Figure 1.

Definitions of cell type and state. A) In the hematopoietic system, cell types have been typically defined by a combination of cell surface marker expression and functional output in in vitro and in vivo assays. B) Within cell types, multiple cell states are possible, including quiescence, active cycling, senescence, and in some cases, resting and activated states. C) Population‐level characterization enables molecular definition of the differences between cell types, in this hypothetical example using principal components analysis (PCA) of RNA‐seq data. This, however, does not reveal heterogeneity within these phenotypically defined populations. Through (D) single‐cell analysis, it is possible to explore this heterogeneity, even in rare cell populations such as HSCs, revealing novel cell phenotypes—cell types and states—within a “homogeneous” population of cells. Abbreviations: LT‐HSC, long‐term reconstituting HSC; FSR‐HSC, finite self‐renewal HSC; LMPP, lymphoid‐primed multipotential progenitors; CMP, common myeloid progenitor; MK, megakaryocyte; E, eryrthroid; My, Myeloid; T, T‐cell; B, B‐cell; PC, principal component.