Abstract
The early stages of mammalian embryonic development involve the participation and cooperation of numerous complex processes, including nutritional, genetic, and epigenetic mechanisms. However, in embryos cultured in vitro, a developmental block occurs that affects embryo development and the efficiency of culture. Although the block period is reported to involve the transcriptional repression of maternal genes and transcriptional activation of zygotic genes, how epigenetic factors regulate developmental block is still unclear. In this study, we systematically analyzed whole-genome methylation levels during five stages of sheep oocyte and preimplantation embryo development using single-cell level whole genome bisulphite sequencing (SC-WGBS) technology. Then, we examined several million CpG sites in individual cells at each evaluated developmental stage to identify the methylation changes that take place during the development of sheep preimplantation embryos. Our results showed that two strong waves of methylation changes occurred, namely, demethylation at the 8-cell to 16-cell stage and methylation at the 16-cell to 32-cell stage. Analysis of DNA methylation patterns in different functional regions revealed a stable hypermethylation status in 3′UTRs and gene bodies; however, significant differences were observed in intergenic and promoter regions at different developmental stages. Changes in methylation at different stages of preimplantation embryo development were also compared to investigate the molecular mechanisms involved in sheep embryo development at the methylation level. In conclusion, we report a detailed analysis of the DNA methylation dynamics during the development of sheep preimplantation embryos. Our results provide an explanation for the complex regulatory mechanisms underlying the embryo developmental block based on changes in DNA methylation levels.
Highlights
Sheep embryonic development involves a complex and spatiotemporal preimplantation process, starting from the zygote and lasting until the blastocyst is formed (Calarco and McLaren, 1976)
Detailed analysis indicated that the DNA methylation level decreased continuously from the oocyte to the 16-cell stage, and increased from the 16-cell stage to the blastocyst stage (Figures 1A,B)
The development of the sheep embryo is a complex and dynamic process that is influenced by numerous factors
Summary
Sheep embryonic development involves a complex and spatiotemporal preimplantation process, starting from the zygote and lasting until the blastocyst is formed (Calarco and McLaren, 1976). This developmental process is regulated by genetic factors, the maternal environment, and maternal nutrition (Reik et al, 2001; Ashworth et al, 2009; Chason et al, 2011). In vitro, sheep embryo development undergoes a developmental block stage that can markedly affect embryo culture (Meirelles et al, 2004), and early sheep embryos do not survive beyond the 8-cell stage when grown under artificial conditions. It has been reported that the block period involves the transcriptional repression of maternal genes and transcriptional activation of zygotic genes (Walser and Lipshitz, 2011; Wang et al, 2019), how epigenetic factors regulate developmental block is still unclear
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