STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 63 



mass appeared in tlie form of soluble proteins. It is reported that, as analyzed 

 in the ultracentrifuge, the proteins derived from hepatoma mitochondria lack 

 a component found in normal liver mitochondria (Hogeboom and Sclmeider, 

 1951). 



The RNA concentration of the mitochondria is far less than that of the 

 inicrosomal fraction. The proportion of the total RNA contained in the 

 mitochondria has been reported as 5 to 7 % (see Table VIII, Hogeboom and 

 Schneider, 1955). Claude (1944) lysed the mitochondria of rat liver in distiUed 

 water and showed that the ENA was associated ^vith small microsome-like 

 particulates which were still sedimentable, although the ribose-containing 

 coenzymes were now in a low molecular dialyzable state. Lowe and Lehninger 

 (1955) have reported the presence of high molecular weight KNA in the most 

 carefully fractionated mitochondria of rat hver, although finding that the 

 mitochondria derived from cortisone-treated rats lack such nucleic acid. 

 Significantly, the latter particles are still capable of normal oxidative 

 phosphorylation. 



The lipids of rat liver mitochondria haA^e been analyzed by Swanson and 

 Artom (1950). About half the phosphorus is present in phospholipid, com- 

 prising 80 % of the total lipid, the remainder consisting of neutral fat, 

 cholesterol, and other substances. Lecithin, cephalin, and sphingomyehn have 

 been demonstrated, as have relatively large amounts of inositol (0.5 %). 



6. The Microsomal Fraction 



The term "microsomes" was used by F. R. Lillie as early as 1906 in de- 

 scribing the smallest visible cytoplasmic particles m the developing poly- 

 chaete embryo. The term is now widely used in the sense introduced by 

 Claude m 1940 to describe the submicroscopic basophilic particles present in 

 the ground substance of cell cytoplasm. Some Avorkers are also using the term 

 "chromidia" to apply to this cytoplasmic fraction (Fiala et al., 1956). Most 

 recently, it has been shown that in highly differentiated cells the dense 

 basophilic particles are usually organized on membranes and are oriented 

 internally within the cytoplasmic system of vesicles and tubules, which is 

 given the name of endoplasmic reticulum (Palade and Porter, 1954; Palade 

 and Siekevitz, 1956). This reticulum can be compared to an intracellular 

 vascular system. The extended form of these particle-studded membranes 

 does not readily survive homogenization, and, for the most part, then, isolated 

 microsomal fractions represent much abused artifacts of preparation. In cell 

 types characterized by rapid proUferation, the dense particles may be more 

 or less freely distributed in the cytoplasm (Palade, 1955). A microsomal 

 fraction has been isolated from all cells studied. 



Claude believed that microsomes possess comparable chemical properties, 

 although the isolated particles range from 50 to 300 m/x in diameter, with the 



VOL. 1^6 



