Islets of Langerhans, also called islands of Langerhans are irregularly shaped patches of endocrine tissue located within the pancreas of most vertebrates. They are named for the German physician Paul Langerhans, who first described them in 1869. The normal human pancreas contains about 1,000,000 islets. The islets consist of four distinct cell types, of which three produce important hormones (alpha cells producing glucagon; beta cells producing insulin and delta cells producing somatostatin). Hormones produced in the islets of Langerhans are secreted directly into the blood flow. The fourth component PP cells secretes pancreatic polypeptide. The combined mass of the islets is 1 to 1.5 grams.