Electron microscopy: cytology of cell fractions.

It should be evident from this brief account that electron microscopy of thin sections is an invaluable asset in the study of fractions isolated by differential centrifugation. I have tried to indicate how the integrity of particles, purity of fractions, and the existence of new particles can be established through its use. I have also suggested the desirability of a common terminology for all cytology -classic, electron-microscopic, and biochemical. Some have expressed the opinion that neutral terms such as alpha, beta, and gamma membranes (59) are more useful than ergastoplasm, Golgi apparatus, and so forth. As helpful as such neutral terms may be in describing intracellular structures, they do not appear to me to substitute for historically rooted cytological names. Note added in proof: Since this article went to press there has appeared an important article by G. E. Palade and P. Siekevitz (60). These authors consider that the vesicles without granules found in the microsome fraction were "probably derived from the smooth surfaced parts of the endoplasmic reticulum." The latter were found to be continuous with the granule-studded membranes; "the two varieties of profiles represent local differentiation within a common system." The authors confirm the finding of Rouiller (18) and Novikoff et al. (7) of the dense bodies adjacent to the bile canaliculi and describe their presence in the microsome fraction as a "minor component."