The endocrine (i.e. hormone-producing) cells of the pancreas are grouped in the so-called Islets of Langerhans. Discovered in 1869 by the German pathological anatomist Paul Langerhans (1847-1888), the Islets of Langerhans constitute 1-2% of the mass of the pancreas. There are about 1 million of them and their combined weight is 1 to 1.5 gram. Each islet contains a few thousand cells and is 0.2-0.5mm in diameter.

Hormones produced in the Islets of Langerhans are secreted directly into the blood flow by (at least) four different types of cells:

• 65-80% of the islet cells are insulin-producing beta cells.
• The second most abundant cell type is the glucagon-releasing alpha cells (15-20%).

Additionally, Islets of Langerhans contain

• somatostatin-producing delta cells (3-10%)
• and pancreatic polypeptide-containing PP cells (1%).

Islet cells can influence each other through paracrine and autocrine communication, and beta-cells are coupled electrically to beta-cells (but not to other cell-types!).

The paracrine feed-back system of the Islets of Langerhans has the following structure:

• Insulin: Activates beta-cells, inhibits alpha-cells.
• Glucagon: Activates beta-cells and delta-cells.
• Somatostatin: Inhibits alpha-cells and beta-cells.

Electrical activity of pancreatic islet-cells has been studied using the patch-clamp technique , and it has turned out that the behaviour of cells in intact islets differs significantly from the behaviour of dispersed cells.