The islets of Langerhans, also known as pancreatic islets, are clusters of endocrine cells in the pancreas found in most vertebrates. Named after the German doctor Paul Langerhans who discovered them in 1869, a typical human pancreas contains about 1.25 million of these islets. They consist of four types of cells, with three (alpha, beta, and delta cells) producing essential hormones, while the fourth type (C cells) has no known function.
These islets are crucial for hormone production, which helps the body break down and digest food. Among the various cells, beta cells are particularly important as they produce insulin. Insulin is a hormone that helps the body use glucose for energy and regulates blood sugar levels. It plays a vital role in carbohydrate, fat, and protein metabolism by facilitating glucose absorption by body cells, preventing glucose release from the liver, and promoting amino acid absorption by muscle cells. Insulin release is primarily triggered by increased blood glucose levels, such as after eating. A failure to produce or release enough insulin can lead to diabetes mellitus.
The islets also contain alpha cells, which release glucagon, a hormone that encourages the liver to produce glucose and release fatty acids. This process supports insulin release and inhibits glucagon release. Delta cells produce somatostatin, which inhibits insulin, somatotropin, and glucagon, though its full role in metabolism is not completely understood. Somatostatin is also produced by the hypothalamus to prevent growth hormone release from the pituitary gland.
Glucagon release is stimulated by protein intake, low blood sugar, and exercise, while carbohydrate intake inhibits it. Glucagon raises blood glucose levels by promoting glycogen breakdown and gluconeogenesis in the liver, which is crucial during fasting and exercise.
Pancreatic polypeptide, produced by F (or PP) cells in the islets, consists of 36 amino acids. Its secretion is stimulated by food, exercise, and fasting, and it may inhibit gallbladder contraction and pancreatic enzyme production, though its role in nutrient metabolism is unclear.
In the pancreas, somatostatin from delta cells helps regulate insulin and glucagon release. These hormones work together to manage nutrient movement in and out of circulation, affecting the absorption, use, and storage of amino acids, glucose, and fatty acids. The close proximity of beta, alpha, and delta cells in the islets is essential for their function. Somatostatin and glucagon have a paracrine relationship, influencing each other’s production and the rate of insulin release. Somatostatin also inhibits various gastrointestinal hormones, affecting digestion and enzyme secretion.
