Transmission electron micrograph (TEM) of the ultrastructure of an acinus in the exocrine component of the pancreas. The acinus is a collection of enzyme-secreting cells arranged in a cluster at the end of a duct into which the enzymes are released. These enzymes (proteases, lipases, trypsinogens and many others) serve digestive roles when they enter the duodenum via a pancreatic duct. The defining features of the pancreatic acinar cells are the many granules of stored inactive enzymes (zymogen granules) and rough endoplasmic reticulum concentrated near the cell nuclei. Magnification: x3,000 when printed at a width of 10cm.