A single cell atlas defines perinatal factors that drive murine bone marrow development

Processes that direct initial colonization and maturation of the bone marrow remain elusive, despite lifelong importance to hematopoiesis. Bone marrow mesenchymal and stromal cell (BMSC) maturation establishes supportive niches prior to hematopoietic stem cell (HSC) recruitment. We define mouse BMSC progenitor identities and temporal emergence in a developmental single cell atlas spanning fetal life through 18 months of age. We clarify Cxcl12-abundant reticular (CAR) cell and osteoblast development, including temporal emergence of signaling modalities that direct HSC quiescence and regenerative capacity. CAR cells are absent until birth, with a developmental block during fetal maturation related to transcriptional changes during perinatal life. Temporal changes in Early B Cell Factor (Ebf) 1-3 expression and activity correlate with CAR formation and niche establishment, including Ebf2 repression and induction of Ebf3 activities that direct metabolic changes. Changes in systemic physiology underlie these transcriptional changes, including perinatal induction of lipid metabolism, inflammation, and hypoxic signaling. These systemic factors direct CAR cell emergence after birth, providing temporal and developmental resolution underlying the colonization and maturation of the bone marrow environment.