Kinetics of Adult hematopoietic stem cell differentiation in vivo

Adult hematopoiesis has been studied in terms of progenitor differentiation potentials, whereas its kinetics in vivo is poorly understood. We combined inducible lineage tracing of endogenous adult hematopoietic stem cells (HSC) with flow cytometry and single-cell RNA sequencing to characterize early steps of hematopoietic differentiation in the steady state. Labeled cells, comprising primarily long-term HSC and some short-term HSC, produced megakaryocytic lineage progeny within one week, in a process that required only 2-3 cell divisions. Erythroid and myeloid progeny emerged simultaneously by 2 weeks, and included a progenitor population with expression features of both lineages. Myeloid progenitors at this stage showed diversification into granulocytic, monocytic and dendritic cell types, and rare intermediate cell states could be detected. In contrast, lymphoid differentiation was virtually absent within the first 3 weeks of tracing. These results show that continuous differentiation of HSC rapidly produces major hematopoietic lineages and cell types, and reveal fundamental kinetic differences between megakaryocytic, erythroid, myeloid and lymphoid differentiation. We combined inducible lineage tracing of endogenous adult hematopoietic stem cells (HSC) with flow cytometry and single-cell RNA sequencing to characterize early steps of hematopoietic differentiation in the steady state.