Placental transfer and distribution of 241Am in the rat

The placental transfer and distribution of 241Am in the feto-placental system were studied in pregnant rats. Rats were injected intravenously with 241Am citrate at 15 or 18 days of gestation. Groups injected at 15 days of gestation were sacrificed 2, 24, 48, or 120 hr after injection, and the group injected at 18 days was sacrificed 24 hr after. The radioactivities of 241Am in fetus, fetal membrane, and placenta were determined, and its distribution in the feto-placental system was investigated by high-speed autoradiography using a silver-activated zinc sulfide-coated membrane as an intensifying screen. The deposition of 241Am in feto-placental units increased with the number of days of gestation. Results of autoradiography revealed that major deposition sites of 241Am in the fetus are the skeleton and liver. Heavy deposition of 241Am in the yolk sac splanchnopleure and its existence in the exocoelom strongly suggest that the yolk sac placenta plays an important role in the placental transfer of this nuclide.