Stage-specific effects of cadmium on preimplantation embryo development and implantation in the mouse

The effects of cadmium (Cd) exposure during the preimplantation period of pregnancy on the subsequent development and implantation of mouse embryos were examined. Injection of a high dose of Cd (38 mumol Cd/kg body wt.) on day 2 (D1 = vaginal plug), when the embryo is at the two-cell stage, had little effect on the initiation and maintenance of pregnancy when examined on D8. The initiation of implantation (localized sites of increased uterine vascular permeability) in a similarly treated group of mice was assessed in the morning of D5, and these sites were absent in 62% of the animals examined. Thus, Cd treatment on D2 delayed temporarily, but did not prevent implantation and was not embryolethal. In marked contrast, the same dose of Cd administered on D4 caused pregnancy failure in all mice examined on D8. No implantation sites were detected on D5 and the few blastocysts recovered were degenerating. To explore the mechanisms underlying these in vivo stage-specific effects of Cd, preimplantation embryos (two-cell, four-cell, eight-cell and morulae) were exposed in vitro to a high concentration of Cd (50 microM) for 8 h followed by reculture to monitor their potential to develop to the blastocyst stage. Two-cell embryos were remarkably resistant to Cd, but toxicity increased with development, and morulae readily degenerated after Cd exposure. Analysis of the accumulation of 109Cd (50 microM) by preimplantation embryos showed little or none in two-cell embryos, but rapid accumulation and efflux of this metal by blastocysts. Removal of the zona pellucida had no influence on Cd accumulation. Nifedipine (500 nM), a potent voltage-gated calcium channel blocker, and zinc (Zn; 100-fold molar excess) each significantly reduced (approximately 50% in 2 h) Cd accumulation by blastocysts, whereas N-ethylymaleimide (NEM; 20 microM) increased it. These results provide evidence that pregnancy failure after Cd exposure during the preimplantation period reflects a direct embryotoxic effect of Cd, although maternal injury by Cd may also contribute. Resistance to Cd at the two-cell stage (D2) reflects a lack of uptake of this metal, whereas sensitivity to Cd at the blastocyst stage (D4) reflects the ability to accumulate Cd.