Tissue-scale mapping reveals a central role of hepatoblasts in the regulation of fetal liver hematopoiesis and stem cell maintenance.

During embryonic development, fetal liver (FL) tissues transiently provide a fertile microenvironment for the maturation and expansion of fetal progenitors and hematopoietic stem cells (HSCs). Nonetheless, the cellular composition of FL HSC niches remains underexplored. Using 3D microscopy, bulk mRNA sequencing (mRNA-seq), flow cytometry, and transgenic mouse models, we mapped the spatiotemporal dynamics of niche cells and HSCs during FL development. At peak hematopoiesis, hepatoblasts, endothelial cells, and mesenchymal stromal cells pervasively expressed pro-hematopoietic cytokines throughout FL tissues. Yet, hepatoblasts were spatially dominant, abundantly expressed niche factors, and regulated HSC expansion and erythropoiesis through production of KIT ligand. Subsequent FL remodeling driven by hepatoblast differentiation led to the rapid downregulation of supportive cues and contraction of hematopoietic niches, coinciding with HSC exit toward emergent marrow tissues. Spatial mapping further showed that HSCs and progenitors were dispersed across the parenchyma, following no clear spatial biases within the FL microanatomy yet exhibiting local clustering.