Pancreas, gallbladder and biliary tree

Embryology

Development of the pancreas occurs in tandem and in close association with that of both the duodenum and the biliary system. Ventral and dorsal evaginations of the endodermal epithelium form at the junction of the foregut and midgut, and proliferate into the relevant mesogastria, which contain splanchnopleuric mesenchyme. The dorsal pancreatic bud is seen in stage 13 (31–33 days post fertilization) and the ventral pancreatic bud arises in stage 14 (32–34 days post fertilization) from the neohepatic diverticulum in close approximation with the developing bile duct. The ventral pancreatic bud and the bile duct rotate from a position within the ventral mesogastrium to one in the dorsal mesogastrium. By stage 17 (39–41 days post fertilization), the ventral and dorsal pancreatic buds have fused. Three-dimensional reconstruction of the ventral and dorsal pancreatic buds has confirmed that the dorsal bud forms the anterior part of the head of the pancreas, the pancreatic body and tail, and the ventral bud forms the posterior part of the head of the pancreas and the posterior part of the uncinate process. At this stage, the hepatic ducts and gallbladder are also lengthening. The dorsal mesoduodenum is affected by movements of the dorsal mesogastrium (with which it is continuous cranially) and the formation of the lesser sac, and the pancreas continues to grow retroperitoneally and extend into the splenorenal ligament.

The ventral pancreas forms the proximal portion of the major duct of Wirschung, which is joined with the common bile duct (CBD) in a Y configuration forming the hepatopancreatic ampulla of Vater. Persistence of the ventral pancreatic bud is thought to contribute to the formation of choledochal cysts due to the anomalous union of the pancreatic and biliary ducts. Indications for surgical management of choledochal cysts include high-risk features of malignancy (nodule or thickening of the cyst) or presence of stones, debris or obstruction due to the cyst. The majority of choledochal cysts (types I and IV) are treated with total extrahepatic excision followed by Roux-en-Y hepatico­jejunostomy or hepaticoduodenostomy.

The hepatopancreatic ampulla then opens on the medial wall of the second part of the duodenum at the major duodenal papilla. The distal component of the major pancreatic duct and the entire minor pancreatic duct of Santorini are formed by the dorsal pancreas. Conventionally, the minor duct fuses with the major duct, forming a common channel. This entire region is known as the pancreaticobiliary junction, and in some cases, the CBD and pancreatic duct may unite outside the duodenal wall to form an abnormally long common channel.

Fusion of the pancreatic ducts occurs late in development or in the postnatal period, and 85% of infants have patent accessory ducts. Failure of fusion of the primordial dorsal and ventral pancreas results in pancreas divisum, whereby the body, tail and parts of the pancreatic head are drained by the minor duct of Santorini. Individuals with this anatomical variation may have a greater predisposition for recurrent pancreatitis secondary to a heightened tendency for stone and stricture formation, although this remains controversial. For surgeons, this varied anatomy is important, as up to 10% of patients may have their major pancreatic duct drainage occurring through the minor duct. The minor duct courses near the gastroduodenal artery (GDA), which is commonly ligated during duodenal surgery, and care must be taken to avoid injuring this structure in procedures that do not involve resection of the pancreatic head.

Surgical surface anatomy

The pancreas lies in the retroperitoneum, sequestered from the plethora of abdominal organs. This location partly accounts for the insidious nature of pancreatic diseases and makes their management challenging. The pancreas is a smooth, soft gland with a lobulated surface and a shape conventionally described as a flattened tongue. The pancreas proper is divided into four anatomical zones: head, neck, body and tail ( Fig. 63.1 ). The uncinate process may also be considered as a fifth zone with its own unique anatomical relationships. For the purposes of this chapter, the uncinate process will be considered as part of the pancreatic head. Major pancreatic divisions are described in relation to the superior mesenteric artery (SMA) and vein (SMV), which course posteriorly to the pancreas. The pancreatic head lies laterally to the right of these vessels and the neck lies anteriorly, essentially linking the head to the pancreatic body and tail.

When approached anteriorly, the pancreatic head and neck are obscured by the stomach, greater omentum and transverse colon (see Fig. 63.1B ). Anterosuperiorly, the pancreatic head and neck are near the gastric antrum and pylorus, and subsequently surrounded by the second and third portions of the duodenum, resulting in a ‘bare area’ that is devoid of any peritoneum. Inferiorly, the pancreatic head shares a close relationship with the transverse colon. Furthermore, the pancreatic neck serves as a landmark for the origin of the transverse mesocolon and the middle colic artery, which arises from the SMA. In addition, the inferior border of the pancreas lies superiorly to the third and fourth portions of the duodenum, leading to the ligament of Treitz and the duodenal-jejunal flexure.

The anterior surface of the pancreas is covered by a layer of visceral peritoneum, separate from the peritoneum that surrounds the neighbouring anterior structures. Access to the anterior pancreatic head and neck may be achieved through several routes when considering its anatomical relationships. Division of the gastrocolic ligament, which runs as an avascular plane between the stomach and transverse colon along the greater omentum, permits entry into the omental bursa (or lesser sac). Further dissection of the peritoneum posteriorly to the stomach will expose the pancreas. This is a route favoured by many surgeons when performing pancreatic resections or requiring access to this area for pancreatic trauma. Access to the lesser sac, and therefore the pancreatic head and neck, may be obtained through the stomach, an approach that is relevant when managing penetrating injuries of the stomach, and exploited clinically in patients requiring endoscopic drainage of symptomatic pancreatic pseudocysts secondary to pancreatitis. Finally, another route of access to the pancreatic head and neck is through the transverse mesocolon. Clinically, this approach carries no significant advantage over the others, and access through this structure risks injury to the middle colic vessels that supply the colon. However, this close anatomical relationship is of importance, particularly in patients with pancreatic neoplasia and retrocolic Roux-en-Y anastomoses to the stomach, as this limb may become obstructed by large pancreatic tumours. More commonly, exposure of the pancreatic head and neck is achieved through removal of the greater omentum from the transverse colon, allowing for a wide exposure of the entire pancreas.

Posteriorly, the pancreatic head and neck are covered by a layer of tissue referred to as the fusion fascia of Treitz. This loose connective tissue provides a boundary between the pancreatic parenchyma proper and the structures of the retroperitoneum: specifically, the superior mesenteric vessels, splenic vessels and portal vein (PV). Furthermore, the major posterior pancreaticoduodenal arcades supplying the pancreas are located between the fascia and the posterior surface of the pancreas.

In procedures such as pancreaticoduodenectomy, colloquially referred to as the Whipple procedure, one of the key early steps involves mobilization of the pancreatic head and neck. As these structures are intimately associated with the duodenal C-loop, mobilization of the entire complex is required, and this is referred to as the Kocher manœuvre. This manœuvre starts along the longitudinal plane of the second part of the duodenum (D2) by incising the peritoneum between D2 and the inferior vena cava posteriorly. Dissection in this space permits exposure of the posterior duodenum (important for exposure in retroduodenal perforations), as well as the posterior pancreatic head and neck. In performing the Kocher manœuvre, the fusion fascia of Treitz remains on the pancreatic side and leaves a bare caval side. Therefore, the dissection plane remains avascular as the pancreaticoduodenal arcades are carried with the pancreas. Posteriorly, the inferior vena cava, left renal vein and aorta are exposed as the pancreas is rotated off the retroperitoneum, with eventual exposure of the SMA and SMV. Identification of the SMV inferiorly is critical in operations on the pancreatic head and neck, as it conventionally serves as the location of pancreatic transection. A quick way to find the SMV is by identifying the middle colic vein located in the transverse mesocolon and following it proximally until it joins with the right gastroepi­ploic vein, forming the trunk of Henle, which inserts into the SMV. However, the surgeon should exercise caution when following this approach because a variety of small venous tributaries may be encountered. At the level of the pancreatic neck, the SMV commonly joins the splenic vein to form the PV, which then courses superiorly into the hepatoduodenal ligament, and eventually the liver. In most cases, the anterior surface of the SMV and PV has no vascular branches, and this allows for safe dissection between these structures and the posterior surface of the pancreatic neck. This is also important for management of retropancreatic PV, SMV and SMA injuries, as exposure may be achieved quickly by negotiating this avascular plane along with subsequent transection of the pancreatic neck.

Clinical anatomy and variants