Dong, R.; Li, H.; He, X. C.; Wang, C.; Perera, A.; Malloy, S.; Russell, J.; Li, W.; Petentler, K.; Mao, X.; Yang, Z.; Epp, M.; Hall, K.; Scott, A.; Smith, S.; Hembree, M.; Wang, Y.; McKinney, S.; Haug, J.; Unruh, J.; Slaughter, B.; Kang, X.; Li, L.

Previous studies of hematopoietic stem cells (HSCs) primarily focused on single cell-based niche models, yielding fruitful but conflicting findings. Here we report our investigation on the fetal liver (FL) as the primary fetal hematopoietic site using spatial transcriptomics. Our study reveals two distinct niches: the portal-vessel (PV) niche and the sinusoidal niche. The PV niche, composing N-cadherin (N-cad)High Pdgfr+ mesenchymal stromal cells (MSCs), endothelial cells (ECs), and N-cad-Low Albumin+ hepatoblasts, maintains quiescent and multipotential FL-HSCs. Conversely, the sinusoidal niche, comprising ECs, hepatocytes, and potential macrophages, supports proliferative FL-HSCs biased towards myeloid lineages. Unlike prior reports on the role of Cxcl12, with its depletion from vessel-associated stromal cells leading to 80% of HSCs\' reduction in the adult bone marrow (BM), depletion of Cxcl12 via Cdh2CreERT (encoding N-cad) induces FL-HSCs relocation from the PV to the sinusoidal niches, resulting in an increase of myeloid-biased HSCs. Similarly, we discovered that adult BM encompasses two niches within different zones, each composed of multi-cellular components: trabecular bone area (TBA, or metaphysis) supporting deep-quiescent HSCs, and central marrow (CM, or diaphysis) fostering relatively proliferative HSCs. This study transforms our understanding of niches by shifting from single cell-based to multicellular components within distinct zones, illuminating the intricate regulation of HSCs tailored to their different cycling states.