Somewhere along embryonic development, individual cells begin following divergent paths of gene expression. But exactly where cellular fates start to point in different directions has been unclear. Presumably, the earliest branching occurs before implantation, as embryos at the implantation stage already contain at least two kinds of cells—cells that will give rise to the body and cells that will give rise to the placenta.
To reveal new details of preimplantation development in mammals, scientists based at the University of Cambridge and European Bioinformatics Institute of the European Molecular Biology Laboratory (EMBL-EBI) tracked gene expression in mouse embryos. These scientists found that divergent developmental paths emerge as early as the second day after conception, when embryos consist of no more than four cells.
The scientists presented their work March 24 in the journal Cell, in an article entitled, “Heterogeneity in Oct4 and Sox2 Targets Biases Cell Fate in 4-Cell Mouse Embryos.” This article identifies targets of master pluripotency regulators Oct4 and Sox2 as being highly heterogeneously expressed between blastomeres of the four-cell embryo, with Sox21 showing one of the most heterogeneous expression profiles.
“Live-cell tracking demonstrates that cells with decreased Sox21 yield more extra-embryonic than pluripotent progeny,” the article’s authors wrote. “Consistently, decreasing Sox21 results in premature upregulation of the differentiation regulator Cdx2, suggesting that Sox21 helps safeguard pluripotency.”
In other words, the totipotent state of the fertilized egg doesn’t last long. After just two cell divisions, some cells are in a pluripotent state, biased toward becoming body cells, whereas other cells are biased toward extraembryonic cell fates, that is, toward becoming cells of the placenta.
“Sox21 is elevated following increased expression of the histone H3R26-methylase CARM1 and is lowered following CARM1 inhibition, indicating the importance of epigenetic regulation,” the authors continued. “Therefore, our results indicate that heterogeneous gene expression, as early as the 4-cell stage, initiates cell-fate decisions by modulating the balance of pluripotency and differentiation.”
One of the article’s authors, Magdalena Zernicka-Goetz from the Department of Physiology, Development & Neuroscience at the University of Cambridge, reflected on the findings as follows: “We know that life starts when a sperm fertilizes an egg, but we're interested in when the important decisions that determine our future development occur. We now know that even as early as the four-stage embryo—just two days after fertilization—the embryo is being guided in a particular direction and its cells are no longer identical.”
Another author, John Marioni of EMBL-EBI, the Wellcome Trust Sanger Institute, and the Cancer Research UK Cambridge Institute, added his comments: “We can make use of powerful sequencing tools to deepen our understanding of the molecular mechanisms that drive development in individual cells. Because of these high-resolution techniques, we are now able to see the genetic and epigenetic signatures that indicate the direction in which early embryonic cells will tend to travel.”