Exocytosis involves highly localized membrane fusions.

Secretory cells of many types store their secretions in membrane-bound granules, the contents of which are released, on demand, by exocytosis. During exocytosis the granule and plasma membrane must fuse. Electron microscopic studies of chemically fixed tissues (Palade & Bruns, 1968; Tandler & Poulsen, 1976; Lawson et al., 1977; Pinto da Silva & Nogueira, 1977) suggest that membrane fusion is initiated by apposition of these two membranes, sometimes over a rather large area, to form a ‘pentalaminar’ structure. This area of contact then undergoes a structural rearrangement to form a single bilayer that is continuous with both secretory granule and plasma membranes (the so-called trilaminar structure). This bilayer is then thought to break forming an aqueous pore that connects the granule interior with the extracellular space. This scheme has been reviewed by Palade (1975). More recently, rapid arrest of exocytosis by quickfreezing has shown that fusion of granule and plasma membranes actually begins at a highly localized region. In amebocytes (Ornberg & Reese, 198 1) fusion is preceded by a pedestal-like depression in the plasma membrane. Contact of this pedestal with the granule membrane, over an area as small as 0.2pm in diameter, is thought to lead to fusion and formation of a very narrow pore. This pore rapidly expands as granule contents are expelled. Our previous studies have shown that in mast cells (Chandler & Heuser, 1980) and in neutrophils (Chandler e f al., 1983) exocytosis also begins with formation of a small pore. This is true both for fusion of granule and plasma membranes at the cell surface and for fusion of neighbouring granules within the cell interior. Here I present data in these same systems that not only confirm the presence of these highly localized contacts before fusion, but also show that fusion may involve a population of small vesicles as well as the secretory granules themselves. Rat peritoneal mast cells were suspended in phosphatebuffered saline (Chandler & Heuser, 1980), stimulated by exposure to 8 pg of the histamine releaser 48/80/ml, and 5200s after stimulation rapidly frozen by contact with a liquid-helium-cooled copper block (Heuser et af. , 1979). Rabbit peritoneal neutrophils were suspended in modified Tyrodes buffer (Chandler et af. , 1983), treated with 5pg of