Following the injection of pregnant mice on the tenth day of gestation with either 200 μCi/kg 3H-thymidine or 200 μCi/kg 3H-bromodeoxyuridine, the half-life of labelled nucleotides in the acid-soluble fraction was found to be of the order of 30 min in various maternal tissues and placentas, and approximately 60–80 min in the embryos. There appears to be no discrimination in turnover rate between thymidine (TdR) and bromodeoxyuridine (BrdU). The administration of a single injection containing 200 μCi/kg 3H-BrdU plus 500 mg/kg BrdU (a known teratogenic dose) resulted in increased labelled nucleotide half-lives in all of the tissues except the embryos. These increased half-lives ranged from 149 min in the maternal liver to 75 min in the kidneys. The turnover rate of labelled nucleotides in the embryos did not change significantly from those observed following a trace dose of 3H-BrdU.
The time course of incorporation of label into the DNA of the various tissues was determined following the injection of the trace doses of 3H-TdR or 3H-BrdU. Significant amounts of DNA label were found in all tissues studied except maternal liver and kidneys, the bulk of the incorporation occurring within one hour of administration. The DNA specific radioactivity had begun to decline by 8 h after injection but was still demonstrable at 48 h. When the teratogenic dose of BrdU was administered along with the 3H-BrdU, the time course of incorporation of label was very similar in all tissues, with the exception of the embryo and maternal intestine, to that observed following injection of the trace dose of 3H-BrdU. The fact that radioactivity found in the DNA fractions after the injection of 3H-BrdU was due to labelled bromouracil, was established by thin-layer chromatography of DNA bases.
At a time in mouse development when BrdU is capable of producing congenital defects in vivo, it is present at its presumed intracellular site of action, the DNA molecule. It is suggested that this demonstration supports the contention that, to study the role of teratogens in the interference with normal differentiation, it is essential to show their presence at their suspected site of action during morphogenesis.