The nucleic acid content of fetal rat liver.

It has been shown by various investigators that embryonic tissues are richer in nucleic acid than are adult tissues (1). The distribution of the two types of nucleic acids (ribonucleic acid (RNA) and desoxyribonucleic acid (DNA)) is also known to be different in embryonic and adult tissues and in different tissues. For instance, muscle and heart are low in ribonucleic acid concentration, whereas liver and pancreas have a high content of this nucleic acid. The ratio of RNA to DNA is smaller in the liver of sheep embryos than in that of the adult animal. Caspersson and others (2) have suggested that these changes are in some way related to protein synthesis for purposes of growth or of secretion. Systematic determinations of the changes in nucleic acid concentration in embryonic mammalian liver have not been conducted. Masing (3), working with rabbit liver, found that a decrease in the nucleoprotein phosphorus per unit of nitrogen of the liver occurred from the 4th week of gestation to the adult period; the ages of the animals were, however, not accurately determined. Dumm (4) showed a decrease of “residual phosphorus” in the fetal liver of rats during the later period of gestation; the author did not determine separately the RNA-P and DNA-P. Dumm’s results, expressed as residual phosphorus, may be accepted as being due to nucleoprotein phosphorus, because the phosphoprotein phosphorus (the only other phosphorus fraction present in residual phosphorus) is always small in comparison with that for nucleic acids. In the present experiments fetal rat livers from the 16th to the 21st prenatal day were employed. The nucleic acids in the tissue were fractionated into RNA and DNA. It will be seen that the ratio RNA:DNA is relatively constant from the 16th to the 19th day and that it increases at the 20th day.