An endodermal subpopulation generates neural and mesodermal fates in the posterior chick embryo

The discovery of neuromesodermal progenitors (NMPs) -- a bipotent progenitor population in the tailbud that gives rise to traditionally ectodermal and mesodermal tissues -- has disrupted the classical view that progenitors of the three distinct germ layers are exclusively segregated during gastrulation. However, until now the notion of lineage restriction of the endoderm to traditional gastrointestinal and respiratory tissues has largely remained intact. Here, we describe our discovery of a unique subpopulation in the chick endoderm that initially lines the ventral surface of the posterior organizer (Hensens node), but at the trunk-to-tail developmental switch, undergoes an FGF-dependent epithelial-to-mesenchymal transition, invading the tailbud and subsequently differentiating into a remarkably broad range of cell types including somites, notochord, and neural tube. Strikingly, ablation of this endodermal cell population results in a severe ([~]50%) reduction in axis elongation rate. Through single cell RNA sequencing and in situ hybridization chain reaction, we conclude that these cells lose their endodermal identity upon ingression, giving rise to NMPs that are biased toward mesodermal fates. Lineage tracing reveals that the node endoderm harbors a mixed multipotent population of progenitor cells capable of generating progeny that span endoderm and mesoderm or endoderm and ectoderm. These findings illustrate a previously unappreciated endodermal source of NMPs, and further demonstrates the breakdown of traditional lineage restriction of germ layers in the posterior embryo.