46 MICROSOMAL PARTICLES 



RECENT INFORMATION ON CYTOPLASMIC BASOPHILIA AND RNP PARTICLES 



During the past 5 years considerable progress has been made in the study of 

 cell organization through the extensive use of electron microscopy and cell- 

 fractionation procedures. Accordingly it is of interest to see how the hypothesis 

 formulated in 1953 has fared through this period of rapid development. 



The assumption that the small particulate component of the cytoplasm is 

 the structural substrate of basophilia has remained in good agreement with 

 the large majority of the findings made on new and very numerous cell types 

 of animal and plant origin (see for examples [59, 60]). The only exception so 

 far encountered is represented by the heart muscle of the turtle [61], in which 

 a slightly larger particulate was found in an acidophil cytoplasm that gave a 

 positive test for glycogen. Consequently it was postulated that particulate glyco- 

 gen might be mistaken for RNP particles in certain cell types, especially in 

 muscle [61]. The actual isolation of this particulate material from various 

 muscle fibers could settle the question, but so far it has not been accomplished. 

 It should be pointed out, however, that under prevailing technical conditions a 

 certain amount of confusion of the type suggested cannot be excluded. Because 

 of the dimensions involved, we are examining only the gross morphology of 

 the small particles, and in so doing we are not helped thus far by any charac- 

 teristic detail of structure. If particles of different chemical composition and of 

 different fractions happen to have the same general size and shape, we cannot 

 avoid lumping them together in a common category. Morphological expressions 

 that can be distinguished at the present level of practical resolution are un- 

 doubtedly less numerous than functional characteristics or macromolecular 

 species. It is exactly for this reason that morphological information should be 

 supplemented, wherever possible, by biochemical and metabolic data. 



Ribonucleoprotein particles, with a sedimentation constant of 70 to 80 S and 

 a calculated or measured diameter of 10 to 15 m/x, have been isolated from 

 many new and old sources such as liver [62, 65], yeast [63], ascites cells [56], 

 and pea seedlings [64]. They have been described in terms of their gross 

 chemistry [62-65], biochemical activities [56], and physicochemical properties 

 [62-65]. A perusal of this symposium shows that recently RNP particles have 

 also been isolated from a variety of bacterial cells. In general, there is good 

 agreement between these findings and our observations, but there is little or no 

 information about the existence, frequency, and topography of the particles 

 in situ. 



Finally it is still debated whether the microsomal RNA is mainly located in 

 the attached particles or is also present in large amounts in the microsomal 

 membranes as originally assumed by Kuff et al. [66]. As already mentioned, 

 an exclusive RNA location in the attached particles cannot be claimed because 

 the particles do not account for ~ 20 per cent of the microsomal RNA in the 

 liver and for ~40 per cent in the pancreas. Recently Chauveau et al. [67] 

 found that there is no good correlation between the frequency of vesicles with 



