Pancreas

Structure/Morphology

The pancreas is an accessory digestive organ with both exocrine and endocrine glandular components.

Acinar cells are surrounded by loose connective tissue to form clusters of lobular acini, the basic exocrine unit that makes up roughly 98% of pancreatic tissue. Each acinus is drained by a single intralobular duct which will join with the ducts of neighboring acini to form interlobular ducts.

The endocrine islets of Langerhans make up roughly 1–2% of pancreatic tissue. These clusters of cells produce hormones that are secreted directly into the dense capillary beds that permeate the islets. Islets consist of alpha, beta, delta, epsilon, and F cells (Standring, 2016).

Besides these basic functional units, the pancreas has a dense neurovasculature and an acinar ductal system which merges to form the pancreatic duct.

Anatomical Relations

The pancreas sits retroperitoneal with a peritoneum on the anterior surface and the lesser sac anterior to that. On the posterior surface, there is no peritoneum, but instead connective tissue fascia and the body wall vasculature, and other retroperitoneal organs. Overall it forms a roughly J-shaped structure much of which is surrounded by the C-shaped duodenum.

Specifically, the pancreas is broken down into four major regions, the tail, body, neck, and head.

- The tail extends from the body of the pancreas to the lateral tip near the spleen. This portion is more mobile and is somewhat variable in position. Generally, it’s bordered by the spleen laterally, the left kidney, splenic artery, and splenic vein posteriorly, and the splenic flexure of the colon anteriorly.

- The body is the main longitudinal portion of the pancreas, running from the tail to the neck. The pancreas as a whole is gently bent such that the body is more anterior than either the head or tail. The medial border of the body roughly overlies the superior mesenteric artery and abdominal aorta. The body is bordered anteriorly by the stomach and inferiorly by portions of the jejunum. The splenic artery and vein run along the posterior wall.

- The neck is a small region that connects the body to the head of the pancreas. It’s largely the portion that lies anterior to the superior mesenteric and portal vein. Posterior to the neck, the splenic and superior mesenteric veins fuse to form the portal vein. Anterior to the neck is the pyloric region of the stomach.

- The head of the pancreas fills the space bound by the first, second, and third parts of the duodenum. It lies to the right of the great vessels, connected to the rest of the pancreas at the neck. A portion of the peritoneum overlying the head of the pancreas, close to the pylorus, is fused to and continuous with the greater omentum.

The superior, right, and inferior borders of the head of the pancreas are surrounded by the first, second, and third parts of the duodenum, respectively. The fourth part of the duodenum borders the medial surface of the uncinate process of the head.

As the head follows the descending and horizontal duodenum, it moves gradually posteriorly so that as it approaches the ascending duodenum it lies posterior to the superior mesenteric vasculature. This medial projection of the head, the uncinate process, sits posterior and inferior to the neck, right up against the inferior vena cava and the aorta.

Lying anterior to the head of the pancreas is the transverse colon. The bile duct lies against or in the posterior wall of the head, and it’s within the head that the bile duct and the pancreatic duct merge to form the hepatopancreatic ampulla (of Vater). Also posterior to the head of the pancreas is the inferior vena cava.

Function

The pancreas serves both exocrine and endocrine functions.

The exocrine functions are performed by acinar cells. These cells secrete zymogens which are inactive forms of enzymes that digest proteins, fats, and carbohydrates. Acinar cells also secrete water and bicarbonate. Secretions are directed into intralobular ducts which flow through progressively larger ducts to the pancreatic duct and ultimately to the descending duodenum.

Enterochromafin cells located in acini also secrete several hormones.

The endocrine function is mediated by the islets of Langerhans which include alpha, beta, delta, epsilon, and F-cells. Beta cells secrete insulin which helps decrease blood glucose levels. Alpha cells secrete glucagon which raises blood glucose levels. Delta cells secrete somatostatin which reduces secretion of insulin and glucagon. Epsilon cells secrete ghrelin which is associated with feelings of hunger. F cells help regulate gastrointestinal secretions and motility (Jain, 2012; Standring, 2016).

Arterial Supply

The pancreas is highly vascularized with blood from several major arteries.

From the celiac trunk, the splenic artery runs along the posterior surface of the pancreas towards the tail. Along the way it gives off several branches that serve the pancreas, chiefly the dorsal pancreatic artery, the great pancreatic artery, and the artery to the tail of the pancreas. These arteries travel largely on posterior surfaces of the pancreas although a branch of the dorsal artery runs laterally along the inferior edge of the body of the pancreas.

Also originating with the celiac trunk and the gastroduodenal artery, the superior pancreaticoduodenal artery serves the head of the pancreas. It divides into anterior and posterior superior pancreaticoduodenal arteries to serve anterior and posterior regions respectively.

The inferior pancreaticoduodenal artery, branch of the superior mesenteric artery, also serves the head of the pancreas. It too divides into anterior and posterior branches which run along the uncinate process towards the head and typically anastomose with the superior pancreaticoduodenal arterial branches.

Venous Drainage

The pancreas drains blood into the portal circulation. Blood from the body and tail typically drains through small veins into the splenic vein. Blood from the neck drains directly into the portal vein.

Blood from the posterior head drains into the portal vein directly. From the anterior superior portion of the head, it drains into the anterior superior pancreaticoduodenal vein which merges with either the right gastric or right gastroepiploic vein, and from there to the superior mesenteric vein.

Inferior portions of the head and the uncinate process may drain through inferior pancreaticoduodenal veins into the superior mesenteric vein (Standring, 2016).

Innervation

Innervation of the pancreas includes the autonomic nervous system (sympathetic and parasympathetic), extrinsic sensory innervation (visceral afferents), and enteric efferent innervation (Standring, 2016).

The vagus nerve (CN X) provides parasympathetic innervation to the pancreas with postganglionic parasympathetic neurons. Parasympathetic innervation is associated with glandular secretion.

Sympathetic innervation is derived from the sympathetic chain or the aortic plexus. These efferents are supplied by postganglionic neurons from the celiac plexus. Preganglionic sympathetic fibers originate in the T6-T12 spinal cord levels and travel in the thoracic splanchnic nerves (Standring, 2016). Sympathetic innervation is associated primarily with vascular tone.

Visceral afferent nerves from the pancreas travel back to the CNS along the thoracic splanchnic nerves carrying pain sensation, or via the vagus nerve carrying other sensory information.

The enteric system from the stomach and duodenum sends efferent fibers to the pancreas (Standring, 2016).

Lymphatic Drainage

Lymphatic drainage from the pancreas largely parallels the arterial supply. Lymph from the tail and body drains through nodes along the splenic artery towards celiac and preaortic nodes. Lymph from the head, neck, and uncinate process drain via pancreaticoduodenal nodes and hepatic nodes associated with the pancreaticoduodenal and hepatic arteries. This lymph also drains to preaortic nodes such as the superior mesenteric and celiac nodes (Standring, 2016).

List of Clinical Correlates

- Pancreatic cancer

- Pancreatitis

- Diabetes mellitus

References

Jain, D. (2012) Endocrine pancreas. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/pancreasendocrine.html opens in new tab/window (Accessed: August 7th 2020).

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.