Gross Gland

The pancreas is an encapsulated, retroperitoneal structure of the posterior, upper abdomen that is situated behind the peritoneal layer lining the base of the lesser sac. Intra-abdominal organs overlying the pancreas include the stomach and transverse colon. The gland weighs approximately 100 g and ranges from 20 to 25 cm in length and tapers in diameter from 5 to 3 cm as one progresses distally. It is divided anatomically into four sections that correspond with the gross appearance of the gland as opposed to functional components: the (1) head and uncinate process, (2) neck, (3) body, and (4) tail (Fig. 12-3). The uncinate process is the inferior portion of the pancreatic head and corresponds to the ventral bud following rotation and fusion with the dorsal bud.

The head of the pancreas is located within the C-loop of the duodenum and is adherent to it through the first, second, and third portions (D1-D3). The pancreatic head and duodenum share a common blood supply; therefore, surgeons contemplating operations involving either structure must account for the adjacent organ. Approximately 50 percent of the pancreatic mass is located in the head and its uncinate process. The neck of the pancreas lies anterior to the origin of the portal vein as it derives from the junction of the superior mesenteric and splenic veins. The body of the pancreas extends from the left of the portal vein and is the portion of the gland that is exposed if the floor of the lesser sac is divided. Anterior to the left kidney is the tail of the pancreas. The pancreatic tail is actually positioned in close proximity to the splenic hilum; therefore, great care must be exercised to avoid the complications of pancreatic injury when splenectomy is performed. On the other hand, the spleen is usually included with the specimen during distal pancreatic resections due to a common blood supply.

Ductal System

In most individuals, the exocrine pancreas is drained by the main duct of Wirsung that lies in the center of the gland and spans the length of the organ. It is 1 mm in diameter in the tail and gradually increases to approximately

Figure 12-3 The figure depicts the anatomic sections of the pancreas. The head is located within the C-loop of the duodenum. The uncinate process represents the embryologic ventral bud. The neck lies anterior to the origin of the portal vein. The body extends lateral to the portal vein. The tail is in close proximity to the splenic hilum.

Figure 12-3 The figure depicts the anatomic sections of the pancreas. The head is located within the C-loop of the duodenum. The uncinate process represents the embryologic ventral bud. The neck lies anterior to the origin of the portal vein. The body extends lateral to the portal vein. The tail is in close proximity to the splenic hilum.

3.5 mm in the head. Numerous secondary ducts join the major duct along its length, thereby draining the tail, body, neck, and uncinate process. The majority of pancreatic exocrine products exit via the main duct and enter the duodenum through the ampulla of Vater, located 10-12 cm distal to the pylorus. The superior head of the pancreas is drained by the minor duct of Santorini that, in most, is in continuity proximally with the main pancreatic duct and empties distally into the duodenum via the minor papilla, which is situated a couple of centimeters proximal to the ampulla of Vater

The release of pancreatic exocrine secretions into the gastrointestinal tract is controlled by a series of sphincters. The distal main pancreatic duct has an individual sphincter, as does the distal common bile duct prior to merging to form the aforementioned common channel. The sphincter of Oddi surrounds the common channel at the ampulla of Vater and prevents the reflux of duodenal contents into the pancreaticobiliary tree. Complex neural and hormonal signals control each of the sphincters.

Arterial Supply

The pancreas has a rich blood supply that is derived from major branches of the aorta, specifically the celiac and superior mesenteric arteries.2 The head of the gland is supplied by a series of arcades that often collateralize with other vessels. The anterior and posterior pancreaticoduodenal arcades are supplied directly by the gastroduodenal artery from above and the inferior pancreaticoduodenal artery from below. Arterial blood flow to the body and tail of the pancreas originates from the splenic artery which courses along the superior aspect of the gland en route to the spleen. Numerous branches of the splenic artery enter the gland along the body and tail including the dorsal, great, and caudal pancreatic arteries that exit along the inferior surface of the gland and collateralize with the inferior pancreaticoduodenal artery.

Venous Drainage

As with most organ systems, venous drainage of the pancreas generally mirrors the arterial supply. The head is drained by the anterior and posterior venous arcades, which empty into the suprapancreatic portal vein. Drainage of the body and tail occurs via venous tributaries that enter the splenic vein. It is of note that all venous blood exiting the pancreas eventually enters the portal vein; therefore, endocrine products of the gland are subject to first pass effect by the liver.

Innervation

Both sympathetic and parasympathetic components of the autonomic nervous system innervate the pancreas. The former originates from the thoracic sympathetic chain, and the latter from the vagus nerves. Both the exocrine and endocrine functions of the pancreas are under sympathetic and parasym-pathetic control. Although a detailed account of the interactions between the nervous system and pancreas is beyond the scope of this text, in general, parasympathetic nerves stimulate pancreatic secretion (exocrine and endocrine), whereas the sympathetic system inhibits these functions.3

The pancreas also has a complicated intrinsic nervous system that influences control over exocrine and endocrine functions by means of a group of peptide neurotransmitters.3 In addition, afferent sensory fibers are present and are thought to mediate abdominal pain associated with chronic pancreatitis and pancreatic cancer.

Lymphatic Drainage

The lymph nodes and channels that drain the pancreas comprise an intricate network that empties into several regional lymphatic groups. Drainage of the pancreatic head occurs through the celiac and superior mesenteric nodes. The body and tail are drained by nodes that lie adjacent to the splenic blood vessels as they course along the superior border of the gland. These lymphatics then empty into larger, named nodal groups. The complex nature of the pancreatic lymphatic system renders curative resection of pancreatic malignancies an uncommon occurrence because of lymphatic metastases noted at laparotomy as well as undetected, residual lymphatic disease left in situ during pancreatectomy.

Microscopic Exocrine Pancreas

The pancreas has separate systems for its exocrine and endocrine functions. At least 80 percent of the pancreatic mass is devoted to synthesizing, secreting, and delivering exocrine products to the gastrointestinal tract. The significant components include acinar and centroacinar cells as well as the ductal apparatus. An acinus is a terminal ductule and is lined by centroaci-nar cells and surrounded by acinar cells. The centroacinar cells regulate fluid and electrolyte secretion, whereas the acinar cells secrete digestive enzymes (see Physiology). The terminal ductules combine to form first the intralobular ducts and then the interlobular ducts, the latter draining into the main pancreatic duct.

Endocrine Pancreas

Although the endocrine component of pancreatic function is considered the most vital aspect of the gland, merely 2 percent of the pancreatic mass is devoted to this function. Named after the German pathologist who discovered them in 1869, the islets of Langerhans contain the cells that are responsible for the production and release of pancreatic hormones. The 106 islets found in the average human pancreas are dispersed throughout the gland, although most are located in the body and tail.

Islets of Langerhans contain five major cell types, each of which produces at least one significant hormone: a (alpha), ยก3 (beta), 8 (delta), 81 (delta 1), and pancreatic-polypeptide-producing (PP) cells. The most abundant cells are the insulin-producing 3 cells which comprise 75 percent of an islet's mass and are located in the core of the islet. The glucagon-producing a cells occupy a peripheral position and represent approximately 20 percent of an islet's mass. Glucose homeostasis is controlled by both a and 3 cells which generate glucagon and insulin, respectively (see Physiology). Hormonal products of the other cell types include somatostatin from S cells, vasoactive intestinal peptide (VIP) from the 81 population and pancreatic polypeptide from PP cells. Interestingly, the concentration of 3 and 8 cells in islets throughout the pancreas is consistent, whereas the concentration of the a and PP populations varies as a function of location.4

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