Sites of O2− production in neutrophils as a function of time after activation.35 (Top) O2− production starts in secretory vesicles (clumped precipitate; single arrow). The membranes of these vesicles are distinct from the plasma membrane, because none of the O2−-forming vesicles contain the ferritin (punctate precipitate, double arrow) that was added to the extracellular medium during the incubation and then was internalized by pinocytosis. (Center) The O2−-forming secretory vesicles later fuse with pinocytotic vesicles to form secondary vesicles. The oxidase in the membranes of these secondary vesicles continue to deliver O2− into the vesicle lumina. (Bottom) O2−-forming secretory vesicles also fuse with the plasma membrane itself, leading to the secretion of O2− into the extracellular environment. (Reprinted with permission from Kobayashi T, Robinson JM, Seguchi H: Identification of intracellular sites of superoxide production in stimulated neutrophils. J Cell Sci 111:81, 1998, published by The Company of Biologists, Ltd, Cambridge, UK.35)
