(a) Immediately after secretory proteins are made on ribosomes of the rough ER, they are found in the lumen of the rough ER. Transfer vesicles transport them to Golgi vesicles, where they are concentrated and packaged into secretory vesicles containing granules of zymogens (pancreatic enzyme precursors, such as chymotrypsinogen). Several of these coalesce to form a storage vesicle; these accumulate under the apical surface, which faces the lumen of an acinus. Hormone or nerve stimulation triggers fusion of the vesicles with the plasma membrane, releasing the vesicles’ contents into the lumen (exocytosis). (b) Once released from acinar cells, the inactive precursors move through ductules and the pancreatic duct to the intestine, where they are proteolytically activated into digestive enzymes.