ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 143 



cells. i^'-'*" Still other references are cited in reference 80 and in addition the reader 

 is referred to two reviews.'*' ■ '*^ 



III. The Nucleolus and Nucleolar PNA 



1. General Remarks 



Reliable biochemical information at the present time concerning the nu- 

 cleolus is scanty, although there is an extensive literature concerning the 

 cytological aspects of this cell particulate. Some of the findings are contro- 

 versial, and there is even widespread disagreement as to what intranuclear 

 inclusion bodies should be termed nucleoli. No attempt will be made here 

 to cover work on the cytology of the nucleolus except insofar as to give the 

 very limited amount of cytological information which is necessary for an 

 understanding of such biochemical investigations as have been published 

 up to the present time. 



According to the views of Caspersson and Schultz,^^^'^*^ nucleoli are sup- 

 posed to be formed near chromocenters, which consist of heterochromatin, 

 or portions of chromosomes which tend not to disperse during interphase, 

 but rather to remain in a more or less condensed state. According to this 

 concept, the nucleolus would seem to be structurally independent of the 

 chromosomes but probably dependent on heterochromatin for the synthesis 

 of some of its constituents such as PNA. Caspersson^*^ has recognized diffi- 

 culties in defining the nucleolus, but his own definition is not wholly satis- 

 factory. 



It is known that the eggs of amphibia and certain insects possess large 

 numbers of nucleoli which would seem to fall into the category just de- 

 scribed, and these nucleoli are said to migrate to the nuclear membrane 

 and there to empty part or all of their contents through the membrane 



16S I. J. Lorch, J. F. Danielli, and S. Horstadius, Exptl. Cell Research 4, 253 (1953). 

 "» J. F. Horstadius, I. J. Lorch, and J. F. Danielli, Exptl. Cell Research 4, 263 (1953). 

 1" J. Brachet, Symposia Soc. Exptl. Biol. 6, 173 (1952). 

 '" N. Linet and J. Brachet, Biochim. et Biophys. Acta 7, 607 (1951). 

 "' F. Vanderhaeghe, Arch, intern, physiol. 60, 190 (1952). 

 "< M. B. Chantrenne-Van Halteren, Arch, intern, physiol. 60, 187 (1952). 

 1^6 J. Brachet, Experientia 8, 347 (1952). 

 •'« J. Brachet, Biochim. et Biophys. Acta 9, 221 (1952). 

 1" E. Urbani, Biochim. et Biophys. Acta 9, 108 (1952). 

 "» E. Urbani, Arch, intern, physiol. 60, 189 (1952). 

 ''" J. Brachet and H. Chantrenne, Arch, intern, physiol. 60, 547 (1952). 

 '«» J. Brachet and H. Chantrenne, Nature 168, 950 (1951). 

 !«■ J. R. G. Bradfield, Biol. Revs. 25, 113 (1950). 



"2 K. I. Altman and A. L. Bounce, Ann. Rev. Biochem. 21, 29 (1952). 

 1" J. Schultz, Cold Spring Harbor Symposia Quant. Biol. 12, 179 (1947). 

 i»* T. Caspersson, "Cell Growth and Cell Function." W. W. Norton and Co., New 

 York, 1950. 



