PhD Student Hannover Medical School Medical School Hanover, United States
Abstract: The human embryo undergoes gastrulation 14-21 day's post-fertilisation, establishing the body plan and initiating organogenesis. Difficulty accessing samples has hindered our understanding of this critical window, particularly the developing tissue crosstalk and the morphological changes that drives. Recent in vitro models have made significant progress in replicating the co-development of the cell types involved. However, one of the first morphological processes in human organogenesis, the caudal invagination of the primitive gut tube dorsal to the cardiac mesoderm, remains unexplored. In 2021, we described the human heart-forming organoid (HFO), which recapitulates the co-development of heart, foregut and vasculature in a robust and reproducible manner. To study the developmental mechanisms from pluripotent to proper heart and foregut patterning at day 10, we performed daily 10x Visium HD spatial RNAseq on multiple HFO sections, these where then spatially contextualised using 3D confocal imaging of whole mount immunofluorescence stained and cleared HFOs. This revealed a primitive streak like Brachyury+ population surrounding an ectodermal core at day 3, which provided a starting point for endodermal migration, with definitive endoderm (FOXA2+/SOX2-) cells migrating inwards and gaining SOX2 expression, committing to the anterior foregut (AFE) fate (FOXA2+/SOX2+). From the same location, day 6 HNF4a+ posterior foregut endoderm (PFE) cells migrated outwards. HFOs therefore mimic the migration and development of the foregut and its anlagen in a human model, which has previously only been possible in model organisms. We next questioned HFOs ability to mimic in vivo genetic and compound induced teratogenic phenotypes. Firstly, we show that GATA4-/- HFOs replicated the loss of PFE seen in GATA4-/ mouse embryos. Secondly, HFOs reproduced the characteristic anti-angiogenic effects and cardiac abnormalities seen in clinical data from thalidomide exposed pregnancies. Notably we also saw inhibition of AFE migration, highlighting the models ability to study transient teratogenic processes. Together, we provide independent experimental evidence that HFOs model key developmental processes, enabling a deeper mechanistic understanding of previously inaccessible areas of human development.
