The effect of Imuran on implantation and early embryonic development in rats.

Pregnant rats were injected daily with 2, 4, and 20 mg/kg of Imuran from the first day of gestation until sacrificed on Days 6-12. Gravid uteri, as well as maternal organs, were removed and examined histologically. According to the histologic findings, the treatment with the high dose of 20 mg/kg/day Imuran induced injuries to the blastocyst. Starting from Day 8, the trophoblastic layer lost its continuity and maternal blood invaded into the blastocystic cavity resulting in destruction of embryonic tissues. This degeneration progressed until evident resorption on Day 12. Smaller doses, such as 2 or 4 mg/kg of Imuran, did not result in early trophoblastic damage. The effect seemed to be the result of the antimetabolic effect of Imuran acting on the proliferating trophoblast through the maternal blood.