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Cover   Exocytosis is now the subject of intense molecular dissection. The factors that induce first the recognition, then the close approach, and finally the fusion of secretory vesicles with the plasma membrane are being purified, mutated, and deleted. The first direct evidence for exocytosis was obtained in the mid-1950s by George Palade, who was studying the secretion of digestive enzymes, most of them in their inactive or "zymogen" form, from exocrine cells of the guinea pig pancreas. This micrograph (from Functional Changes in the Structure of Cell Components. Subcellular Particles, Washington, DC: American Physiological Society, 1959) shows portions of three exocrine cells that form an acinus (a compartment with a central lumen into which the cells can secrete). In one cell a secretory granule filled with zymogen (z₁) is discharging into the acinar lumen (lm) (which is in continuity with a branch of the pancreatic duct). The membrane of the zymogen granule appears to be continuous with the plasmalemma of the cell (small arrows). The three exocrine cells surround this duct, and microvilli (long arrows) in cross or oblique section are seen as lightly staining compartments against the dense background provided by the discharged zymogen. This micrograph also demonstrates what has become known as "piggyback exocytosis," in which a second zymogen granule (z₂) discharges by fusing with another granule that is already in the process of exocytosis, rather than directly with the plasmalemma of the exocrine cell.
  In subsequent years, other examples of exocytosis have been captured (although not as many as one might have predicted). One example is Figure 10 in Palade's Nobel lecture (Science, 189, 347-358, 1975). The image shown here was the first confirmation that zymogen granules are transport vesicles that secrete their content by fusion with the plasmalemma; it is justly famous, even though specimen preservation appears by modern standards to be less than optimal. The concepts suggested by this image allowed more precise hypotheses to be proposed and tested, leading to our current understanding of secretion. J. Richard McIntosh and Kathryn Howell