3083 – THE MURINE BONE MARROW NICHE DOES NOT SUPPORT A DIVERSE ARRAY OF HEMATOPOIETIC STEM AND PROGENITOR CELLS UNTIL BIRTH

Hematopoietic stem cells (HSCs) are thought to migrate from the fetal liver (FL) to the fetal bone marrow (FBM) beginning around embryonic day 15.5 (E15.5) to E16.5. The dynamics of hematopoiesis in FBM is largely unexplored. To better understand hematopoiesis in the FBM, we catalogued the phenotype and function of hematopoietic stem and progenitor cells (HSPCs) in murine fetal and neonatal BM throughout ontogeny. Surprisingly, the most prevalent HSPC population within the Lineage-Sca-1+c-Kit+ compartment in E15.5-E18.5 FBM were MPP2s. However, right at birth (E19-P0) the frequency of other HSPCs markedly rose, more closely mirroring that seen in adult BM. At E16.5, E18.5, and P0, FBM MPP2s displayed a dramatic loss of both CFU and in vivo repopulating activity, relative to FL and adult MPP2s. Thus, we show for the first time that early FBM MPP2s—which are potentially seeded directly from the FL—are an immunophenotypically identical but functionally distinct population of HSPCs compared to their FL and adult MPP2 counterparts. To better understand the FBM and neonatal BM niche, we used 10x Genomics to acquire the single cell gene expression profiles of hematopoietic progenitors and stromal cells across ontogeny (E16.5, E18.5, P0, P6, P14, and adult). Our analyses reveal pronounced transcriptional heterogeneity within multiple HSPC subtypes and a dramatic reorganization of hematopoietic and stromal cell lineages across development. In sum, our study suggests that the BM niche does not support a diverse array of HSPCs until E19-P0. We are now focused on using our single cell gene expression resource to identify putative factors and cells in the neonatal BM critical to establishing a supportive niche for HSPCs.