Ethical and legal restrictions limit human implantation knowledge. Alternative tools, such as retrospective clinical studies, in vitro three dimensional cell culture modelling, and in vivo implantation animal modelling provide some insight. While in vivo implantation murine models have proven useful, none are recognised as standard. Therefore, the overall research objective was to gain an understanding of improved standardization of in vivo implantation murine models, suitable to explore implantation and preliminary studies surrounding In Vitro Fertilization (IVF) intervention potential.

A selection of models, termed optimal or suboptimal by their level of implantation, were examined and compared to natural mouse pregnancy. Embryo transfer (ET) outcomes on day 17 post coitus (pc) and the ability of the most suitable optimal and suboptimal models to detect consequences of manipulated embryos, an IVF intervention, were explored. Significant inconsistencies within suboptimal models led to investigating the beginning of endometrial receptivity within natural pregnancy, and synchronous and asynchronous uterine ET. Assessed time points were set to capture the pre receptive and receptive endometrial phases. Examining at least three mice per time point, achieving 80% statistical power, ensured statistical robustness.

This research extended current knowledge in three ways. Firstly, ETs performed at two hourly intervals, demonstrated significant influence of the distinct moments of coitus, ET and autopsy on implantation rate consistency. Previous research, which specified the day of transfer rather than hours pc, was unable to reveal such influence. Finer control of coitus, ET and autopsy demonstrated improved implantation rate consistency, necessary for model standardization. Implementation within wider research could improve statistical robustness and facilitate greater comparison between research groups.

Secondly, assessed time points extended beyond previous the literature, which did not report endometrial receptivity within natural pregnancy prior to 92 hours pc. This study reported endometrial receptivity by 88 hours pc. No mean zero baselines were also observed, suggesting endometrial receptivity could begin prior to 88 hours pc. A four hour difference is significant, given murine endometrial receptivity is considered at least 12, but less than 24 hours long, translating to a potential 33% increase in known implantation opportunity. Data collection surrounding the beginning of endometrial receptivity within natural pregnancy, and synchronous and asynchronous uterine ET, revealed reasons for implantation rate inconsistencies, not previously observed in single-point data sets; contributing towards the first point.

Thirdly, this is the first report of early endometrial receptivity, following the transfer of mature pre-implantation embryos directly into uteri, whose endometrium would not normally be exposed to embryos at that time. Only one other study, which performed oviductal transfers, demonstrated similar phenomena. Due to the lack of uterine ET evidence, investigators reasoned oviductal transport of pre-implantation embryos was responsible. However, uterine ET bypasses oviductal transport. Therefore, early endometrial receptivity was likely due to the premature uterine presence of mature pre implantation embryos and not dependent on their oviductal transport.

This research provided robust insight into improved in vivo implantation murine model standardization. Model consistency could be improved by strictly controlling the time of coitus, ET and autopsy. Research suggested endometrial receptivity began prior to 88 hours pc. Findings provided further evidence that pre-implantation embryos can influence the timing of endometrial receptivity. Exploiting findings could provide opportunities to expand implantation knowledge and explore commercial benefits of IVF-interventions with more efficiency and reliability.