Post-gastrulation amnioids (PGAs) as a stem cell-derived model of week 2-4 of human extra-embryonic development

The amnion, an essential extra-embryonic tissue in mammalian embryos, is thought to provide crucial signaling, structural and nutritional support during pregnancy. Despite its pivotal importance, studying primate amnion formation and function has been hampered by the lack of accurate in-vitro models. Here, we present a human embryonic stem cell-derived 3D model of the post-gastrulation amnion (PGA) that faithfully recapitulates extra-embryonic development up to 4-weeks post-fertilization closely mimicking the functional traits of the human amniotic sac. PGAs self-organize forming the amnion and the yolk sac and are surrounded by extra-embryonic mesoderm, which forms a connecting stalk. Using PGAs, we show that GATA3 is required and sufficient for amniogenesis and reveal an autoregulatory feedback loop governing amnion formation, whereby extra-embryonic signals promote amnion specification. The reproducibility and scalability of the PGA system, with its precise cellular composition and structural integrity, opens new avenues for investigating embryo-amnion interactions beyond gastrulation and offers an ideal platform for large-scalegenetic, pharmacological, and biomechanical studies.