Transmission electron micrograph (TEM) of the ultrastructure of pancreatic acinar cells. The dominant features are the basal location of rough endoplasmic reticulum and the many circular granules that store digestive enzymes. These are called zymogen granules and upon stimulation of the cells are released into the branching duct system that permeates the tissue of the pancreas. Digestive enzymes pass through the duct system and enter the duodenum where they act to breakdown dietary products delivered from the stomach. Pancreatic acinar cells show a high level of protein (enzyme) synthesis reflected by the abundant supply of rough endoplasmic reticulum. Magnification: x4,000 when printed at a width of 10cm.