The effects of lead and X-rays, alone or in combination, on blastocyst formation and cell kinetics of preimplantation mouse embryos in vitro.

Two-cell embryos in late G2-phase (cytofluorometrically determined) at 32 hours post-conception were treated in vitro with PbC12 (0 . 1 or 1 . 0 micrograms per ml). An additional group was X-irradiated 1 hour later (0 . 94 Gy) with or without lead treatment. Lead alone impaired formation and hatching of blastocysts. The combined treatment increased this deleterious effect on preimplantation development but synergism was not observed. Cell proliferation was disturbed by lead alone but to a higher degree by lead plus X-rays. A pronounced reduction of cell number per embryo was found during the period after reaching the morula stage although the labelling index increased. Apparently cell death occurred. Unlabelled S-phase cell nuclei (comparison between autoradiographic results and cytofluorometric DNA-determinations on the same isolated cell nuclei) and cell nuclei with a hyperploid (0 . 1 microgram PbC12 + 0 . 94 Gy) or hypoploid (1 . 0 microgram PcC12 + 0 . 94 Gy) DNA-content may have contributed to this cell loss. Labelling of cell nuclei which according to their DNA-content were not in S-phase was only observed when 0 . 1 microgram PbC12 alone or in combination with X-rays was used. This effect may be due to unscheduled DNA-repair synthesis or to the induction of chromosome aberrations (acentric fragments or non-disjunction).