Totipotency segregates between the sister blastomeres of two-cell stage mouse embryos

Abstract

Following fertilization in mammals, it is generally accepted that totipotent cells are exclusive to the zygote and to each of the two blastomeres originating from the first mitotic division. This model of totipotency was inferred from a minority of cases in which blastomeres produced monozygotic twins in mice. Was this due to experimental limitation or biological constraint? Here we removed experimental obstacles and achieved reliable quantification of the prevalence of dual totipotency among mouse two-cell stage blastomeres. We separated the blastomeres of 1,252 two-cell embryos, preserving 1,210 of the pairs. Two classes of monozygotic twins became apparent at the blastocyst stage: 27% formed a functional epiblast in both members (concordant), and 73% did so in only one member of the pair (discordant) – a partition that proved insensitive to oocyte quality, sperm-entry point, culture environment and pattern of cleavage. In intact two-cell embryos, the ability of sister blastomeres to generate epiblast was also skewed. Class discovery clustering of the individual blastomeres’ and blastocysts’ transcriptomes points to an innate origin of concordance and discordance rather than developmental acquisition. Our data place constraints on the commonly accepted idea that totipotency is allocated equally between the two-cell stage blastomeres in mice.

Asymmetric Expression of LincGET Biases Cell Fate in Two-Cell Mouse Embryos

“…Our data provide further evidence that the heterogeneous expression of LincGET at the late two-cell stage could act as an upstream regulator to influence subsequent lineage fate. In fact, although two-cell blastomeres are generally considered to be totipotent, there are previous pieces of evidence and recent emerging reports showing that when two-cell blastomeres are separated, in the majority of cases, only one of the two-cell blastomeres develops into a mouse (Casser et al, 2017;Katayama et al, 2010;Morris et al, 2012;Papaioannou and Ebert, 1995;Tsunoda and McLaren, 1983). These data demonstrate inequality in the totipotency of two-cell blastomeres.…”

An integrated genome-wide multi-omics analysis of gene expression dynamics in the preimplantation mouse embryo

“…Metaphase II (MII) oocytes of B6C3F1 mice aged 8-10 weeks were collected from the oviductal ampullae after gonadotropin priming with 10 IU each PMSG and hCG injected 48 hours apart, as described Casser et al 2017).…”

Single-embryo and single-blastomere immunoblotting reports protein expression heterogeneity in early-stage preimplantation embryos

“…On the one hand, it is thought that embryonic plasticity supports blastomere symmetry up to the 8-cell embryo, where embryos can compensate for the loss of one blastomere as early as the two-cell stage 41 . On the other hand, studies point to early asymmetry, where sister blastomeres show consistent bimodal expression of genes related to differentiation, suggesting that the involved factors may not be inherited equally by all blastomeres 6 .…”

“…The initiating events and specific proteins involved in the first cell fate commitment within preimplantation blastomeres still remain open questions in developmental biology. While functional studies and embryonic plasticity suggest that blastomeres remain equivalent until the compacted morula [1][2][3] , growing evidence of inter-blastomeric differences in early-stage embryos point to heterogeneous configurations at even the earliest multicellular stages [4][5][6][7][8][9][10][11][12][13][14] . Although measurement tools with single-embryo and single-blastomere resolution, including RNA-Seq, have greatly advanced our knowledge, companion protein expression and state measurements within single embryos are required to test and validate these transcript-based predictions.…”