Pancreatic alpha cell. Coloured transmission electron micrograph (TEM) of a hormone-secreting (endocrine) alpha cell, found in the islets of Langerhans of the pancreas. Alpha cells make up 15-20% of the cells in the islets. They produce glucagon, a hormone that regulates blood sugar levels by increasing the amount of glucose available. The glucagon is carried into the bloodstream by granules (green). The nucleus (round, brown) and mitochondria (purple) can also be seen. Magnification: x10000 when printed 10cm wide.