Toward the dissection of hematopoietic stem cell fates and their determinants.

Hematopoietic stem cell (HSC) functions have long been difficult to study under physiological conditions. Recently, genetic in vivo approaches have been developed for lineage tracing of differentiating progeny emerging from HSC over time (output), and for high-resolution, endogenous barcoding to uncover the lineages that HSC contribute to (fate). Such fate measurements have in principle led to the recognition of three major fate groups of HSC: multilineage, myelo-erythroid-restricted, and inactive, that is, no or no known progeny, in addition to a minor group of megakaryocyte-restricted HSC. The most recent RNA-barcoding experiments have begun to directly link fate measurements with transcriptome reading in HSC clones and single HSC, which yielded insights into transcriptional signatures associated with fate patterns. Here, we discuss these findings in light of the structure of the hematopoietic differentiation hierarchy, and we provide an outlook on strategies to dissect molecular determinants of HSC fates.