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15.11A: Overview of Pancreatic Islets

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    7776
  • Pancreatic islets, also called the islets of Langerhans, are regions of the pancreas that contain its hormone-producing endocrine cells.

     

    LEARNING OBJECTIVES

     

    Differentiate among the types of pancreatic islet cells

     

    KEY TAKEAWAYS

    Key Points

     

    • The pancreatic islets are small islands of cells that produce hormones that regulate blood glucose levels. Hormones produced in the pancreatic islets are secreted directly into the bloodstream by five different types of cells.
    • The alpha cells produce glucagon, and make up 15–20% of total islet cells. The beta cells produce insulin and amylin, and make up 65–80% of the total islet cells. The delta cells produce somatostatin, and make up 3–10% of the total islet cells.
    • The gamma cells produce pancreatic polypeptide, and make up 3–5% of the total islet cells. The epsilon cells produce ghrelin, and make up less than 1% of the total islet cells.
    • The feedback system of the pancreatic islets is paracrine, and is based on the activation and inhibition of the islet cells by the endocrine hormones produced in the islets.

     

    Key Terms

     

    • endocrine: Produces internal secretions that are transported around the body by the bloodstream.
    • paracrine: Describes a hormone or other secretion released from endocrine cells into the surrounding tissue rather than into the bloodstream.
    • exocrine: Produces external secretions that are released through a duct.

    The pancreas serves two functions, endocrine and exocrine. The exocrine function of the pancreas is involved in digestion, and these associated structures are known as the pancreatic acini.

    The pancreatic acini are clusters of cells that produce digestive enzymes and secretions and make up the bulk of the pancreas. The endocrine function of the pancreas helps maintain blood glucose levels, and the structures involved are known as the pancreatic islets, or the islets of Langerhans.

    This is an illustration of the pancreas with a detailed view of a pancreatic islet with endocrine cells. The islet is surround by the pancreatic acini and pancreatic duct.

     

    Pancreatic islets or islets of Langerhans: The islets of Langerhans are the regions of the pancreas that contain its endocrine (hormone-producing) cells.

    The pancreatic islets are small islands of cells that produce hormones that regulate blood glucose levels. Hormones produced in the pancreatic islets are secreted directly into the blood flow by five different types of cells.

    This is a photo taken through a microscope of pancreatic tissue. The small cells in the middle are beta cells, and the surrounding larger cells are alpha, delta, gamma, and epsilon cells.

     

    Pancreatic tissue: The small cells in the middle are beta cells, and the surrounding larger cells are alpha, delta, gamma, and epsilon cells.

    The endocrine cell subsets are:

    • Alpha cells that produce glucagon and make up 15–20% of total islet cells. Glucagon is a hormone that raises blood glucose levels by stimulating the liver to convert its glycogen into glucose.
    • Beta cells that produce insulin and amylin and make up 65–80% of the total islet cells. Insulin lowers blood glucose levels by stimulating cells to take up glucose out of the blood stream. Amylin slows gastric emptying, preventing spikes in blood glucose levels.
    • Delta cells that produce somatostatin and make up 3–10% of the total islet cells. Somatostatin is a hormone that suppresses the release of the other hormones made in the pancreas.
    • Gamma cells that produce pancreatic polypeptide and make up 3–5% of the total islet cells. Pancreatic polypeptide regulates both the endocrine and exocrine pancreatic secretions.
    • Epsilon cells that produce ghrelin and make up less than 1% of the total islet cells. Ghrelin is a protein that stimulates hunger.

    The feedback system of the pancreatic islets is paracrine—it is based on the activation and inhibition of the islet cells by the endocrine hormones produced in the islets. Insulin activates beta cells and inhibits alpha cells, while glucagon activates alpha cells, which activates beta cells and delta cells. Somatostatin inhibits the activity of alpha cells and beta cells.