PALADE 47 



attached particles and the amount of RNA present in microsomal pellets. They 

 indicate, however, that particles appear after treating predominantly mem- 

 branous pellets with DOC, and they suggest that the particles could be masked 

 by incorporation into the membrane under certain uncontrolled conditions. It 

 is evident that we should know more about the behavior of these particles 

 under various metabolic conditions, and about their reaction to various suspen- 

 sion media used in tissue fractionation, before arriving at an understanding 

 of these conflicting pieces of evidence. 



CONCLUSIONS AND COMMENTS 



In conclusion it can be said that small (~ 15 m/*), dense particles have been 

 found, either free or attached to the membrane of the endoplasmic reticulum, 

 in practically all animal and plant cells thus far examined. Comparable parti- 

 cles, usually unattached to membranous structures, exist in bacterial cells. The 

 distribution of these particles is largely similar to that of cytoplasmic basophilia, 

 by implication to that of cytoplasmic RNA. 



Ribonucleoprotein particles of relatively small size (10 to 20 mn and 40 to 

 80 S) have been isolated by various procedures, such as ultracentrifugation and 

 electrophoresis, from a variety of animal, plant, and bacterial sources. Evidence 

 that the small, dense particles seen in the intact cell consist of ribonucleoprotein 

 has been obtained for the liver (rat) and the pancreas (guinea pig). Accord- 

 ingly the assumption that these particles are the structural substrate of basophilia 

 has been verified for two cell types only. For all the others, it remains what 

 it has been, a hypothesis to be tested by further work. Integrated studies pro- 

 viding an adequate coverage from cell to pellets are evidently needed for more 

 kinds of tissues. 



Although it appears that the RNP particles are cytoplasmic components of 

 widespread occurrence, probably basic structural elements in the organization 

 of the cell, many points in their history are still uncertain or controverted, and 

 many pertinent questions remain unanswered. For instance, though there is 

 good agreement about the presence of RNA in particles, there is still doubt 

 about the presence or absence of RNA in membranous structures, primarily in 

 the membrane of the endoplasmic reticulum. Considerable variation occurs, 

 apparently connected with the methods of preparation, in the RNA content of 

 these particles; accordingly one would like to know the procedure by which 

 the situation in situ is more closely approximated. There is morphological, 

 physicochemical, and metabolic diversity among these particles, but very little 

 is known about the way in which the various differences are correlated, or 

 about the significance of this diversity. In this respect one may wonder whether 

 particles with different locations and activities represent distinct, fully developed 

 cell organs, or whether they correspond to successive stages in the differentiation 

 of a single or a few cell organs. What seems to be particularly disturbing at 

 present is the meagerness of our information about the functional role of these 



