56 Mammalian pre-implantation embryos at the single cell level: The bovine as a model for early human embryonic development

Abstract

Single-cell transcriptomics and gene editing on human pre-implantation embryos have proved that mechanisms previously identified and well characterised in mouse pre-implantation development may not hold true in the human embryo. However, ethical and legal concerns limit the availability of surplus human embryos for research, resulting in the need for novel animal models. Bovine embryos share morphological and temporal resemblance with human early development; still, key molecular and cellular developmental mechanisms need to be further explored. In the present study, we performed a comparative single-cell RNA sequencing analysis across multiple pre-implantational developmental stages of in vitro-derived bovine, human (Petropoulos et al. 2016 Cell 165, 1012-1026; https://doi.org/10.1016/j.cell.2016.03.023), and mouse embryos (Cheng et al. 2019 Cell Rep. 26, 2593-2607, https://doi.org/10.1016/j.celrep.2019.02.031; Posfai et al. 2017 eLife 6, e22906, https://doi.org/10.7554/eLife.22906; Chen et al. 2016 Genome Res. 26, 1342-1354). In total 497 blastomeres corresponding to embryonic days (E) E4-E8 were prepared using Smart sEqn 2, Illumina HiSEqn 2500 sequencing with 50-bp single-end reads. Embryonic stage was defined by cell numbers and morphology as multicellular stage (Mu, ~E4), morulae (M; E5), early blastocysts (EB, ~6), mid-expanded blastocysts, (MB, ~E7), and late expanded/hatched blastocysts (LB, ~E8). We found top differentially expressed genes elucidating how lineage specification and pluripotency is controlled in the early bovine embryo. Our transcriptomic data showed that the first lineage segregation in the pre-implantation bovine embryo occurs after cavitation at E6 in the early blastocyst, suggesting a similarity with the early human blastocyst. In addition, E7 showed distinctive and more mature inner cell mass (ICM) and trophectoderm (TE) profiles. Different from the mouse, where the first lineage segregation of TE-ICM is suggested to occur at the morula stage due to CDX2-OCT4 antagonism, we found that the expression of OCT4 protein cattle expanded blastocysts is not restricted to the ICM, similar to what has been reported to occur in the human embryo. Interestingly, we also found that the second lineage segregation, which segregates the epiblast from the primitive endoderm within the ICM, starts in bovine at E8 expanded hatched late blastocysts, suggesting a potential difference with humans, where TE-ICM and epiblast-primitive endoderm has previously been reported to occur almost concurrently. We are now complementing our findings with genome editing in bovine embryos by generating knockout embryos of different target genes part of an evolutionarily conserved signaling pathway to study their effects in the early pre-implantation embryo. We aim to elucidate how early lineage specification and pluripotency establishment occurs while offering theoretical support for efficient derivation and culture of bovine embryonic stem cells.