3.25 in. x 4 in. Lantern Slides and Cell fractionation confirmed that newly synthesized pancreatic acinar cell secretory proteins moved from the RER to the periphery of the Golgi in the lumen of a smooth-surfaced membrane bounded compartment. While EM autoradiography told us a great deal about the route and timetable of intracellular transport of nascent proteins, limitations of resolution of the technique (~0.1 micron) precluded a definitive localization to a cell compartment. This was provided by cell fractionation in which rough and smooth microsomes could be obtained in relative purity from pulse labeled pancreatic slices and further fractionated into lumenal content of the microsomes and membranes. The data clearly showed that nascent proteins were always in transit through the cell in the lumen of vesicles derived from the RER and Golgi complex, were not associated with the membranes, and did not travel in soluble form through the cytosol. The top band in this linear sucrose gradient (density of 1.14 to 1.27 gm/cc) contained primarily smooth/Golgi microsome while the largest band in the bottom half of the gradient consisted of relatively pure rough microsomes.
3.25 in. x 4 in. Lantern Slides, Images such as these indicated that secretory granules closely and selectively approach the apical membrane of the pancreatic acinar cell where exocytosis takes place, fuse with the apposed membrane, and release the content of secretory proteins into the acinar lumen without broaching the intracellular environment. The resulting omega figure is transitory, suggesting that the excess granule membrane is retrieved/recycled. Note the actin coat around the newly inserted granule membrane. The reference below expands on this observation 33 years later., and Original Magnification: x48,000
