The classical hematopoietic hierarchy roadmap illustrates that hematopoiesis is a stepwise process, from multi-, oligo-, and unipotent progenitors to mature hematopoietic cells.
An updated view of hematopoietic hierarchy reveals that hematopoiesis is a continuous differentiation process, and that the differentiation of megakaryocyte lineage can bypass the intermediate steps.
The discovery of lineage-biased HSCs remodels the balanced hierarchy roadmap.
Hematopoiesis is maintained by distinct stem/progenitor cell populations in native and stress states, indicating that the hematopoietic hierarchy roadmap is flexible to adapt to different physiological conditions.
The classical roadmap of hematopoietic hierarchy has been proposed for nearly 20 years and has become a dogma of stem cell research for most types of adult stem cells, including hematopoietic stem cells (HSCs). However, with the development of new technologies such as omics approaches at single-cell resolution, recent studies in vitro and in vivo have suggested that heterogeneity is a common feature of HSCs and their progenies. While these findings broaden our understanding of hematopoiesis, they also challenge the well-accepted hematopoietic hierarchy roadmap. Here, we review recent advances in the hematopoiesis field and provide an updated view to incorporate these new findings as well as to reflect on the complexity of HSCs and their derivatives in development and adulthood.