354 MAHLON B. HOAGLAND 



varying amounts of the fragmented membranes of the endoplasmic reticu- 

 lum. Thus microsomes are centrifugally heterogeneous chiefly because the 

 "particle" size is determined by the extent of damage to the membranous 

 component to which the ribosomes are attached. When measures are taken 

 to remove the lipoprotein membranous component by treatment with a 

 lipid solvent, considerably more uniform material is obtained. 



It has been well documented that whole microsomes vary in ultracentrif- 

 ugal sedimentation characteristics depending both on the tissue of origin 

 and the metabolic activity of the tissue. A valid generalization seems to 

 emerge that cells which are rapidly proliferating (such as tumors, embry- 

 onic, and regenerating mammalian cells) yield microsome fractions with a 

 relatively wider and lower average size distribution; while adult differ- 

 entiated mammalian cells contain more rapidly sedimenting microsomes. 

 The relatively more rapid sedimentation properties of the latter particles 

 is in part due to the fact that a greater proportion of the ribosomes are 

 still attached to the endoplasmic membrane; hence, they are by definition 

 microsomes (cf. Petermann et al. 16h ). Accompanying the change to a more 

 self-reproducing synthetic machinery in mammalian tissues is a tendency 

 to find the separation of ribosomes from these membranes resulting in a 

 random dispersion of ribosomes in the cytoplasm. 32 ' 37 



Since the ribosomes are, as we shall see, able to synthesize protein auton- 

 omously we may ask — what is the role of the membranous component of 

 the endoplasmic reticulum? Palade has said: "What is known so far about 

 the fine structure of bacterial cells suggests that internal membranous sys- 

 tems, like the endoplasmic reticulum, are not necessary for the organization 

 and function of a simple type of self-sustaining cell. Such membranous sys- 

 tems appear in more elaborate cell forms and could therefore be regarded 

 as superstructures. We do not know what special problems are solved by 

 their introduction, but we may wonder whether they are not connected 

 with an increase in cell volume, subsequent difficulties in diffusion, and 

 relative decrease in available surface. " 16a 



Palade 38 and Siekevitz and Palade 39 • 40 have obtained preliminary evi- 



32 G. E. Palade, J. Biophys. Biochem. Cytol. 1, 59 (1955). 



33 A. F. Howatson and A. W. Ham, Cancer Research 15, 65 (1955). 



34 M. L. Petermann, N. A. Mizen, and M. G. Hamilton, Cancer Research 16, 620 

 (1956). 



35 J. W. Littlefield and E. B. Keller, J. Biol. Chem. 224, 13 (1957). 



36 E. Hay, J. Biophys. Biochem. Cytol. 4, 583 (1958). 



37 B. Munger, Am. J . Anat. in press. 



38 G. E. Palade, in "Subcellular Particles" (Teru Hayashi, ed.), p. 64. Ronald Press, 

 New York, 1959. 



39 P. Siekevitz arid G. E. Palade, J. Biophys. Biochem. Cytol. 4, 557 (1958). 



40 P. Siekevitz and G. E. Palade, Federation Proc. 18, 1282 (1959). 



