PALADE 37 



usual light optics. 3 In his early reports he described the new components as 

 "small particles" or "small granules"; later he chose the term "microsomes" 

 (small bodies) [6, 7], which met with favor and has remained in common use 

 ever since. From his centrifugation data, 4 Claude calculated that the micro- 

 somes measured ~ 50 to 200 m/* in diameter, and on the strength of his 

 chemical analyses he defined them as "phospholipide-ribonucleoprotein com- 

 plexes" [4, 5]. In this way and thus defined the microsomes entered the bio- 

 chemical thinking of our times. 



Claude's discovery was followed by an extensive period of biochemical re- 

 search which confirmed and greatly extended his findings. For practical rea- 

 sons, the research effort was concentrated almost exclusively on liver, and as a 

 result the voluminous literature thereby produced (for reviews see [8, 9, 10]) 

 applies primarily to liver microsomes, not, as usually assumed, to microsomes in 

 general. According to this literature, the dominant biochemical feature of the 

 microsomal fraction is its high RNA content: ~ 40 to 50 per cent of the RNA 

 of the tissue brei is usually recovered in the microsomes, together with ~ 15 per 

 cent of its proteins. Consequently the microsomal RNA/protein ratio is high— 

 until recently higher than that of any other cell fraction. It should be men- 

 tioned, however, that, with all this concentration, the RNA does not represent 

 more than ~ 10 per cent of the microsomal dry weight. Another apparently 

 characteristic feature of the fraction is its large content and high concentration 

 of phospholipides : ~ 50 per cent of the phospholipides of the tissue are re- 

 covered in the microsomes. As far as biochemical activities are concerned, the 

 microsomes are distinguished by high concentrations of diphosphopyridine 

 nucleotide-cytochrome c reductase [11], cytochrome m or b- [12, 13], and glu- 

 cose-6-phosphatase [14], and especially by their ability to incorporate labeled 

 amino acids into their proteins both in vivo [15-19] and in vitro [20-22]. Ac- 

 cording to current interpretations, the last property indicates that the micro- 

 somes are, or contain, the sites of protein synthesis of the cytoplasm. 



In contrast with the active and diversified work on the biochemical aspects 

 of the problem, research on the identity of the microsomes, or of their pre- 

 cursors, inside the intact cell made little progress because of a number of tech- 

 nical limitations. Of the instruments available for morphological investigation, 



3 The microsomes can be seen as distinct particles in the dark-field microscope [5], and 

 as a shimmering mass of indistinct small bodies in the light microscope, especially under 

 phase contrast optics. 



4 For Claude the microsomes were the fraction sedimented in 1 hour at 18,0(% from the 

 supernatant of the "large granule" (mitochondrial) fraction. The medium he used in pre- 

 paring tissue "extracts" was either water or dilute phosphate buffer or 0.15 M NaCl. When 

 sucrose solutions were introduced in cell-fractionation procedures, the centrifugal force was 

 increased to compensate for the higher density and viscosity of the new medium. At 

 present the microsomes are usually separated by centrifuging a mitochondrial supernatant 

 for 1 hour at ~ 100,00%, irrespective of the sucrose concentration in the suspending 

 medium. Since this concentration varies from 0.25 to 0.88 M, the microsome fractions 

 described in the literature are not strictly comparable to one another. 



