Nucleic acid synthesis in preimplantation mouse embryos : Autoradiographic evidence of uridine and deoxyuridine utilization in DNA synthesis.

Mouse embryos were collected at the 2-cell stage, cultured in vitro in the presence of3H deoxyuridine or uridine for 6 or 4 h and autoradiographed.Deoxyuridine is actively incorporated into the DNA of cleaving mouse embryos indicating the existence of thymidylate synthetase activity at least at the 4-cell stage and presumably already before this.RNAase treatment of embryos squashed on slides shows a weak but obvious incorporation of uridine into DNA of cleaving mouse embryos, from the 4-cell stage onwards; this incorporation is totally inhibited by hydroxyurea. The reduction of ribonucleotides to deoxyribonucleotides is a metabolic pathway already required for cleavage, as shown by hydroxyurea experiments.The second polar pody, known to incorporate thymidine, is unable to incorporate either deoxyuridine or uridine.