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Module 7: Figure pancreas

Structure of insulin-secreting β-cells located in the islets of Langerhans.

The islets of Langerhans, which make up about 2% of the pancreas volume, are richly supplied with blood vessels. They consist of a nest of closely packed insulin-secreting β-cells (60–70%), together with fewer glucagon-secreting α-cells. These islets are distinct from the surrounding exocrine pancreatic acinar cells that release enzymes and fluid into pancreatic ducts. The β-cells contain granules lined up on cytoskeletal elements and directed towards the surface facing the capillaries, which have fenestrae similar to those seen in other endocrine organs, i.e. the kidney and intestine. These fenestrae ensure a rapid dispersal of insulin into the blood, and also enable the β-cells to continuously monitor the blood level of glucose and circulating hormones. Individual β-cells are connected by both desmosomes and by gap junctions. The latter ensure that the β-cells are electrically coupled, and this is critical for secretory efficiency.