Mapping Human Liver Development using Single-Cell Transcriptomics to Reveal Pathways Directing Stem Cell Fates
The liver is an organ with a diversity of essential functions including xenobiotic removal, bile acid production, storage of iron and vitamins, and metabolism of glucose and fatty acids. Hepatocytes, comprising the liver parenchyma, fulfil these functions and form an extensive network with non-parenchymal cholangiocytes, endothelial cells, Kupffer cells, and hepatic stellate cells. Disorders affecting hepatocytes are life threatening and increasing in prevalence12, with end-stage treatment relying upon a limited supply of liver donations for transplant. Consequently, the production of hepatocyte-like cells (HLCs) from human pluripotent stem cells (hPSCs) for clinical applications such as cell-based therapies and toxicology screens has become a critical research focus. Therapeutic advances, especially in regenerative medicine, are currently hampered by the lack of knowledge concerning how human hepatic cells develop. Here, I addressed this limitation by describing the developmental trajectories of different cell types comprising the human fetal liver using single-cell transcriptomics (scRNA-seq). These analyses revealed that sequential cell-to-cell interactions direct functional maturation of hepatocytes, with non-parenchymal cells playing critical, supportive roles during organogenesis. We also uncovered a novel progenitor of sinusoidal endothelial cells and hepatic stellate cells in the early liver, thus providing an explanation for the similar, complementary roles these cells place in liver extracellular matrix formation and vascularisation in support of hepatoblast growth and differentiation during development. We utilised this information to derive bipotential hepatoblast organoids and used this novel model system to validate the importance of key signalling pathways and developmental cues. Currently, hPSC-derived hepatocytes do not directly resemble their primary tissue counterparts and have limited clinical applicability. We used scRNA-seq to compare HLCs with primary development to identify important regulators involved in late-stage maturation of HLCs and allow us to improve the derivation of these cells in vitro. This marks an important application of development biology to inform stem cell differentiation for the production of clinically-relevant cells.