A single-cell hematopoietic microenvironmental atlas reveals progressive maturation of bone marrow vascular niche

The interaction between hematopoietic stem and progenitor cell (HSPC) and its vascular niche is essential for supporting the homeostasis and reconstitution of hematopoietic system in adult bone marrow (BM), but a comprehensive atlas covering this HSPC-vascular niche crosstalk in multiple developmental stages and species is lacking. Here, we integrated single-cell transcriptomic data of HSPC and its vascular niches from fetal liver until aged BM, covering two species, two organs, and six developmental time points. Comparative analyses revealed dramatic differences in the gene expression, enriched pathway, and cell-cell communication between human fetal and adult BM. Notably, many of these differences were conserved between humans and mice. Multi-timepoint profiling of murine BM vascular niches revealed a stepwise maturation of gene expression, including critical niche factors such as SCF and CXCL12. Furthermore, analysis of this dynamic vascular niche atlas highlighted organ-specific features between fetal liver and BM niches, significant transcriptional changes in aged BM endothelial cells, and identified midkine as a previously unknown niche factor. Functional validation showed that transplanting HSPC into midkine knockout mice or treating with a midkine inhibitor (iMDK) enhanced hematopoietic reconstitution. In contrast, recombinant midkine suppressed HSPC differentiation. Together, our work presents a cross-species and multi-stage atlas of HSPC-vascular niche interactions, offering valuable insights into the dynamic changes of vascular niche through lifelong HSPC development and a platform to identify unknown niche factors.