Pancreas Anatomy and Physiology

Anatomy

1. Embryology
• the pancreas forms from 2 pouches, a larger dorsal pouch and a smaller ventral pouch
• the dorsal pouch forms first, directly from the duodenal endoderm, and forms the bulk of the pancreatic tissue
• the ventral pouch develops as an outpouching from the hepatic diverticulum and maintains a close relationship with the common bile duct
• as the duodenum rotates to assume its C configuration, the ventral pouch rotates clockwise
• at the 8th week of gestation, the ventral and dorsal pancreatic pouches fuse, as do their respective duct systems
• the ventral pouch becomes the uncinate process and the inferior aspect of the pancreatic head
• the dorsal pouch becomes the superior aspect of the pancreatic head as well as the neck, body, and tail of the gland
• the main pancreatic duct, the duct of Wirsung, joins the CBD in an intrapancreatic location and empties through the ampulla of Vater at the major duodenal papilla
• the proximal aspect of the dorsal pancreatic duct, the duct of Santorini, may empty into the duodenum through a separate minor papilla located ~ 2 cm above the major papilla


1. Congenital Anomalies
1. Heterotopic Pancreatic Tissue
• development of pancreatic tissue outside the confines of the main gland
• most commonly found in the stomach, duodenum, small intestine, and Meckel’s diverticulum
• firm, yellow, irregular submucosal nodules that may vary from several millimeters to several centimeters in size
• becomes clinically evident because of complications: intestinal obstruction, usually as a result of intussusception, and ulceration and hemorrhage


2. Pancreas Divisum
• failure of fusion of the two primordial duct systems
• occurs in ~ 5% to 10% of the population
• major portion of the pancreas is drained by the duct of Santorini via the minor papilla
• a small duct of Wirsung drains through the major papilla
• the significance of pancreas divisum is controversial
• in some series of patients with idiopathic pancreatitis, the incidence of pancreas divisum approaches 25%
• some have speculated that pancreas divisum, when associated with stenosis of the minor papilla, may cause pancreatitis



3. Annular Pancreas
• histologically normal pancreatic tissue completely or partially encircles the 2nd portion of the duodenum
• thought to arise from failure of normal clockwise rotation of the ventral pouch
• often associated with other serious congenital anomalies
• obstructive symptoms are the indication for surgery
• a bypass – duodenojejunostomy or gastrojejunostomy – is the indicated procedure, not resection or division of the obstruction



2. Gross Anatomy
• the pancreas lies in the retroperitoneum, posterior to the stomach and lesser omentum
• covered by peritoneum anteriorly
• posteriorly the pancreas is in close proximity to the right renal vein, inferior vena cava, aorta, superior mesenteric vein and artery, and splenic vein and artery
• divided into 5 portions: the head, uncinate process, neck, body, and tail
• the neck is anterior to the superior mesenteric vein
• the head is to the right of the neck and lies within the duodenal C loop
• the uncinate process arises from the inferior aspect of the head and extends posterior to the superior mesenteric vein and ends at the right margin of the superior mesenteric artery
• the body is to the left of the neck
• the tail is to the left of the body and extends into the splenic hilum



1. Blood Supply
1. Arterial Supply
1. Head
• shares a joint blood supply with the 2nd portion of the duodenum
• supplied by a collateral network originating from the anterior and posterior branches of the superior and inferior pancreaticoduodenal arteries
• the superior pancreaticoduodenal artery arises from the gastroduodenal artery
• the inferior pancreaticoduodenal artery arises from the superior mesenteric artery


2. Neck, Body, and Tail
• generally supplied by a variable number of branches from the splenic artery
• within the posterior substance of the gland lies the inferior pancreatic artery, which connects to the splenic artery via the transversely-oriented dorsal, great and caudal arteries




2. Venous Drainage
• parallels the arterial anatomy
• eventually drains into the portal vein


2. Lymphatic Drainage
• extensive, with multiple lymph node groups draining the pancreas
• from the head of the gland, nodes in the pancreaticoduodenal groove drain into the subpyloric, portal, mesocolic, mesenteric, and aortocaval nodes
• lymphatics in the body and tail drain into splenic nodes or into celiac, aortocaval, mesocolic, or mesenteric nodes



3. Innervation
1. Sympathetic Innervation
• postganglionic sympathetic fibers, arising from the celiac ganglia, innervate the pancreas and serve as the principal pathways for pain of pancreatic origin
• chemical splanchnicectomy is often performed for relief of pancreatic pain, with variable success


2. Parasympathetic Innervation
• originates from the posterior vagus nerve
• innervates the pancreatic islets, acini, and ducts


3. Microscopic Anatomy
• 2 distinct organs reside within the pancreas, an exocrine organ and an endocrine organ


1. Exocrine Pancreas
• comprised of the acini and ductal systems
• each acinus drains into a minor pancreatic duct
• the minor ducts terminate in the major pancreatic duct



2. Endocrine Pancreas
• endocrine cells are contained within the islets of Langerhans
• the islets are spherical collections of cells scattered throughout the pancreas
• ~1,000,000 islets per gland
• each islet has an extensive blood supply and is composed of several different cell types
• each cell type produces a single hormone
• insulin-producing α cells are the most abundant (~70%), followed by glucagon-producing β cells (20% to 25%)
• δ cells (5%) produce somatostatin, which appears to function as a paracrine modulator of islet cell function
• a small minority of islet cells produce pancreatic polypeptide (PP), vasoactive intestinal peptide (VIP), and gastrin