(A) Secretory proteins become segregated and highly concentrated in secretory vesicles by two mechanisms. First, they aggregate in the ionic environment of the trans Golgi network; often the aggregates become more condensed as secretory vesicles mature and their lumen becomes more acidic. Second, excess membrane and lumenal content present in immature secretory vesicles are retrieved in clathrin-coated vesicles as the secretory vesicles mature. (B) This electron micrograph shows secretory vesicles forming from the trans Golgi network in an insulin-secreting β-cell of the pancreas. An antibody conjugated to gold spheres (black dots) has been used to locate clathrin molecules. The immature secretory vesicles (open arrow), which contain insulin precursor protein (proinsulin), contain clathrin patches. Clathrin coats are no longer seen on the mature secretory vesicle, which has a highly condensed core (solid arrow). (Courtesy of Lelio Orci.)