Highlights d Identification of fetal-associated endothelial cells and macrophages in HCC d Embryonic-like reprogramming in a subset of FOLR2 + TAM1s d Conserved GRN in mouse embryonically seeded, human fetal-liver, and TAM1 macrophages d Shared onco-fetal ecosystem between human fetal liver and HCC

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Highlights d Human fetuses in 2 nd trimester show T cell diversity with effector-memory phenotype d Fetal organs show diverse bacterial genera that can be cultured and propagated d Bacterial structures with mucin-like threads are visualized in 14-weeks EGA fetal gut d Fetal bacteria induce syngeneic memory T cell activation in fetal mLN T cells