Impaired steel factor responsiveness differentially affects the detection and long-term maintenance of fetal liver hematopoietic stem cells in vivo

Abstract

The results of previous studies have shown that the development of hematopoiesis during fetal life can occur in the absence of Steel factor (SF) signaling. On the other hand, impairment of this mechanism can severely compromise the ability of cells from adult bone marrow to regenerate hematopoiesis on their transplantation into myeloablated recipients. This apparent paradox could result from changes during ontogeny in the responsiveness of hematopoietic stem cells to regulators that may substitute for SF as well as from differences in the availability of such factors during embryogenesis and in the myeloablated adult. To investigate these possibilities, we studied the effect of W41 and W42 mutations on the numbers, phenotype, and posttransplant self-renewal behavior of primitive hematopoietic cells present in the fetal liver (FL) of 14.5-day-old mouse embryos. In W41/W41 FL, day-12 spleen colony-forming units and long-term culture-initiating cells appeared both quantitatively and qualitatively similar to their counterparts in the FL of +/+ embryos. W41/W41 FL also contained near normal numbers (≃50% of controls) of transplantable lymphomyeloid stem cells with competitive reconstituting ability in myeloablated adult +/+ recipients (as assessed for up to at least 16 weeks posttransplant). Moreover, both the original phenotype of these W41/W41 competitive repopulating units (CRUs) and their clonal posttransplant output of mature progeny were normal. Similarly, when myeloablated adult +/+ mice were cotransplanted with 5 x 104 +/+ FL cells and a sevenfold to 70-fold excess of W41/W41 FL CRUs, the contribution of the +/+ FL CRUs to the circulating white blood cell count present 5 weeks later was markedly reduced as compared with that of mice that received only +/+ FL cells. However, over the next 3 months, the proportion of mature white blood cells that were derived from +/+ precursors increased significantly (P