Increased sister-chromatid exchange frequency in preimplantation mouse embryos after maternal cyclophosphamide treatment before implantation.

The effect of a toxic agent in vivo on sister-chromatid exchange (SCE) frequency of preimplantation mouse embryos and bone marrow cells was determined using combined in vivo treatment and in vitro culture in the presence of 5-bromo-2-desoxyuridine (BrdU) for differential staining of the chromatids. In mice exposed to cyclophosphamide (CPA) on day 2 of pregnancy SCE frequency was increased dose-dependently both in embryos and bone marrow cells 1 h after treatment. It returned to control values in bone marrow cells obtained 24 h after exposure but was still significantly increased in the embryos. A closer time-related evaluation of SCE on day 2 of gestation showed a significant increase in SCE in bone marrow cells and in embryos obtained 20-60 min after CPA treatment. Furthermore, SCE frequency was the most sensitive toxicological endpoint to detect embryotoxic effects of CPA treatment before implantation, since it was significantly increased in embryos exposed to 5 mg/kg CPA on day 2 of pregnancy while embryolethality at term and both cytogenetical (structural chromosomal aberrations, micronuclei) and developmental parameters (cell number, differentiation in culture) before implantation did not indicate any toxic effect.