The effect of RU486 on transport, development and implantation of mouse embryos.

Pregnancy was interrupted in Swiss-Rockefeller mice by a single subcutaneous injection of the antiprogesterone RU486 given postcoitally. A dose of 0.1 mg/animal injected on day 1, produced partial inhibition of pregnancy, a notable delay in embryonic development and a slight retention of embryos in the oviducts. When the same dose was injected on day 2, 3 or 4, no implantation sites were seen at autopsy performed on day 12 of pregnancy. Treatment with 0.5 or 1 mg/animal on day 1 produced the loss of a large proportion of the embryos from the genital tract by day 4 of pregnancy and suppressed implantation completely. These observations indicate that preimplantation phenomena in mice are highly dependent on progesterone action.