Pancreas

The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. The pancreas is covered with a very thin connective tissue capsule that extends inward as septa, partitioning the gland into lobules. This organ has both exocrine and endocrine tissue. The exocrine portion is organized in grape-like clusters of acini, which are small sacs surrounding the terminal ends of pancreatic ducts. The cells lining the acini and ducts secrete products that make up pancreatic juices, which play a major role in digestion. The endocrine portion of the gland consists of circular islets interspersed between acini, which secrete glucagon, insulin, and somatostatin.

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Development

The pancreas is a leaf-shaped organ that lies transversely on the posterior abdominal wall. 

  • Derived from the foregut
  • Ventral pancreatic bud contributes to the:
    • Pancreatic head
    • Uncinate process
    • Main pancreatic duct
  • Dorsal pancreatic bud alone becomes the:
    • Pancreatic head
    • Accessory pancreatic duct
    • Body
    • Tail

Gross Anatomy

Structure and anatomic relations

The pancreas is a leaf-shaped organ with 5 major parts:

  • Head:
    • Right lateral end of the pancreas
    • Lies within the C-shaped concavity of the duodenum
    • Contains the main pancreatic duct and the common bile duct
    • Most common location for pancreatic malignancies
    • Uncinate process of the head: 
      • End of the pancreatic head that curves down, back, and medially to form a hook-like process
      • Posterior to the superior mesenteric vessels
  • Neck: 
    • Relatively short region connecting the head to the body
    • Splenic vein runs posterior to the neck, where it joins with the superior mesenteric vein (SMV), forming the hepatic portal vein.
  • Body: 
    • Elongated portion of the organ extending from the neck to the tail
    • Lies transversely along the posterior abdominal wall at the level of the L1–L2 vertebrae
    • Posterior to the stomach
  • Tail: 
    • Left lateral end of the organ
    • Lies in front of the left kidney 
    • “Points toward” the hilum of the spleen within the splenorenal ligament
  • Note: The SMA and SMV/hepatic veins run posterior to the neck/body but anterior to the uncinate process
Anatomic relationships of the pancreas to surrounding organs

Anatomic relationships of the pancreas to surrounding organs:
Note that the liver and stomach are light gray and that the intestines have been removed completely in order to allow better visualization of this posterior organ.

Image by BioDigital, edited by Lecturio

Ducts

The exocrine glands within the pancreas secrete their products into a network of ducts. These ducts ultimately drain into the main pancreatic duct.

  • Main pancreatic duct (Wirsung duct): 
    • Combines with the common bile duct at the hepatopancreatic ampulla (i.e., ampulla of Vater)
    • Ampulla enters the descending part of the duodenum at the major duodenal papilla:
      • Contains the hepatopancreatic sphincter (i.e., sphincter of Oddi) 
      • Sphincter regulates the secretion of bile and pancreatic fluid into the intestines.
      • Controlled by the autonomic nervous system
  • Accessory pancreatic duct (duct of Santorini): 
    • A branch off the main pancreatic duct
    • Empties into the duodenum at the minor duodenal papilla (located just above the major duodenal papilla)
    • Allows pancreatic juice to be released into the duodenum even when bile is not

Microscopic Anatomy and Function

There are 2 primary functional and histologic tissue types in the pancreas. The exocrine pancreas secretes digestive enzymes into the intestinal lumens, and the endocrine pancreas secretes hormones into the bloodstream.

Exocrine pancreas

Exocrine secretions are secretions into a lumen; in the case of the exocrine pancreas, its products, collectively known as pancreatic juices, are secreted into the intestinal lumen via the pancreatic duct. 

General structure:

  • Makes up approximately 80%–90% of the organ’s tissue
  • Made of up grape-like clusters of acini: 
    • Functional units of exocrine pancreas
    • Form small sacs around terminal ends of pancreatic ducts
    • Made up of centroacinar cells, which:
      • Produce and secrete enzymes and zymogens into these small ducts
      • Have dense granules at their apical ends (containing secretory products)
  • Ducts made up of columnar epithelial cells
  • Pancreas divided into lobules by septae (connective tissue)

Pancreatic juice:

The fluid secreted into the ducts consists of:

  • Water and electrolytes
  • Enzymes:
    • Pancreatic amylase → digests starch by breaking bonds in polysaccharides
    • Pancreatic lipase → digests fat by hydrolysis
    • Ribonuclease and deoxyribonuclease → degrades RNA and DNA, respectively
  • Zymogens (enzymes that are altered after secretion to become active): 
    • Functions: digest proteins
    • Zymogens secreted by the exocrine pancreas:
      • Trypsinogen → trypsin 
      • Chymotrypsinogen → chymotrypsin 
      • Procarboxypeptidase → carboxypeptidase
  • Sodium bicarbonate:
    • Secreted by ductal epithelial cells
    • Neutralizes hydrochloric acid (HCl) from the stomach

Endocrine pancreas

Endocrine secretions refer to hormone secretions directly into the bloodstream. The endocrine pancreatic tissue is arranged in circular pancreatic islets, or islets of Langerhans:

  • Functional units of the endocrine pancreas
  • Well-vascularized circular groups of cells 
  • Interspersed throughout the pancreas, in between acini
  • Make up approximately 10%–20% of the organ’s tissue
  • 3 different cell types each produce a different hormone: 
    • Alpha cells → secrete glucagon, which ↑ blood glucose levels by:
      • Stimulating glycogenolysis and gluconeogenesis
      • Inhibiting glycolysis and glycogen synthesis
    • Beta cells → secrete insulin, which ↓ blood glucose by stimulating:
      • Peripheral glucose uptake
      • Glycolysis and glycogen synthesis
    • Delta cells → secrete somatostatin, which inhibits:
      • Secretion of multiple hormones, including insulin, glucagon, gastrin, cholecystokinin, growth hormone, thyroid-stimulating hormone (TSH), and prolactin 
      • Gastric acid secretion
      • Bile secretion

Neurovasculature

Arterial blood supply

  • Head and neck supplied by the pancreatic arcade, which consists of:
    • Anterior and posterior superior pancreaticoduodenal arteries:
      • Branch off the gastroduodenal artery (a branch off the hepatic artery, which is off the celiac trunk)
      • Anastomoses with the inferior pancreaticoduodenal arteries
    • Inferior pancreaticoduodenal arteries: 
      • A branch off the SMA
      • Splits into anterior and posterior divisions (which anastomose with the superior pancreaticoduodenal arteries)
  • Body and tail: 
    • Pancreatic branches of the splenic artery
    • Inferior pancreatic artery: runs along the body and tail
  • Anastomotic branch: connects the pancreaticoduodenal arteries to the inferior pancreatic artery
Arterial supply of the pancreas

Arterial supply of the pancreas

Image by Lecturio.

Vascular drainage

Venous drainage is through pancreatic veins. These veins drain into the:

  • Pancreaticoduodenal veins → superior mesenteric vein (from the head and neck)
  • Splenic vein (from the body and tail)

Lymphatic drainage

  • Drained by lymphatic vessels that follow the arterial supply
  • Vessels empty into the pancreaticosplenic nodes and the pyloric nodes. 
  • These nodes drain into the superior mesenteric and celiac lymph nodes.

Innervation

The pancreas is innervated primarily by the autonomic nervous system, though pancreatic secretions are predominantly regulated by other hormone products and chemical signals (e.g., glucose levels, secretin, cholecystokinin).

  • Sympathetic: T6–T12 thoracic splanchnic nerves and the celiac plexus
  • Parasympathetic: vagus nerve

Clinical Relevance

  • Acute pancreatitis: acute inflammatory process that can be mild or a life-threatening condition. Acute pancreatitis occurs when the digestive enzymes are activated before they are released into the small intestine, causing inflammation. Most individuals present with an acute onset of persistent, severe epigastric abdominal pain radiating to the upper back. The most common causes are gallstones and alcohol abuse. 
  • Diabetes mellitus: diseases of abnormal carbohydrate metabolism that are characterized by hyperglycemia. Type 2 diabetes is by far the most common type of diabetes in adults, and it presents with peripheral insulin resistance and progressive loss of insulin secretion from beta islet cells. Type 1 diabetes is characterized by autoimmune destruction of the pancreatic beta cells, leading to absolute insulin deficiency.
  • Glucagonoma: glucagon-secreting neuroendocrine tumor, originating from the α cells in the pancreatic islets. Most glucagonomas are malignant, and many are part of the autosomal dominant condition MEN1. Presentation is often with diabetes, a characteristic rash called necrolytic migratory erythema (NME), weight loss, anemia, deep-vein thrombosis, and neuropsychiatric symptoms.
  • Insulinoma: insulin-secreting neuroendocrine tumor originating from the β cells of the pancreatic islets. Insulinoma more commonly presents as a solitary benign tumor, but can sometimes be associated with MEN1. Presentation is with fasting hypoglycemia, which may manifest as episodes of diaphoresis, palpitations, tremor, and confusion. 
  • Exocrine pancreatic insufficiency: complication of pancreatic disease in which the pancreas does not secrete sufficient pancreatic juices for digestion, resulting in maldigestion of fat and protein, weight loss, bloating, flatulence, and steatorrhea (loose, greasy stools). Exocrine pancreatic insufficiency can be caused by chronic pancreatitis, cystic fibrosis, pancreatic duct obstruction, or loss of secretin due to surgical resection of stomach and/or bowel.
  • Exocrine pancreatic cancer: highly lethal malignancy consisting mostly of invasive pancreatic ductal adenocarcinoma (PDAC) that arises from the ductal cells of the exocrine pancreas. Risk factors include tobacco smoking, obesity, diabetes, age, and race. Clinical presentation may include abdominal pain, jaundice, anorexia, asthenia, weight loss, Courvoisier sign (enlarged but nontender gallbladder due to common bile duct obstruction), splenomegaly, and GI hemorrhage.

References

  1. Le T, Bhushan V, Sochat M, et al. (Eds.) (2021). First aid for the USMLE step 1, 31st ed. p. 370. McGraw-Hill Education.
  2. Fernandez-del Castillo C. (2021). Clinical manifestations, diagnosis, and staging of exocrine pancreatic cancer. UpToDate. Retrieved August 17, 2021, from https://www.uptodate.com/contents/clinical-manifestations-diagnosis-and-staging-of-exocrine-pancreatic-cancer
  3. Drake RL, Vogl AW, Mitchell AWM. (2020). Gray’s Anatomy for Students, 4th ed. Churchill Livingstone/Elsevier. 
  4. Talathi S, Zimmerman R. (2021). Anatomy, abdomen and pelvis, pancreas. StatPearls. Retrieved Aug 19, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/26567/ 
  5. Saladin KS, Miller L. (2004). Anatomy and physiology, 3rd ed., pp. 962–964. McGraw-Hill Education.

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