Generating different genetic expression patterns in the early embryo: insights from the mouse model

The divergence of two differentiating extraembryonic cell types (trophectoderm and primitive endoderm) from the pluripotent epiblast population (the source of fetal progenitor cells) by the blastocyst stage of mouse development relies upon the activation and execution of lineage-specific gene expression programmes. While our understanding of the central transcription factor ‘effectors’ directing these cell-fate choices has accumulated rapidly, what is less clear is how the differential expression of such genes within the diverging lineages is initially generated. This review summarizes and consolidates current understanding. I introduce the traditional concept and importance of a cell’s spatial location within the embryo, referencing recent mechanistic and molecular insights relating to cell fate. Additionally, I address the growing body of evidence that suggests that heterogeneities among blastomeres precede, and possibly inform, their spatial segregation in the embryo. I also discuss whether the origins of such early heterogeneity are stochastic and/or indicative of intrinsic properties of the embryo. Lastly, I argue that the robustness and regulative capacity of preimplantation embryonic development may reflect the existence of multiple converging, if not wholly redundant, mechanisms that act together to generate the necessary diversity of inter-cell-lineage gene expression patterns.Following conception, the early mammalian embryo undergoes a series of cell divisions. The divisions not only increase the number of cells but also lead to the generation of three different cell classes. These cell classes ensure that the early embryo is able to implant into the mothers womb and continue successful development until birth. In order to generate these three types of cells, specific genes need to be activated or shut down to ensure they can develop properly and permit further embryogenesis. The process of acquiring the correct genetic expression patterns is controlled by specialized proteins called transcription factors. As the three specialized types of cells arise, they require the correct set of transcription factors but little is known about how this process is regulated. In this review summary, I introduce some recent evidence about how this is achieved and how the potential processes involved can contribute to the robust nature of early mammalian embryo development.VIDEO LINK:http://sms.cam.ac.uk/media/1401242