The pancreas develops from both ventral and dorsal pancreatic anlages, which arise from the endodermal epithelium of the duodenum. The ventral anlage becomes the dorsocaudal portion and uncinate process of the head of the pancreas, and the dorsal anlage becomes the ventrocephalic portion of the head and body and tail of the gland.
The duct of the ventral anlage and of the dorsal anlage distal to the junction of both ducts serve as the main pancreatic duct (duct of Wirsung). The duct of the dorsal anlage proximal to the junction becomes the accessory pancreatic duct (duct of Santorini). The pancreatic duct system is arranged on the main channel principle, that is, 15 to 30 side branches of about equal thickness draining from both above and below into a main channel, namely the Wirsung duct.10 Secondary ducts enter the main pancreatic duct in a herringbone pattern.
The duct of Wirsung is the main pancreatic duct and opens into the duodenum at the papilla of Vater (major papilla). It drains the major part of the gland and begins in the tail by the convergence of several small ducts (secondary ducts). The main pancreatic duct begins at the terminus of the tail of the gland. It runs closer to the posterior surface and midway between the superior and inferior margins through to the tail and body and runs nearer to the dorsal and superior surfaces of the gland than to the ventral and inferior surface in the head. Its course is horizontal in the tail and body and it moves posteriorly and inferiorly to form an arch (convex to right) in the head. It often receives secondary ducts that drain the posteroinferior part of the head of the pancreas (ventral pancreas), including the uncinate process. The diameter of the duct of Wirsung averages 3 mm11 and is 4.8 mm in the head, 3.5 mm in the body, and 2.4 mm in the tail, respectively.
The accessory pancreatic duct of Santorini, which embryologically corresponds to the head part of the dorsal duct, is present in 99% of all humans. This duct drains the anterosuperior part of the head and may open into the duodenum at the minor papilla, about 2 cm cephalad to the papilla of Vater (major papilla). It also anastomoses with the duct of Wirsung near the neck of the pancreas. When pancreatitis is mainly located in the pancreas drained by the Santorini duct, such pancreatitis is termed groove pancreatitis.
In the head of the pancreas, the lower 3 to 5 cm part of the CBD runs in a groove in the posterior surface of the head of the pancreas and may seem to be embedded in the pancreatic tissue. In all but a few of the cases of the latter type, however, the duct lies in a cleft in the pancreas and is overlapped by the pancreatic tissue. The bile duct usually passes down close to, or runs in direct contact with, the left border of the second part of the duodenum; but it may be as much as 2 cm from the duodenal wall before it begins to incline to the right to reach and then pass through the wall of the second part of the duodenum. Its progress to the right may be in a straight line or toward the duodenum almost at a right angle.
In the head, the main pancreatic duct inclines caudally and dorsally and passes to the left caudal side of the intrapancreatic portion of the CBD, with which it usually unites while running obliquely through the duodenal wall. It finally opens into the major duodenal papilla (the papilla of Vater), a prominence located on the posteromedial wall of the second portion of the duodenum. Because of the anatomical intimacy of the distal bile duct and the pancreas head, the distal CBD tends to be at high risk of invasion by pancreatic carcinoma, which develops in the head of the pancreas and often produces obstructive jaundice.
The ampulla of Vater is the vase shaped expansion of the duodenal wall through which the CBD and the main pancreatic duct enter the duodenum. The duodenal mucosa and musculature (the muscle of Oddi) form the outer wall of the ampulla and project into the duodenum as a papilla, the major papilla. The ampullary inner surface is lined with a biliary ductal epithelium. There is considerable controversy regarding the opening of the ampulla of Vater, the openings of the CBD and pancreatic duct into the complex. There is even doubt as to the existence of an ampulla in some cases.
In about one-third of all patients, the common bile and main pancreatic ducts enter into a common ampulla, from 3 to 14 mm, from the apex of the duodenal orifice, so that a true "common channel" is formed. When carcinoma arises in the papilla of Vater, the CBD and main pancreatic duct are affected together, thus producing a "double duct sign" on images.
2.5.3 Variations of Main and Accessory Pancreatic Ducts
During the embryologie development of the main and accessory pancreatic ducts, several variations may occur.12-13 The usual pattern of the pancreatic duct is that the accessory pancreatic duct is connected with the main pancreatic duct and then opens into the duodenum. The accessory duct is smaller in caliber than the main duct. There are hypoplasia and aplastic variations of the accessory duct and nonunion of both ducts (Figure 2.2). In these patterns, the main pancreatic duct carries most or all of the pancreatic secretion. In less common patterns, the accessory pancreatic duct carries most or all of the secretion. The variation is pancreas divisum and the details of this condition are described in Section 2.5.4.
A communication of the accessory and main ducts is present in 90% of patients and the patency of the accessory duct orifice is reported in 35
The chief variations of the pancreatic ducts
The chief variations of the pancreatic ducts
Figure 2.2 Main and accessory pancreatic ducts.
Figure 2.3 Prominent Santorini duct. The Santorini duct is larger than the Wirsung duct in diameter.
to 60% of cases.13 The frequency of the variations of the ducts is 60% in the usual configuration, 30% in the suppression of the accessory duct, and 10% in the suppression of the main duct.14 There seems to be at least some agreement that in about 90% of all pancreases, the sole or main excretory channel is the duct (of Wirsung) that opens at the major duodenal papilla. In about 50 to 70% of pancreases, an accessory (of Santorini) is present, thus resulting in the transmission of various amounts of secretion, from none up to all the gland's output into the duodenum at the minor duodenal papilla (Figure 2.3).
Two ducts of Santorini with two minor papilla in the duodenum and with a communication with the major duct system were found in a single case by Berman et al.15 in their series of 130 specimens.
It has already been mentioned that variations in the course of the main pancreatic duct through the head of the pancreas are by no means uncommon. One striking variation in the course of the main pancreatic duct through the head of the pancreas is a complete loop of the duct (Figure 2.4). The looped type of duct was first recognized by Baldwin16 and Rienhoff and Pickrell17 and it was also demonstrated radiographically in autopsy materials by Newman et al.18 They also showed that the duct of Wirsung may occasionally form a loop around the CBD and such a loop is sometimes referred to as the formation of an ansa pancreatica.18 Some of the numerous variations in the course of the duct are illustrated.
Figure 2.4 Loop of main pancreatic duct. Distal portion of the main pancreatic duct shows a loop.
A Santorini system and a Wirsung system may coexist with separate duodenal orifices and without communications within the gland (pancreas divisum) (Figure 2.5). This condition is termed a pancreas divisum (complete and incomplete). In this situation, the duct of Wirsung, normally the
Figure 2.5 Pancreas divisum. Santorini and Wirsung ducts are separated. Intraductal papillary mucinous tumor is seen in a branch of the Santorini duct.
main pancreatic duct, may be small, namely no more than 1 to 2 cm in length, and the duct of Santorini, draining into the duodenum at the minor papilla, provides the main ductal system. When the duct of Santorini is well developed, its course through the head of the gland is in a straight line without the angles or loops that are usually observed in the course of the duct of Wirsung.
It has been postulated by certain workers that, in pancreas divisum, inadequate drainage via Santorini's duct may predispose to pancreatitis. This condition is called dorsal pancreatitis. When pancreatitis is confined to the ventral pancreas, such pancreatitis is named ventral pancreatitis. The anomaly was found in 24 patients by Cotton and Kizu19 by endoscopic pancreatography and episodes of pancreatitis was said to have occurred in 14 of these cases, and 6 others had had episodes of abdominal pain. A failure of fusion of the ducts was found, also by endoscopic retrograde pancreatography, by Mitchell et al.20 in 21 out of 449 (4.7%) successful pancreatograms. Pancreas divisum may be one cause of pancreatitis in patients with idiopathic pancreatitis. A papilllotomy, tube stenting, and sphincterotomy of the minor papilla may effectively prevent a relapse of acute pancreatitis. When the communication of the Santorini and Winslow ducts is incomplete, pancreas head carcinoma should be differentiated from this condition on pancreatography because incomplete communication mimics stenosis by pancreatic cancer.
2.5.5 Termination of the Common Bile Duct and the Main Pancreatic Duct
The main pancreatic and common bile ducts unite in the duodenal wall in 85% of cases to form a common channel that has been named the ampulla of Vater13 after Vater who first noted it. The common channel, 1 to 14 mm in length, opens on the major duodenal papilla with a single opening (Figure 2.6). In rare types of the termination of both ducts, both ducts open on the major duodenal papilla via separate orifices or open into the duodenum at the separate points. Investigations that relied mainly upon dissection showed that between 20 and 30% of cases may have separate biliary and pancreatic orifices on the duodenal papilla. Rienhoff and Pickrell17 found that in 24% of their 250 dissections there was no junction of the pancreatic and bile ducts, but Singh21 found that in 30 of the 100 dissections he carried out the two ducts remained separate. However, injection methods followed by radiography in Millbourn's studies22 showed it was only exceptionally (about 1 in 20 cases) that there were separate entrances. Somewhat similar results were obtained by Berman et al.15 who made vinyl acetate casts of the ducts of the pancreases obtained at routine postmortem examinations and then digested away the
Figure 2.6 Termination of the common bile duct and the main pancreatic duct.
glandular tissue. They found that only 6.2% of their 130 specimens displayed the main pancreatic duct (duct of Wirsung) and the CBD opening independently into the duodenum. Such variations in the results obtained by different workers means that there must be many cases in which it is difficult to discriminate between a common orifice and two ducts that open separately but close together.
A pancreatobiliary maljunction (PBM) is a congenital anomaly that can be defined as a union of the pancreas and biliary ducts that is located outside of the duodenal wall and is beyond the influence of the sphincter of Odii (Figure 2.7 and Figure 2.8). The common channel measures 15 mm or greater in length. PBM is uncommon with the incidences of 1.0% in endoscopic retrograde cholangiopancreatography (ERCP) and 3.3% in biliary surgery in Japan. The frequency of PBM in Western countries is lower than that in Japan in contrast to pancreas divisum. PBM is frequently seen in Asian people, especially in women. PBM is closely related to a congenital bile duct dilatation.23
Because the function of the sphincter muscle of the duodenal papilla does not extend the full length of the common channel, it results in the regurgitation of the pancreatic juice and bile. In most cases, because the
Figure 2.8 Pancreatobiliary maljunction (P-B type). The distal common bile duct drains into the main pancreatic duct forming a long common channel. Congenital choledochal cyst is present.
intrapressure of the pancreatic duct is higher than the intrapressure of the biliary tract, the pancreatic juice regurgitates into the biliary tract continuously and the biliary mucosa is continuously susceptible to damage as a result of a continued presence of infected bile and activated pancreatic enzymes. Regurgitated phospholipase A2 of the pancreatic juice activates lisolecithin in the bile, which injures the bile duct and evokes bile duct cancer. This eventually causes cancer to occur in the biliary mucosa and other pathological changes including gallstones (28.5 to 38.1%) or acute pancreatitis (23.4 to 26.6%). When PBM is associated with a congenital choledochal cyst,24 bile duct cancer is often seen in the choledochal cyst (15.6 to 36.0%).25 When PBM is not associated with congenital choledochal cyst, gallbladder carcinoma is sometimes seen due to a condensation of the bile containing pancreatic juice in the gallbladder.26
Choledochocele occurs in patients older than those with PBM (Figure 2.9). Choledochocele has sometimes been reported to show dysmotility by sphincter of Oddi manometry and it is considered to be an acquired disorder produced either by a sphincter of Oddi dysfunction or stenosis. Others say that choledochocele is a congenital condition due to the presence of a duodenal mucosa lining on both sides of choledochocele. Ohtsuka et al.27 reported the malignant potential of choledochocele by the bile with stagnating pancreatic juice in the choledochocele to be the same mechanism as that observed in PBM.
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