Both Nodal signalling and stochasticity select for prospective distal visceral endoderm in mouse embryos

Anterior–posterior (A–P) polarity of mouse embryos is established by distal visceral endoderm (DVE) at embryonic day (E) 5.5. Lefty1 is expressed first at E3.5 in a subset of epiblast progenitor cells (L1(epi) cells) and then in a subset of primitive endoderm cells (L1(dve) cells) fated to become DVE. Here we studied how prospective DVE cells are selected. Lefty1 expression in L1(epi) and L1(dve) cells depends on Nodal signaling. A cell that experiences the highest level of Nodal signaling begins to express Lefty1 and becomes an L1(epi) cell. Deletion of Lefty1 alone or together with Lefty2 increased the number of prospective DVE cells. Ablation of L1(epi) or L1(dve) cells triggered Lefty1 expression in a subset of remaining cells. Our results suggest that selection of prospective DVE cells is both random and regulated, and that a fixed prepattern for the A–P axis does not exist before the blastocyst stage.