Abstract

The development of an entire organism from a single fertilized egg is arguably one of the most fascinating processes in biology. In eutherian mammals the whole process starts before embryo implants into the uterus. From around the 8-cell stage, originally identical embryonic cells – blastomeres gradually become committed to one of the first three cell lineages: the trophectoderm (TE), the primitive endoderm (PrE), which both contribute to the placenta, and the pluripotent epiblast (EPI), which gives rise to the foetus and later to the whole adult organism. At the blastocyst stage, TE consists of a single layer of cells (“epithelium”) enclosing a fluid-filled cavity and an adjacent inner cell mass (ICM). By the late blastocyst stage, the PrE appears as a noticeable layer of cells on the surface of the ICM lining the cavity, with deeper cells constituting the EPI . Although the emergence of all three embryonic lineages is a common phenomenon among mammals, some apparent differences can be observed during lineage specification between different mammalian species. It is therefore paramount to gain the insight into early differentiative events, to pinpoint the processes and mechanisms that are universal for various mammals.