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Abstract Study question Are there cellular, molecular and/or epigenetic risks associated with cryopreservation that may affect embryo development and compromise the health of future newborns? Summary answer Cryopreservation significantly impacted mice pre-implantation embryo morphokinetics, altered transcriptomic profiles, and cell lineage distribution. However, global DNA methylation remained unaffected by cryopreservation. What is known already Cryopreservation is currently widely used in assisted reproduction. However, epidemiological studies suggest a negative impact in the long-term health of children born after frozen-embryo transfer (FET). Recent population-based register studies have reported: 1) an increased risk of infant mortality from birth up to 1 year of life in singletons conceived using cryopreserved embryos vs natural conceptions; 2) high birth weights for gestational age associated with FET; 3) an increased risk of obstetrical complications in singletons conceived using cryopreserved embryos; 4) a higher incidence of childhood cancer and imprinting disorders associated with FET. Study design, size, duration F1 hybrid (B6/CBA) mice zygotes were collected following uterus dissection (n = 200 embryos/collection), and half of those were vitrified for comparison. Each embryo’s morphokinetics was assessed through time-lapse imaging using GERI, at 37 °C, 6% CO2, 5% O2, up to 105 hours. Extended culture beyond blastocyst stage was also performed. Fresh and cryopreserved blastocysts at E5.0 were cultured up to E9.0-E10 in static conditions (IVC1 and IVC2 medium). Participants/materials, setting, methods Fresh and cryopreserved embryos at several developmental stages were used for: 1) immunofluorescence staining, 2) gene expression analysis through RT-qPCR and 3) single-cell RNAseq. Global methylation activity, cell lineage distribution and single-cell transcriptome were assessed using one of the previously described methodologies. Main results and the role of chance The morphokinetics of preimplantation embryos were examined by tracking the time required to reach 4-cell, 8-cell, compactation, morula, full blastocyst, and expanded blastocyst stage. A comprehensive analysis revealed notable delays in the developmental rhythm of cryopreserved embryos compared to fresh embryos (p 0.01). Applying single-cell RNA sequencing (scRNAseq), transcriptomic profiles were generated for fresh and cryopreserved embryos, covering the zygote to blastocyst stages. Notably, distinct transcriptomic signatures for fresh and frozen embryos emerged within separate clusters at different developmental stages. These findings suggest a potential functional impact of cryopreservation on embryo development. To complement this, gene expression analysis of DNA methyltransferases and MAT enzymes was conducted through RT-qPCR at 8-cell and blastocyst stages. No significant differences were detected between fresh and cryopreserved conditions. Furthermore, immunofluorescence staining of nuclear 5mC and DNMT1 in 8-cell embryos and blastocysts demonstrated similar levels between fresh and cryopreserved embryos. Additionally, differential staining of embryonic and extraembryonic cells (OCT4, GATA6 and GATA3) in extended embryos (E9 and E10) revealed distinct cell distribution patterns between fresh and cryopreserved conditions. Limitations, reasons for caution Our research, conducted using a mouse experimental model, has limited applicability to humans due to considerable species differences in embryonic development. Additional research is needed to validate our findings and establish their relevance to human embryos, facilitating translational approaches. Wider implications of the findings Our study highlights a noticeable functional impact of vitrification on mouse embryos. Given the global incorporation of this method in fertility clinics and its potential association with increased disease risks, our findings emphasize the need for increased research to reduce uncertainties regarding the health of children born from cryopreservation methods. Trial registration number Not applicable
Barroso et al. (Mon,) studied this question.