Caption

Pancreatic secretory cell. Coloured transmission electron micrograph (TEM) of a slice through an enzyme-secreting acinar cell from a pancreas. Most of the pancreas is made of cells like this, which produce large quantities of digestive enzymes in a secretion called zymogen. The zymogen is stored in large round secretory granules (zymogen granules, blue), before being exported from the cell and carried by pancreatic ducts to the small intestine. Zymogen is secreted as an inactive 'proenzyme', which is only activated once it reaches the intestine, to prevent self-digestion of the pancreas.