Microdensitometric and autoradiographic comparison of the DNA contents of foetal and adult rat liver nuclei

Summary Glare-corrected, scanning Feulgen microdensitometry and ³H-thymidine autoradiography were applied to squash preparations of rat 18-day foetal and maternal liver cells, and to smears of maternal blood. No significant differences were found between the mean Feulgen-DNA contents of autoradiographically unlabelled diploid foetal and maternal hepatocytes. The Feulgen-DNA contents of other unlabelled foetal and maternal hepatocytes were also as predicted by the DNA-constancy hypothesis, i.e. were twice or four times that of diploid cells. Small (less than about 4%) but statistically significant discrepancies in the mean Feulgen-DNA contents of foetal haemopoietic cells and adult leucocytes were attributable to uncorrected residual distribution and chromatic errors in the microdensitometry.None of the 371 maternal nuclei measured had Feulgen-DNA contents substantially (i.e. more than ±10%) different from a modal value. About 12% of these nuclei were classified as labelled. Evidence was found suggesting a significantly non-random distribution of background grains in the autoradiographs, which would materially affect the proportion of cells incorrectly classified. After taking this factor into account there seems no reason to suppose that the apparently labelled adult nuclei were in fact synthesising DNA.Of 376 foetal cells measured, 107 had inter-modal Feulgen-DNA contents. Eleven of these were classified as unlabelled. All the inter-modal cells were however probably in the S-phase of the cell cycle, statistical variation in autoradiographic grain distribution accounting for those appearing to be unlabelled.Our results are consistent with the DNA-constancy hypothesis, and are at variance with previous claims for the existence of metabolic DNA in adult and/or foetal rat hepatocytes.