MEDIMAGE / SCIENCE PHOTO LIBRARY MEDIMAGE / SCIENCE PHOTO LIBRARY
Pancreatic secretory cells. Coloured transmission electron micrograph (TEM) of a slice through a cluster of enzyme-secreting acinar cells 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, yellow), 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.
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