Unstainable DNA in cell nuclei. Comparison of ten different fluorochromes.

Ten fluorochromes with specificity for DNA were used to compare the stainability of nuclei of exponentially growing, nondifferentiated Friend leukemia (FL) cells with that of dimethylsulfoxide-induced, fully differentiated FL cell nuclei. Decreased accessibility of DNA to several dyes, particularly pronounced in the case of some intercalators, was observed in differentiated cells. Dye binding was also compared for both sets of nuclei following extraction of nuclear proteins, mostly histones, with 0.1-N HCl. Acid extraction of nuclear proteins increased the accessibility of DNA to varying degrees, depending upon the fluorochrome. In most cases, the differences in fluorescence between differentiated and nondifferentiated nuclei stained with most intercalating dyes was abolished by acid treatment. The results are discussed in terms of the mode of interaction between DNA and the various fluorochromes and the factors associated with chromatin structure, which may affect or be associated with different degrees of proliferative activity.