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The major limitations to the widespread use of high‐dose chemotherapy or radiotherapy followed by autologous or allogeneic transplantation are the scarcity of stem cell donors and the depletion of the autologous stem cell reservoir. Cord blood is a readily available source of stem cells, which, however, might be limited in number. For this reason, up to now, cord blood transplantation has been restricted to children. Therefore, a major goal for experimental and clinical hematology is the identification of mechanisms and conditions that support the expansion of transplantable hematopoietic stem cells. Two systems have been described to identify in vitro these progenitor cell populations in both mice and humans: A) long‐term culture‐initiating cells (LTC‐IC), so named because of their ability to support the growth of hemopoietic colonies (colony‐forming cell [CFC]) for five to six weeks when cocultured on stromal layers, and B) the generation of hematopoietic progenitors CFC from stroma‐free liquid cultures for extended periods of time, which is another indirect evidence for the presence of primitive stem cells. The two systems detect largely overlapping but not identical cell populations of progenitor cells; thus, the identification of the growth factor requirements for the maintenance and amplification of both systems is relevant.  The studies presented here demonstrate that CD34 +  cord blood cells can be grown in stroma‐free liquid cultures for extremely prolonged periods of time (up to six months). During such a period, hemopoietic precursors and committed progenitors belonging to all of the hematopoietic lineages are continuously and massively generated. Such a massive expansion is sustained by an increasingly larger expansion of primitive stem cells (CFU‐BI and LTC‐IC). The presence of both FL and thrombopoietin (TPO) was necessary and sufficient to support this phenomenon. The addition of KL ± interleukin 6 (IL‐6) does not appear to substantially modify the extent of LTC‐IC expansion.  FL and TPO appear to be two unique growth factors that preferentially support the self‐renewal of primitive stem cells; the additional presence of KL and IL‐6 seems to enhance the proliferative potential of at least a subpopulation of daughter stem cells which can undergo at least three differentiation pathways.

The role of c‐Mpl ligands in the expansion of cord blood hematopoietic progenitors