Cleavage and Gastrulation in the Mouse Embryo

After fertilisation of the oocyte, the mouse zygote divides three times before undergoing compaction, which allows polarisation of the blastomeres, and the first lineage specification between trophectoderm (destined to become placenta) and inner cell mass (ICM). The second lineage specification occurs at the blastocyst stage, when ICM cells become either epiblast (which will give rise to the embryo proper) or primitive endoderm (an extraembryonic lineage). After implantation, the proximal (site of attachment with the uterus)‐distal axis is established, quickly followed by the determination of the anterior–posterior axis. Formation of the three main germ layers, ectoderm, mesoderm and endoderm, then occurs on the posterior side at the primitive streak, site of gastrulation. However, recent advances in mouse embryo live imaging have shown that lineage specification was more flexible than previously thought. It has also become possible to visualise the early stages of human embryo development.