Fenestrae in the Rough Endoplasmic Reticulum of the Exocrine Pancreatic Cells

Since its original description, the endoplasmic reticulum (ER) has been considered to be a system of flattened cisternae interconnected with canalicular bridges or tubules (1) . This three-dimensional model of the ER was inferred mainly from images obtained on spread cells in toto where the ER appears as a continuous branching network . Subsequently, Palade (2) reported that the ER cisternae were not only branched and connected with bridges but also fenestrated . However, since thin sections for electron microscopy give only a limited sampling of an entire ER sheet, the number and the distribution of the fenestrae could not be accurately assessed . By exposing large areas of cellular membranes, the technique of freeze etching retrieves the threedimensional image lost by sectioning, and this communication will present data from freezeetched preparations showing clear images of fenestrae in the ER cisternae of the exocrine pancreatic cells of several species .