CBD = common bile duct; DOR = dorsal pancreas; MPD = main pancreatic duct; PV = papilla of Vater; R-VEN = right lobe of ventral pancreas.

of the pancreas nor the minor papilla was observed. Based on the distribution of the ventral and dorsal pancreas after fusion of both anlagen, the term 'aplasia of the body and tail of the pancreas' should be reserved for conditions such as those reported by Ghon and Roman [28]. Therefore, this anomaly is not present when both the duct of Santorini and the minor papilla are present. Congenital aplasia of the body and tail of the pancreas derives from a defect of the dorsal pancreatic anlage and should not be considered a type of acquired atrophy [29].

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