HEMATOPOIESIS Unique Differentiation Programs of Human Fetal Liver Stem Cells Shown Both In Vitro and In Vivo in NOD/SCID Mice

Comparative measurements of different types of hematopoietic progenitors present in human fetal liver, cord blood, and adult marrow showed a large (up to 250-fold), stage-specific, but lineage-unrestricted, amplification of the colony-forming cell (CFC) compartment in the fetal liver, with a higher ratio of all types of CFC to long-term culture-initiating cells (LTC-IC) and a lower ratio of total (mature) cells to CFC. Human fetal liver LTC-IC were also found to produce more CFC in LTC than cord blood or adult marrow LTC-IC, and more of the fetal liver LTC-IC–derived CFC were erythroid. Human fetal liver cells regenerated human multilineage hematopoiesis in NOD/SCID mice with the same kinetics as human cord blood and adult marrow cells, but sustained a high level of terminal erythropoiesis not seen in adult marrow-engrafted mice unless exogenous human erythropoietin (Epo) was injected. This may be due to a demonstrated 10-fold lower activity of murine versus human Epo on human cells, sufficient to distinguish between a differential Epo sensitivity of fetal and adult erythroid precursors. Examination of human LTC-IC, CFC, and erythroblasts generated either in NOD/SCID mice and/or in LTC showed the types of cells and hemoglobins produced also to reflect their ontological origin, regardless of the environment in which the erythroid precursors were generated. We suggest that ontogeny may affect the behavior of cells at many stages of hematopoietic cell differentiation through key changes in shared signaling pathways. r 1999 by The American Society of Hematology.