The islets of Langerhans are the regions of the pancreas that contain its endocrine (i.e. , hormone-producing) cells. Discovered in 1869 by German pathological anatomist Paul Langerhans at the age of 22, the islets of Langerhans constitute approximately 1 to 2% of the mass of the pancreas. There are about one million islets in a healthy adult human pancreas, which are distributed throughout the organ; their combined mass is 1 to 1.5 grams.