ABSTRACT
Hematopoietic stem cells (HSCs) reside at the apex of hematopoiesis, can maintain this function for life, and are the functional units in clinical bone marrow transplantation. Limitations in HSC numbers not only restricts their clinical use but also several embodiments of experimental research. Recently, a highly defined ex vivo culture system was shown to support several hundredfold expansion of functional in vivo HSC activity. Here, we further explored this system. Over a 3-week culture period, only 0.1% of initially seeded HSCs retained their original phenotype, which contained virtually all functional long-term HSC activity in cultures. Despite this low frequency, net HSC expansions were large due to the extensive proliferation in cultures. Limited numbers of expanded HSCs allowed for long-term multilineage engraftment of unconditioned hosts, with expanded HSCs rapidly returning to quiescence in this in vivo setting. The ex vivo differentiated progeny from candidate HSCs was rich in progenitor cell activity and allowed for radioprotection from even single cultured HSCs. Finally, clonal barcoding and competitive repopulation experiments demonstrated that successful HSC expansion emanated from rare HSC clones that was only partially predicted by phenotype, underscoring the large impact of cellular heterogeneity for HSC biology.
Competing Interest Statement
The authors have declared no competing interest.
