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Microsomes and Ribonucleoprotein Particles 



GEORGE E. PALADE 



The Laboratories of the Rockefeller Institute for Medical Research 



I should like to present in the following pages a short history of the develop- 

 ment of our present concepts on microsomes and ribonucleoprotein (RNP) 

 particles. I consider the historical background of this field of research interesting 

 in itself; moreover, I believe that its knowledge may throw some light on the 

 actual relations between microsomes and RNP particles as well as on some basic 

 principles of biological organization. 



THE MICROSOMES 



The discovery of the microsomes was a by-product of work done on virus- 

 induced tumors by Albert Claude at the Rockefeller Institute in the late 1930's. 

 Trying to purify a tumor-inducing fraction obtained by differential centrifuga- 

 tion from breis of Rous sarcomata, 1 Claude found as expected that the prepara- 

 tion was rich in ribonucleic acid (RNA) [1], and was inactivated by various 

 agents known to affect nucleoproteins and nucleic acids [2]. At the same time, 

 however, he unexpectedly discovered that cell fractions, similar in their gross 

 chemistry to the tumor-inducing preparations, could be isolated from chick 

 embryos [3], and from a variety of tissues, adult as well as embryonic, and 

 normal as well as tumorous [4, 5]. From the beginning Claude was convinced 

 that these fractions consisted of pre-existing cell structures, not of cytoplasmic 

 aggregates artificially produced by tissue grinding. After some hesitation, 2 he 

 [4, 5] arrived at the conclusion that the structures involved were new cell 

 components of widespread occurrence which had eluded detection by light 

 microscopy because they were too close to, or below, the limit of resolution of 



1 Chicken tumor I. 



2 For a while he assumed that the fraction consisted of mitochondria or mitochondrial 

 fragments. 



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