Cytophotometric factors causing apparent differences between Feulgen DNA contents of different leukocyte types

A CENTRAL issue in the study of both metazoan differentiation and the control of cellular activity is the validity or otherwise of the hypothesis1,2that nonreplicating nuclei of a given higher organism contain a constant amount of DNA per haploid set of chromosomes. Differential elimination or increase of DNA has been demonstrated in dipteran polytene chromosomes3, and in the oocytes of Acheta and amphibia4, but apart from these possibly special cases the most studied apparent exception to the constancy rule is probably provided by mammalian leukocytes5–11. Most workers in this field have found the apparent DNA content of the relatively large nuclei of monocytes, as assessed by Feulgen cytophotometry, to be between 5% and 20% higher than that of the smaller, more darkly stained nuclei of lymphocytes and polymorphonuclear leukocytes. The consensus of opinion seems to be that the measured differences are probably due to nonstoichiometry of staining rather than to genuine differences in the amounts of DNA present. But work published to date has in general been performed without certain technical refinements which have recently become available, and we have now investigated the possibility that systematic cytophotometric errors may have been responsible for at least some of the reported differences between cell types.