Exposure to Ethylnitrosourea Before Implantation Induces Congenital Malformations in Mouse Fetuses

Abstract The effects of ethylnitrosourea (ENU) on the development of preimplantation mouse embryos were investigated. ICR mice were treated intraperitoneally with single doses of 25, 50 or 100 mg ENU/kg body weight on day 0, 1, 2 or 3 of gestation, or with single doses of 25, 50 or 75 mg ENU/kg on day 8 of gestation. The uterine contents were examined on day 18 of gestation, and viable fetuses were inspected for external and skeletal malformations. No significant differences were observed in the number of implants between the ENU-treated groups on day 1, 2 or 3 of gestation and controls, while the number of implants in all of the groups treated with ENU on day 0 of gestation was significantly decreased compared to that in the control group. The frequencies of early postimplantation deaths were significantly increased in all of the groups treated with ENU on each gestational day before implantation, compared to the control frequencies. ENU treatment before implantation caused dose-dependent increases in the incidence of externally or skeletally malformed fetuses. Cleft palate, exencephaly and umbilical hernia were the most common types of external malformations in the groups treated with ENU before implantation and in the control group. The skeletal malformations seen in the ENU-treated groups were malformed vertebrae, malformed ribs, and bending of appendicular skeleton. Fused ribs was the most common skeletal malformation seen in the control fetuses. The type distributions of external and skeletal malformations induced by the treatment with ENU before implantation is quite different from those of fetal malformations induced by the treatment with ENU at the organogenesis stage. The results in the present study demonstrate that embryos before implantation in the uterus are susceptible with regard to the induction of congenital malformations by chemicals, and I propose that a large portion of the external malformations in fetuses treated at the preimplantation stages is the result of increased yields of spontaneously-occurring malformations.