Pancreas - Anatomy & Histology
Pancreas
Has both exocrine and endocrine functions.
- Most of the pancreas comprises exocrine cells, called acini, that secrete digestive enzymes; the ducts that drain them secrete alkaline fluid.
- Clusters of endocrine cells, called Islets of Langerhans, secrete hormones that regulate blood glucose metabolism.
Key Anatomical & Histological Features
- The pancreas is lumpy and glandular.
- The duodenum wraps around the head of the pancreas
- The pancreatic tissue primarily comprises acini, which are clusters of secretory cells.
- Visible as a circle of dark purple-staining secretory cells, which form the acinus.
- Acini drain their contents into the main pancreatic duct via smaller ducts; secretions from the glandular cells travel through a series of ducts, ultimately reaching the main pancreatic duct.
- Upon stimulation, zymogen granules release digestive enzymes and pro-enzymes into the lumen.
- Centroacinar cells comprise the initial portion of the excretory duct arising within the acinar lumen, lead to intercalated ducts.
- Intercalated ducts are lined by columnar epithelial cells that contribute an alkaline aqueous solution to the acinar secretions. Though not shown, here, intercalated ducts drain to interlobular ducts, and, eventually, the main pancreatic duct (be aware that some texts use the terms intercalated and intralobular ducts interchangeably, while others state that intercalated ducts drain to intralobular ducts before draining to interlobular ducts).
- The main pancreatic duct and common bile duct merge and drain into the duodenum via the hepatopancreatic duct.
- In some individuals, an accessory pancreatic duct persists and drains more proximately.
- Islet of Langerhans (pancreatic islet) comprises a lighter-staining, roundish cluster of endocrine cells that are in close contact with interweaving capillaries (recall that endocrine cells secrete substances that travel within the blood supply to reach their target tissues).
- Surrounding the Islet is a thin layer of connective tissue capsule, which provides a slight barrier between the endocrine and exocrine cells.
- To maintain homeostatic blood glucose concentrations, the Islet cells engage in communication via neural, hormonal, and cell-to-cell signaling.
- Cell makeup
- Alpha cells comprise approximately 20% of the islet cells; they secrete glucagon, which increases blood glucose concentrations between meals.
- Beta cells comprise approximately 65% of the islet cells; they secrete insulin, which lowers blood glucose concentrations after a meal.
- Delta cells comprise approximately 10% of the islet cells; they secrete somatostatin, which inhibits insulin and glucagon secretion to moderate their effects on blood glucose concentrations.
- F cells, aka, PP cells, which are rare; they secrete pancreatic polypeptides, whose specific functions are uncertain.
Images:
Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)