The preimplantation stages of mammalian development are dedicated to the differentiation of two extraembryonic epithelia, the trophectoderm (TE) and the primitive endoderm (PrE), and their segregation from the pluripotent embryonic lineage, the epiblast. The TE and PrE are responsible for implantation into the uterus and for producing the tissues that will support and pattern the epiblast as it develops into the foetus. PrE and epiblast are formed in a two step process that involves random cell fate specification, mediated by fibroblast growth factor (FGF) signalling, and cell sorting through several mechanisms. In the present work I have addressed aspects of both steps of this process. Chimaera assays showed that epiblast precursors transplanted onto a recipient embryo rarely differentiate into PrE, while PrE precursors are able to switch their identity and become epiblast. Transient stimulation or inhibition of the FGF4-ERK pathway in the chimaeras can modify the behaviour of these cells and restore the plasticity of epiblast precursors. This work shows that epiblast precursors are refractory to differentiation signals, thus ensuring the preservation of the embryonic lineage. I have also found that atypical Protein Kinase C (aPKC) is a marker of PrE cells and that pharmacological inhibition of aPKC impairs the segregation of PrE and epiblast precursors. Furthermore, it affects the survival of PrE cells and can alter the subcellular localisation of the PrE transcription factor GATA4. These data indicate aPKC plays a central role for the sorting of the PrE and epiblast populations and links cell position within the embryo to PrE maturation and survival. Lastly, I have found that aPKC can directly phosphorylate GATA4 in vitro. Knockdown of GATA4 affects cell position within the embryo, whereas aPKC knockdown reduces the number of GATA4-positive cells. These results indicate GATA4 plays an important role in cell sorting during preimplantation development and suggest phosphorylation by aPKC could determine its presence in the nuclei of PrE cells. My work, in the light of the current knowledge, supports a model where the earliest cell fate decisions during mammalian development depend on cellular interactions and not on inherited cell fate determinants. This robust mode of development underlies the plasticity of the preimplantation embryo and ensures the formation of the first mammalian cell lineages, critical for any further progression in mammalian development.