STUDIES ON THE ENDOPLASMIC RETICULUM* I. ITs IDENTIFICATION IN CELLS IN SITU

In 1945, Porter, Claude, and Fullam (1) noticed the presence of a "lace-like reticulum" in the cytoplasm of avian cells cultured in vitro and examined in the electron microscope after appropriate fixation and mounting. The retieulum, which appeared to be part of the ground substance of the cytoplasm, was found to consist usually of interconnected "strands and vesicles" of small dimensions (100 to 150 m#) and relatively low density. The new cytoplasmic component was later studied in more detail, in various avian and mammalian cell types by Porter and Thompson (2, 3), Porter and Kallman (4), and Porter (5), who described it under the name "endoplasmic reticulum" because of its general morphology and intracellular location. Comparable findings were reported by Oberling et al. (6), Martin and Tomlin (7), and by Selby and Berger (8) using similar materials and methods. While observations on the endoplasmic reficulum in cells cultured in vitro continued to accumulate, information concerning corresponding structures in cells in situ 1 remained sparse and unsatisfactory, mainly because, in their case, preparatory procedures were too crude to permit examination at the level of resolution attained with cultured material. In the last few years, however, this impediment has been removed, at least in part, by a general improvement in the techniques for fixation (9), imbedding (10), and microtomy (10, 11). As a result, satisfactory resolution can be obtained consistently at present on sectioned material as demonstrated by a number of recent electron microscope studies. Moreover, in some of these studies, structures in the range of dimensions of the endoplasmic reticulum were actually resolved in the cyto-