Pancreatic cells. Coloured transmission electron micrograph (TEM) of acinar (exocrine) pancreatic cells (red) adjacent to hormone- secreting (endocrine) Islet of Langerhans cells (yellow). Acinar cells secrete zymogens (inactive digestive enzymes) in vesicles called zymogen granules (large purple blobs, left), which then become activated when in the small intestine. Islet of Langerhans cells regulate blood sugar levels by secreting hormones such as insulin and glucagon, that are also transported out of the cell by granules (small red dots, right). Magnification: x2000 when printed 10 centimetres high.