The effects of methylnitrosourea (MNU) on the development of preimplantation mouse embryos were investigated in this study. ICR mice were treated intraperitoneally with single doses of 10, 20, and 30 mg MNU/kg body wt on day 0, 1, 2, or 3 of pregnancy. The uterine contents were examined on day 18 of pregnancy. The fetuses were examined for external and skeletal abnormalities. No significant differences were observed in the number of implantation sites between all the MNU-treated groups and controls. MNU treatment on day 2 or 3 of pregnancy caused dose-dependent significant increases in the incidence of abnormal fetuses over the control level, while treatment on day 0 or 1 failed to cause an increase of abnormalities. Cleft palate, exencephalus, and malformed vertebrae were the most common types of abnormalities. In the embryo transfer experiments, the frequency of fetal abnormalities induced when embryos were transferred from MNU-treated females to untreated pseudopregnant females was significantly higher than that induced when embryos were transferred from untreated females to MNU-treated or untreated pseudopregnant females. The results in the present study confirm and extend the previously proposed hypothesis that the direct effects of MNU on preimplantation embryos make a significant contribution to the induction of fetal malformations.

ICR mice were treated intraperitoneally with mitomycin C (MMC) at a single dose of l‐5mg/kg on day 0, 1, 2 or 3 of pregnancy (vaginal plug = day G). Embryotoxicity and teratogenicity were examined at term. A dose‐dependent increase in fetal mortality and decrease in the number of implants, fetal weight or number of ossified sacral and caudal vertebrae were observed. The incidence of external malformation, umbilical hernia, significantly increased among the fetuses of dams treated with MMC at 5mg/kg on day 1, 2 or 3. Skeletal malformation failed to show significant increase though some fetuses from MMC‐treated mice were attended with multiple defects such as fusion of ribs or axial skeleton. Maternal contribution to and the direct action of MMC on embryonic development were investigated by embryo transfer. In regard to malformation inducement and decrease in implants, the maternal environment was found to have more significant effect than the direct action of this drug but both these factors are significantly responsible for increased fetal mortality and growth retardation.

The sensitive period and the dose‐response characteristics for dexametha‐sone‐induced cleft palate and palatal slit in rats were examined. Pregnant rats were injected subcutaneously with dexamethasone in a single dose of 2–6 mg/kg on one day between day 8 and 16 of the gestational period or once each day for two successive days (days 14 and 15 of gestation). All animals were euthanized on day 20 of gestation and the palatal condition of their fetuses was inspected after fixation under a dissecting microscope. The sensitive stage of palatal slit and cleft palate induction was determined and the dose‐response relationship was analyzed by the log‐probit transformation method. No maternal lethality was elicited in any of the treatment groups. High frequencies of palatal slit and cleft palate were induced when dexamethasone was administered as a single dose on any individual day from day 12 to day 14 of gestation. Omphalocele and general edema were also found to be significantly more frequent in the high‐dose group treated on days 12–14 of gestation. All groups treated showed a reduction in fetal body weight. The response data for palatal slit and cleft palate after administration of dexamethasone fitted to similar linear regression slopes, suggesting a similarity in the underlying mechanisms for palatal slit and cleft palate induction in rats. ED50 values were 3.48 mg/kg for palatal slit and 6.64 mg/kg for cleft palate. These findings indicate that dexamethasone treatment is effective in inducing palatal slit and cleft palate in rats, as well as in mice. Key words: glucocorticoid, dexamethasone, palatal slit, cleft palate, sensitive stage, doseresponse, rats

The induction of abnormal palatal development by three glucocorticoids; prednisolone, triamcinolone acetonide and hydrocortisone was evaluated in rat fetuses.Pregnant rats were injected subcutaneously with either prednisolone (12.5-100 mgkgl day), triamcinolone acetonide (0.25-2 mg/kg/day) or hydrocortisone (1 00 mg/kg/day) on days 14 and 15 of gestation. These females were humanely killed on day 20 of gestation and viable fetuses were inspected for their palatal abnormalities. The frequencies of cleft palate were significantly higher in the group treated with 100 mg/kg/day prednisolone (10.6%), and in the groups treated with 0.5, 1 and 2 mg/kg/day triamcinolone acetonide (8.6%, 26.1% and 58.3%, respectively) than the control frequency of 0%. Triamcinolone acetonide was 70 times as potent as prednisolone in inducing palatal slit, with ED50 value of 1 .O mgkg/day and 70 mgkg/day, respectively. Hydrocortisone showed no potentiality for the induction of cleft palate and palatal slit. Other developmental abnormalities including omphalocele and general edema, late resorption, and growth retardation were induced by triamcinolone acetonide and prednisolone. These findings indicate that triamcinolone acetonide has a significantly higher potentiality for the induction of palatal slit in rats, as well as in mice, compared to prednisolone and hydrocortisone.In a previous study (Ishizuka et al., 1993), it was reported that dexamethasone produces palatal slits with an appreciable frequency in rat fetuses, as well as in mouse fetuses. Palatal slit involved a failure of fusion of the premaxilla and palatal shelves, corresponding to stage 7 in normal palatal closure (Biddle, 1980). In normal development, the premaxilla fuses with the dorsal part of the palatal shelves, from the foremost to near the third ruga; palatal slit does not show such fusion and the oral cavity connected with the nasal cavity