Pancreas

Abstract

The pancreas is a glandular organ located in the retroperitoneum between the duodenal curvature and the splenic hilum. It has both exocrine and endocrine functions and is anatomically classified into four parts: the head, neck, body, and tail. Because it is embryologically derived from the foregut, the pancreas receives arterial supply from the celiac trunk and its branches. The veins of the pancreas drain into the portal vein and the lymphatics eventually drain into the celiac, superior mesenteric, and splenic lymph nodes. The pancreas receives both sympathetic and parasympathetic innervation via the celiac ganglia. Histologically, the pancreas has two functional structures: the exocrine acinar glands and the endocrine pancreatic islets. The exocrine glands produce digestive enzymes, which are secreted into the duodenum, and the endocrine islets produce blood glucose-regulating hormones such as insulin and glucagon, which are secreted into the blood.

Gross anatomy

Overview

Characteristics

  • Glandular organ
  • Length: 12–15 cm
  • Weight: approx. 80 g

Location

Function

  • Endocrine: produces somatostatin, glucagon, insulin
  • Exocrine:
    • Produces digestive enzymes that aid the absorption of macronutrients
    • Produces bicarbonate, which neutralizes chyme

Anatomical subdivisions of the pancreas [1][2]

  • Head
    • Located within the C-shaped duodenal curvature; contains the pancreatic duct and distal common bile duct.
    • Uncinate process of pancreas: an extension of the pancreatic head that is located posterior to the superior mesenteric vessels (the remainder of the pancreas is not)
  • Neck: lies anterior to the portal vein
  • Body: lies anterior to the aorta and extends to the left kidney
  • Tail

The uncinate process is posterior to the superior mesenteric vessels. The head, body, and tail of the pancreas lie anterior to the superior mesenteric vessels.

Pancreatic ducts [1][2]

Tumors in the pancreatic head often cause bile duct obstruction and can manifest with painless jaundice.

Vasculature, lymphatics, and innervation of the pancreas [1][2]

Microscopic anatomy

Exocrine pancreas[1]

Histologically, the exocrine pancreas closely resembles the salivary glands. However, unlike the salivary glands, the pancreatic exocrine glands lack myoepithelial cells in the acini and do not possess striated ducts. Additionally, centroacinar cells are unique to the pancreas.

Endocrine pancreas [1]

Beta cells, which are located on the inside of islets of Langerhans, produce insulin.

Pancreatic ducts [1]

The majority of pancreatic malignancies are located in the head of the pancreas and originate from epithelial cells within the tubules.

Exocrine pancreas

Content of secretions

Cystic fibrosis is caused by a defective chloride channel that impairs pancreatic water and electrolyte secretion. This results in obstruction of the pancreatic ducts, with subsequent maldigestion and malabsorption.

Feedback mechanisms

  • Stimulation
    • Cholecystokinin (CCK): from enteroendocrine cells of the duodenum
    • Secretin: from enteroendocrine cells of the duodenum
  • Inhibition: somatostatin

Acute pancreatitis leads to activation of the pancreatic enzymes within the pancreatic ducts, which also leads to subsequent autodigestion of pancreatic tissue.

Elevations in serum lipase and amylase can be seen in acute pancreatitis.

Endocrine pancreas

Main endocrine cell types and hormones produced [3]

Cell types Substance secreted Function Feedback mechanism
α-cells
  • Glucagon
β-cells
δ-cells
  • Insulin and glucagon secretion
  • Growth hormone secretion
  • ↓ Gastric acid and pepsinogen secretion
  • ↓ Pancreatic and small bowel fluid secretion
  • Gallbladder contraction↓ Biliary drainage
  • Splanchnic vasoconstriction
  • Analog: octreotide
ε-cells
  • Stimulation: fasting, sleep deprivation
  • Inhibition: intake of food
PP cells (formerly γ-cells)
  • ↑ Gastric secretion of gastric acid
  • ↓ Pancreatic exocrine secretions
  • Promotes satiety
  • Stimulation: fasting
  • Inhibition: intake of food

Insulin synthesis and secretion [3][4][5][6]

  1. Insulin gene transcription and translation in the rER of pancreatic β-cellspreproinsulin (precursor protein)
  2. Signal proteases remove the signal peptide of preproinsulinproinsulin
  3. Protein folding and formation of one disulfide bridge in the α-chain and two disulfide bridges between the α-chain and β-chain
  4. Proinsulin is transported to the Golgi apparatus → packaged in membrane-bound secretory granules with proprotein convertases
  5. Proinsulin is cleaved by proteases → mature insulin + C-peptide
  6. Secretion of insulin and C-peptide via exocytosis

C-peptide is released by the β-cells in an equimolar proportion to insulin but is not present in exogenous insulin. It can, therefore, be used to differentiate between endogenous (e.g., insulinoma) and exogenous hyperinsulinism.

References:[4][3][7][8][6][5]

Embryology

Clinical significance

Exocrine pancreas

Endocrine pancreas

Embryology of the pancreas