(164aw) Spatiotemporal Imaging and Analysis of Mouse and Human Liver Bud Morphogenesis

The process of liver organogenesis has thus far served as a paradigm for organ formation. However, there still remains a lack of understanding regarding early mouse and human liver bud morphogenesis and early liver volumetric growth. Elucidating dynamic changes in liver volumes is critical for understanding organ development, implementing toxicological studies, and for modeling hPSC-derived liver organoid growth. New visualization, analysis, and experimental techniques are desperately needed. Here, we combine observational data with digital resources, new 3D imaging approaches, retrospective analysis of liver volume data, mathematical modeling, and experiments with hPSC-derived liver organoids. Image slices were downloaded from the 3D Atlas of Human Embryology, eMouseAtlas, and Toronto Centre for Chemogenomic Mouse Imaging Centre. ImageJ was used to trace the liver bud, the STM, and the gut tube for each aligned section, and these traces were combined with the 3D Viewer plugin to form a 3D model of the liver. Using images from fetal mouse and human from several imaging modalities (MRI, micro-CT, optical projection, H&E stain), we were able to demonstrate distinct spatial features and complete full growth curves, which we modeled using a Gompertz curve. Hepatic cords were able to be visualized at E9.0-E9.5 in mouse and day 25 in human as well as evidence of mesenchymal-liver epithelial interactions. The growth curve models were able to be further analyzed and verified with liver weight, cell count data, as well as partial hepatectomy growth. There was an overall 8.74 x 10⁴-fold-increase in volume between E8.5 to E18. However, 7.5 x 10³ of this fold change occurs just between E9 and E14. This growth is much more exponential when compared to regeneration growth. Interestingly, the cell count data increased at a similar rate as volumetric growth from E10.5 onward, indicating that proliferation is the main source of growth rather than hypertrophy. Visualization of liver-epithelial and septum transversum mesenchyme (STM) interactions suggests extended interactions, which together with new spatial features, may be responsible for extensive exponential growth. These STM interactions are modeled with a novel in vitro human pluripotent stem cell (hPSC)-derived hepatic organoid system that exhibits cell migration. Our methods enhance our understanding of liver organogenesis, with new 3D visualization, analysis, mathematical modeling, and in vitro models with hPSC. Our approach highlights mouse and human differences and provides potential hypothesis for further investigation in vitro and in vivo.