FIGURE 1 The synthesis and movement of guinea pig pancreatic secretory proteins as revealed by electron microscopic autoradiography. After a period of labeling with [³H]leucine, guinea pig pancreatic tissue was fixed, sectioned for electron microscopy, and subjected to autoradiography. The radioactive decay of [³H] in newly synthesized proteins produces autoradiographic grains in an emulsion placed over the cell section (which appear in the micrograph as dense, wormlike granules); these grains mark the positions of newly synthesized proteins. (a) At the end of a 3-minute labeling period, autoradiographic grains are over the rough ER. (b) Following a 7-minute chase period with unlabeled leucine, most of the labeled proteins have moved to the Golgi. (c) After a 37-minute chase, most of the grains are over immature secretory vesicles. (d) After a 117-minute chase, the majority of the grains are over mature zymogen granules.

[James D. Jamieson and George E. Palade.]