Delayed implantation induced by letrozole in mice

Implantation timing is key for a successful pregnancy. Short delay in embryo implantation caused by targeted gene ablation produced a cascading problem in the later stages of the pregnancy. Although several delayed implantation models have been established in wild mice, almost none of them is suitable for investigating the delay on the late events of pregnancy. Here, we report a new delayed implantation model established by the intraperitoneally administration of letrozole at 5 mg/kg body weight on the day 3 of pregnancy. In these mice, initiation of implantation was induced at will by the injection of estradiol (E2). When the estradiol (3 ng) was injected on day 4 of pregnancy (i.e., without delay), the embryo implantation restarted, and the pregnancy continued normally. However, high dose of estrogen (25 ng) caused compromised implantation. We also found that only 67% of the female mice could be pregnant normally and finally gave birth when the injection of estradiol (3 ng) was on day 5 of pregnancy (i.e., one day delay). Most of the failed pregnancies had impaired decidualization, decreased plasma progesterone levels and compromised angiogenesis. Progesterone supplementation could rescue decidualization failure in the mice. Collectively, we established a new model of delayed implantation by letrozole, which can be easily used to study the effect and mechanisms of delay of embryo implantation on the progression of late pregnancy events.