Embryonic liver developmental trajectory revealed by single-cell RNA sequencing in the Foxa2eGFP mouse

The liver and gallbladder are among the most important internal organs derived from the endoderm, yet the development of the liver and gallbladder in the early embryonic stages is not fully understood. Using a transgenic Foxa2eGFP reporter mouse line, we performed single-cell full-length mRNA sequencing on endodermal and hepatic cells isolated from ten embryonic stages, ranging from E7.5 to E15.5. We identified the embryonic liver developmental trajectory from gut endoderm to hepatoblasts and characterized the transcriptome of the hepatic lineage. More importantly, we identified liver primordium as the nascent hepatic progenitors with both gut and liver features and documented dynamic gene expression during the epithelial-hepatic transition (EHT) at the stage of liver specification during E9.5–11.5. We found six groups of genes switched on or off in the EHT process, including diverse transcripitional regulators that had not been previously known to be expressed during EHT. Moreover, we identified and revealed transcriptional profiling of gallbladder primordium at E9.5. The present data provides a high-resolution resource and critical insights for understanding the liver and gallbladder development.

Charting the emergent organotypic landscape of the mammalian gut endoderm at single-cell resolution

To comprehensively delineate the ontogeny of an organ system, we generated 112,217 singlecell transcriptomes representing all endoderm populations within the mouse embryo until midgestation. We employed graph-based approaches to model differentiating cells for spatio-temporal characterization of developmental trajectories. Our analysis reveals the detailed architecture of the emergence of the first (primitive or extra-embryonic) endodermal population and pluripotent epiblast. We uncover an unappreciated relationship between descendants of these lineages, before the onset of gastrulation, suggesting that mixing of extra-embryonic and embryonic endoderm cells occurs more than once during mammalian development. We map the trajectories of endoderm cells as they acquire embryonic versus extra-embryonic fates, and their spatial convergence within the gut endoderm; revealing them to be globally similar but retaining aspects of their lineage history. We observe the regionalized localization of cells along the forming gut tube, reflecting their extra-embryonic or embryonic origin, and their coordinate patterning into organ-specific territories along the anterior-posterior axis.

Sox17 is essential for proper formation of the marginal zone of extraembryonic endoderm adjacent to a developing mouse placental disk†

In mouse conceptus, two yolk-sac membranes, the parietal endoderm (PE) and visceral endoderm (VE), are involved in protecting and nourishing early-somite-stage embryos prior to the establishment of placental circulation. Both PE and VE membranes are tightly anchored to the marginal edge of the developing placental disk, in which the extraembryonic endoderm (marginal zone endoderm: ME) shows the typical flat epithelial morphology intermediate between those of PE and VE in vivo. However, the molecular characteristics and functions of the ME in mouse placentation remain unclear. Here, we show that SOX17, not SOX7, is continuously expressed in the ME cells, whereas both SOX17 and SOX7 are coexpressed in PE cells, by at least 10.5 days postconception. The Sox17-null conceptus, but not the Sox7-null one, showed the ectopic appearance of squamous VE-like epithelial cells in the presumptive ME region, together with reduced cell density and aberrant morphology of PE cells. Such aberrant ME formation in the Sox17-null extraembryonic endoderm was not rescued by the chimeric embryo replaced with the wild-type gut endoderm by the injection of wild-type ES cells into the Sox17-null blastocyst, suggesting the cell autonomous defects in the extraembryonic endoderm of Sox17-null concepti. These findings provide direct evidence of the crucial roles of SOX17 in proper formation and maintenance of the ME region, highlighting a novel entry point to understand the in vivo VE-to-PE transition in the marginal edge of developing placenta.