The minimum requirements for oestradiol to induce uterine sensitivity for implantation and decidualization in mice.

Steroid-containing s.c. silastic capsules with known physiological release characteristics were used to study the control of implantation and the induction of uterine sensitivity in ovariectomized mice. In mated, ovariectomized animals maintained on progesterone, the implantation sites were detected after approximately 12 h of oestradiol exposure, and alkaline phosphatase activity at the implantation sites developed within 21 h. Implantation could be induced in > 60% of animals by 4 h exposure to an oestradiol implant, in which time approximately 1.6 ng oestradiol would have been delivered. Continuous delivery of a low dose of oestradiol near the threshold for implantation induced a full complement of implantation sites in the responding animals. The sensitivity of implantation to low amounts of oestradiol suggests that this response is at least as sensitive as either the induction of vaginal cornification or the stimulation of uterine weight. The minimum time for the induction of uterine sensitivity when oestradiol and progesterone treatment were started simultaneously was approximately 36 h. The use of slow-release oestradiol-containing capsules provides a good model to investigate the roles of oestradiol in initiating and defining the 'implantation window'.