The teratogenic response of mouse embryos to 5‐bromodeoxyuridine

Pregnant ICR mice were administered single intraperitoneal injections of 300, 400, or 500 mg/kg of the thymidine analogue 5‐bromodeoxyuridine during the period of major organogenesis (days 7–11, Witschi stages 12–25). All dosages were teratogenic. The spectrum of malformations present on day 18 followed a definite stage‐dependency. Exencephaly with associated micrognathia and open eyelid were produced only by treatment during the time the anterior neuropore is open (days 7 and 8) with a peak on day 7. Polydactyly was produced by treatment on days 7 through 10 with a peak on day 9; while cleft palate occurred after treatment on days 9 through 11 with a peak on day 10. Experiments with 3H‐BUDR demonstrated that label appeared in the DNA of embryos and maternal liver at a time when the embryo is susceptible to the teratogenic action of BUDR.