Effect of day of exposure on the developmental toxicity of manganese in mice.

Manganese is embryotoxic and fetotoxic in mammals. The aim of this study was to determine whether the day of exposure would modify the developmental toxicity of manganese (II). Pregnant Swiss mice were given single sc doses of 50 mg manganese chloride tetrahydrate/kg on day 9, 10, 11 or 12 of gestation. No maternal deaths, abortions or early deliveries were observed. Dams were killed on gestational day 18 and the uterine contents examined. Embryotoxicity, evidenced by significant increases in number of late resorptions and in percentage of postimplantation loss, was especially relevant in groups dosed on gestational days 9 or 10. Fetotoxicity (reduced fetal body weight and increased incidence of skeletal defects) was also especially remarkable from doses on days 9 or 10 of gestation. However, no teratogenic effects were noted in any group. Although mouse conceptus are adversely affected by sc exposure to manganese on any of the gestational days 9-12, days 9 and 10 of gestation are the most sensitive for manganese-induced embryo/fetal toxicity in mice.