Thin section of a rat pancreatic acinar cell. The section was fixed in 3% formaldehyde, cryoprotected in 30% polypropylene glycol, dehydrated by freeze substitution, and low-temperature embedded in Lowicryl HM20. Cells were immunolabeled for the presence of amylase. Gold particles indicate the rough endoplasmic reticulum (arrows) and zymogen granules (Z). Mitochondria are unlabeled, showing that nonspecific labeling is low. Bar, 200 nm. (Reprinted with permission from Scion Publishing Ltd. © 2006.)