The ability to monitor and manipulate epiblast cells in and around the primitive streak of the mouse embryo is important for investigating how cells maintain potency and how distinct cell fates are established. Here, we report development of a key resource for such studies, a mouse line in which a tamoxifen-inducible Cre recombinase construct replaces an endogenous gene, Nkx1.2, whose expression demarcates the epiblast adjacent to and including the primitive streak. We show that this Nkx1.2CreERT2 line drives transgene expression in the endogenous Nkx1.2 domain. Labelling this caudal epiblast cell population at embryonic day (E) 7.5 confirmed contribution to all three germ layers at later stages. Labelled cells were also retained within the caudal lateral epiblast at E8.5 and E9.5 and E10.5 tailbud. A subset of these cells co-expressed early neural (Sox2) and mesodermal (Bra) markers. These findings support the existence of neuromesodermal progenitors within the Nkx1.2 cell lineage. Consistent with the retention of such bipotent progenitors throughout body axis elongation, labelling Nkx1.2-expressing cells at E10.5 and assessment at E11.5 demonstrated continued contribution to both neural and mesodermal lineages. Furthermore, detailed analysis of Nkx1.2 expression revealed a novel domain in tailbud mesenchyme that is contiguous with neural tissue. The presence of labelled Sox2/Bra co-expressing cells in this mesenchyme cell population suggests that this is the retained neuromesodermal progenitor pool, which gives rise to both new paraxial mesoderm and new neural tissue, generated now via secondary neurulation. Here, we introduce this caudal-most Nkx1.2 domain as the neuromesodermal lip.

Fate mapping caudal lateral epiblast reveals continuous contribution to neural and mesodermal lineages and the origin of secondary neural tube