THE INTERPHASE NUCLEUS 



to be lost when hypertonic sucrose alone is used for this purpose 

 (Arneson et alii^^).* For quantitative estimations on Feulgen-stained 

 material, Lison and Pasteels^^^ aim at an even dispersion of DNA by 

 using a fixative which would be unsatisfactory from the cytological 

 standpoint. To quote these authors, 'Les tissus ont ete fixes a I'alcool- 

 formol qui a pour effet d'homogeneiser les noyaux dans une certaine 

 mesure'. Schrader and Leuchtenberger^^ measured the absorption 

 of Feulgen-stained nuclei of different issues of Tradescantia without even 

 dispersion of the DNA, and found a range of apparent DNA content 

 in various interphase somatic nuclei of the same diameter of more than 

 twofold. Schrader and Leuchtenberger calculated the absolute 

 amounts of DNA by means of the procedure of Di Stefano, but apart 

 from the possible errors involved in these computations their results 

 can be considered as relative values. It might be expected that the 

 nuclei of mammalian cells on which most of these estimations have 

 been made would contain less Feulgen-positive material than those of 

 Tradescantia in which the chromosomes are much larger, and so differ- 

 ences in DNA content between somatic nuclei would be there more 

 readily revealed. In Lilium, Ogur et alii,^^ have found by using methods 

 of bulk analysis that in the nuclei of the pollen grain, the content of 

 DNA is very much higher than in any animal nuclei so far assayed. 



SwiFT*^ used formalin-fixed material for his absorption studies on 

 Feulgen-stained sections of animal tissues, principally from mice and 

 Amphibia, with no previous treatment with sucrose. His results were 

 reproducible to within 15 per cent. In mouse tissues he found three 

 classes of nuclei with respect to their relative DNA content, the ratios 

 between which were approximately 1:2:4, which suggest that these 

 differences were due to heteroploidy. The relative DNA content of 

 nuclei of the smallest class in different tissues was approximately the 

 same. 



When nuclei of the mammalian liver are isolated and analysed in 

 bulk, the figure for the DNA content per nucleus does not stand for a 

 diploid value and must be an average for the various size classes and 

 degrees of heteroploidy. In the adult rat liver, estimates of the propor- 

 tion of diploid nuclei present range from about 10-60 per cent (p 61). 

 Swift found that his second class of nuclei were most abundant in the 

 liver of the adult mouse. Ris and Mirsky^* also find twofold relation- 

 ships between the DNA contents of rat liver nuclei of three size classes ; 

 although Swift*^ states that the volume of a nucleus is not necessarily 

 related to its DNA content. In the regenerating rat liver, the proportion 

 of polyploid nuclei is markedly increased; accordingly, it has been 

 found that the average nuclear DNA content then rises nearly twofold 

 (Price and Laird*'). 



* Schneider and Hogeboom** have recently criticized this assertion. 



37 



