MEK signalling pathway is required for hypoblast specification and migration in ovine.

Early embryo development requires the differentiation of three cell lineages in two differentiation events. The second lineage specification differentiates the inner cell mass into epiblast, which will form the proper foetus, and hypoblast, which together with the trophectoderm will form the extraembryonic membranes and the foetal part of the placenta. MEK signalling pathway is required for hypoblast differentiation in mouse embryos, but its role in ungulate embryos remains controversial. The aim of this work was to analyse the role of MEK signalling on hypoblast specification at the blastocyst stage, and on hypoblast migration during post-hatching stages in vitro in the ovine species. Using well-characterized and reliable lineage markers, and different MEK inhibitor concentrations, we demonstrate that MEK signalling pathway is required for hypoblast specification in the inner cell mass of the ovine blastocyst, and that it plays a role during the hypoblast migration occurring following blastocyst hatching. These results show that the role of MEK signalling pathway on hypoblast specification is conserved in phylogenetically distant mammals.