Single-cell multiomic atlas of healthy pediatric bone marrow reveals age-dependent differences in lineage differentiation driven by stromal signaling

Childhood is a critical period for hematopoietic development and susceptibility to hematologic disease. Here we generated a multimodal single-cell atlas of healthy human bone marrow, capturing mRNA and surface protein expression in 90,710 cells, including over 20,000 hematopoietic stem and progenitor cells (HSPC) and mesenchymal stromal cells (MSC) from nine donors ranging from infancy to young adulthood (2-32 years). Young pediatric (YP) bone marrow (<10 years) was compositionally and molecularly distinct from adolescent and young adult (AYA) bone marrow (≥13 years), with hematopoietic output shifting from B cell dominance in YP bone marrow to myeloid and T cell bias in AYA bone marrow. Spatial transcriptomics of six bone marrow biopsies (0-23 years) confirmed these age-dependent changes. Two lymphoid progenitor (LyP) subsets regulated this lineage shift: CD127⁺ LyP cells with B cell-biased output were enriched before age 10, whereas CD127^- LyP cells with combined lymphoid and myeloid features predominated thereafter. Stromal signaling showed corresponding age-dependent changes, with increased interleukin-7 production by bone marrow MSC in YP compared to AYA, indicating niche-mediated regulation of HSPC lineage potential during ontogeny. This single-cell atlas provides a comprehensive resource for understanding hematopoietic development and early-life origins of hematologic disease.