Heterogeneity in the differentiation behavior of hematopoietic stem cells is well documented but poorly understood. To investigate this question at a clonal level, we isolated a subpopulation of adult mouse bone marrow that is highly enriched for multilineage in vivo repopulating cells and transplanted these as single cells, or their short-term clonal progeny generated in vitro, into 352 recipients. Of the mice, 93 showed a donor-derived contribution to the circulating white blood cells for at least 4 months in one of four distinct patterns. Serial transplantation experiments indicated that two of the patterns were associated with extensive self-renewal of the original cell transplanted. However, within 4 days in vitro, the repopulation patterns subsequently obtained in vivo shifted in a clone-specific fashion to those with less myeloid contribution. Thus, primitive hematopoietic cells can maintain distinct repopulation properties upon serial transplantation in vivo, although these properties can also alter rapidly in vitro.

As shown using clonal assays, the mouse HSC population undergoes quantitative as well as qualitative changes with age, including lineage differentiation, HSC pool size, marrow-homing efficiency, and self-renewal.

IntroductionEach day, the hematopoietic system of the adult mouse produces billions of mature blood cells. The multistep process that underlies the production of these cells is controlled by complex mechanisms that enable changing physiologic demands to be met without overwhelming the system. It is now clear that a small population of cells known as hematopoietic stem cells (HSCs) are ultimately responsible for maintaining the lifelong output of new blood cells. 1 Nevertheless, a molecular understanding of what constitutes an HSC and how its key functions are maintained are still poorly understood.The existence of hematopoietic cells with the individual potential to produce large numbers of multiple blood cell types in vivo for prolonged periods was first suggested by retrospective clonal tracking experiments that used unique chromosomal, 2 and later, retroviral marking approaches. 3,4 The relatively short lifespan of many mature blood cell types and the contrasting longevity of some of the multilineage clones identified in these experiments implied an origin of the clones from undifferentiated cells with an extensive ability to divide and maintain a derivative population of similarly undifferentiated cells. Serial transplantation experiments demonstrated that such cell divisions did occur and thus provided the first definitive evidence of HSC self-renewal. 3,4 These findings prompted a search for functional end points that would allow HSCs to be specifically quantified independent of the presence of other cell types when assayed in limiting dilution transplantation strategies using suitably irradiated congenic hosts. 5,6 A sustained output of at least 1% of all the circulating white blood cells (WBCs) for at least 4 months is now widely assumed to be suitable for this purpose. 7 Interestingly, most analyses of individual pluripotent hematopoietic cells proliferating either in vivo or in vitro have generally found the self-renewal activity actually displayed to be highly variable. 3,4,8,9 How such a variable behavior is related to the molecular state of the initial cells is not well defined and remains a subject of intense interest and investigation. 10 Variations in external cues may be one contributing parameter, at least in vivo, because it is known that self-renewal responses can be directly and rapidly modulated in this way in vitro. 11,12 In addition, there is some evidence of predetermined heterogeneity in HSC self-renewal potential. This is exemplified by the differences in regenerative activity of HSCs from fetal and adult sources 13,14 and the finding of a consistent association of short-term and long-term multilineage WBC outputs with distinct phenotypes of adult bone marrow (BM) cells (eg, according to their expression of CD49b and CD34). 15,16 Taken together, these results suggest that some hematopoietic cells can remain pluripotent for several divisions even though they are already destined to undergo terminal differentiation within a finite period. This is the basis of the concept of durable versu...