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

Area of Science:

• Developmental Biology
• Stem Cell Biology
• Embryogenesis

Background:

• Classical embryology posits strict germ layer segregation (ectoderm, mesoderm, endoderm) during gastrulation.
• Neuromesodermal progenitors (NMPs) were known to arise from the tailbud, contributing to mesodermal and ectodermal tissues.
• The endoderm's lineage restriction to endodermal fates was largely considered intact.

Purpose of the Study:

• To investigate the developmental potential of chick endoderm, particularly in the posterior organizer region.
• To identify novel progenitor populations and their contribution to embryonic tissues.
• To understand the mechanisms regulating germ layer plasticity during posterior development.

Main Methods:

• Single-cell RNA sequencing (scRNA-seq) to analyze gene expression profiles.
• In situ hybridization chain reaction (HCR) for spatial gene expression analysis.
• Experimental ablation of specific endodermal cell populations.
• Lineage tracing experiments to track cell fate.

Main Results:

• A unique subpopulation of chick endoderm cells undergoes FGF-dependent epithelial-to-mesenchymal transition (EMT) at Hensen's node.
• These ingressing endodermal cells invade the tailbud and differentiate into diverse cell types, including somites, notochord, and neural tube.
• Ablation of these cells significantly reduces axis elongation rate (∼50%).
• scRNA-seq and lineage tracing confirm these cells generate NMPs biased toward mesodermal fates, and the node endoderm contains multipotent progenitors spanning multiple germ layers.

Conclusions:

• The endoderm serves as a previously unrecognized source of NMPs.
• These findings demonstrate a breakdown of traditional germ layer restriction in the posterior embryo.
• Endodermal cell plasticity contributes significantly to axis elongation and NMP generation.