Transmission electron micrograph (TEM) of the ultrastructure of the production and packaging of proteins within a pancreatic exocrine cell. Peptides and polypeptides are assembled and folded in the rough endoplasmic reticulum (rER) seen to the left and right of the cell nucleus (lower centre). Packaged within vesicles from the rER, proteins are transported for further processing to Golgi membranes from which dilated vesicles form larger immature secretory vesicles, or granules, seen here as pale grey in density. Mature zymogen granules, seen as numerous circular structures of medium density, represent the final concentration and compaction of proteins. Zymogen granules are stored in the cytoplasm until their contents are secreted by exocytosis. Magnification: x8,000 when printed at a width of 10cm.