Investigating the dynamic signaling pathways involved in the specialization and differentiation of epiblast cells in early human embryos

The human embryonic epiblast is essential for early development, ultimately giving rise to all somatic and germ cell lineages. Despite advances in understanding embryogenesis, the mechanisms regulating intercellular communication during epiblast specification remain incompletely understood. Here, we analyzed single-cell RNA sequencing data spanning pre-implantation, post-implantation, pre-gastrulation, and early-gastrulation stages of human embryos to investigate how signals from extra-embryonic tissues influence epiblast (EPI) development. Our data-driven analysis indicates that multiple signaling pathways, including BMP, WNT, FGF, LIF, and TGF$\beta$, may be involved in the first and second lineage separations, amniotic epithelial (AME) cell development, and primitive streak (PS) formation. We further propose that the inner cell mass (ICM) and EPI could function as signaling hubs, coordinating critical intercellular communication events. Based on RNA expression patterns, extra-embryonic tissues such as the hypoblast, trophoblast (TrB), and extra-embryonic mesoderm (ExM) appear to secrete key signals (BMP4, SELL, WNTs, and PTN) that potentially regulate EPI cell polarization, EPI-AME transdifferentiation, and PS development. Notably, BMP4 expression may follow a dynamic pattern, transitioning from early implantation visceral/parietal endoderm (VE/YE) cells to pre-gastrulation ExM cells and ultimately to CS7 advanced mesoderm cells. Overall, these findings provide a comprehensive overview of putative signaling events mediated by extra-embryonic tissues and underscore their potential roles in orchestrating epiblast development and early lineage decisions in the human embryo, while highlighting the need for further protein-level and functional validation.