Multi-omics and developmental comparison of direct and conventional warming methods in vitrified human and mouse cleavage-stage embryos

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STUDY QUESTION: Does direct warming preserve embryonic developmental competence and molecular integrity as well as conventional multi-step warming in vitrified human and mouse cleavage-stage embryos? SUMMARY ANSWER: Direct warming yielded comparable developmental and molecular outcomes to conventional warming in both mouse and human cleavage-stage embryos. WHAT IS KNOWN ALREADY: Conventional embryo warming uses stepwise cryoprotectant dilution to minimize toxicity and osmotic shock. Direct warming methods have been proposed, but their impact on post-warming development and the molecular comparison remains unclear. STUDY DESIGN, SIZE, DURATION: This is a controlled experimental study conducted over 18 months. A total of 490 vitrified mouse embryos and 15 donated human embryos were tested either in direct or conventional warming group. A subset underwent embryo transfer and follow-up. Parallel transcriptomic and DNA methylation profiling was performed on mouse and donated human embryos. PARTICIPANTS/MATERIALS, SETTING, METHODS: Mouse cleavage-stage embryos (C57BL/6J) were vitrified and randomly assigned to direct (n = 265) or conventional (n = 225) warming. Post-warming survival and blastocyst formation were assessed in vitro. A subset (n = 211) underwent embryo transfer to evaluate implantation, live birth, and postnatal development to Day 21. For molecular analysis, pooled mouse embryos from fresh, conventional, and direct groups were analyzed by bulk RNA-seq and bisulfite sequencing. Ten vitrified human embryos (n = 5 per group) were analyzed individually by scRNA-seq and scBS-seq. All procedures were conducted under standard IVF lab conditions with ethical approval. MAIN RESULTS AND THE ROLE OF CHANCE: Direct warming in mouse achieved comparable survival (95.8% vs 93.8%), blastocyst formation (88.6% vs 88.2%), implantation (82.5% vs 83.0%), and live birth rates (69.6% vs 71.1%) to conventional warming (all P > 0.05). Offspring showed similar growth, developmental milestones, and organ histology. Mouse transcriptome and methylome profiles revealed minimal differences and no significant pathway enrichment. In human embryos, ion channel-related gene variability was observed but without coordinated pathway disruption. Global methylation levels remained within expected developmental ranges. LARGE SCALE DATA: N/A. LIMITATIONS, REASONS FOR CAUTION: While the mouse model enables in vivo validation, species-specific differences in embryo size, membrane properties, and development may limit generalizability. As all embryos were cultured under optimized conditions, caution is advised when extrapolating to diverse clinical settings. WIDER IMPLICATIONS OF THE FINDINGS: To our knowledge, this is the first study to provide multi-level evidence supporting the safety and efficacy of direct warming as an alternative to conventional multi-step embryo warming protocols. By incorporating in vivo reproductive outcomes, postnatal development, and molecular profiling, it strengthens current evidence on the feasibility of this approach. The absence of pathway-level disruptions in transcriptome and methylome datasets suggests that direct warming does not impair essential developmental programs. These findings may support the clinical use of simplified warming procedures, especially for cleavage-stage embryo transfer or in resource-limited settings. However, further clinical studies are warranted to confirm long-term safety in humans. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by Collaborative Research Fund (CRF-C4007-24E), and Early Career Scheme (ECS-26103623) from the University Grants Committee (UGC), and Health and Medical Research Fund (HMRF 12230736) from the Hong Kong Government. The authors report no competing interests. TRIAL REGISTRATION NUMBER: N/A.

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