AbstractMice with mutations at theWlocus have a hemopoietic stem cell defect characterized by an apparent deficiency of spleen colony forming cells (CFU‐S). In the present report, we provide evidence that mutant cells form colonies and we compare the characteristics of the colonies derived from mutant and normal cells. To perform the colony‐derivation studies, marrow cells were transferred into lethally irradiated congenic hosts that differed from the donors in the ubiquitous genetic marker, glucose phosphate isomerase (GPI‐1). Donor GPI‐1 comprised over 50% of the marker in the host spleen and marrow by 12 days post injection, regardless of whether the donor was mutant or normal. To characterize the colonies, serially sectioned host spleens were examined microscopically. Colonies are present by 8 days post‐transplantation regardless of donor genotype, but mutant colonies are distinctly different from normal colonies. The proportion of blast and granulocyte colonies is always greater inW/Wvthan in +/+ recipients. Unlike theW/Wvdonors, the +/+ donors generate primarily erythrocyte colonies at 8, 10, and 14 days and mixed colonies at 12 days post‐injection. Colonies from the mutant mice are generally smaller but visible colonies do appear by 12 days. The results are consistent with the notion that the anemia inW/Wvmice is caused by the early restriction of differentiating cells to a non‐erythrocyte lineage accompanied by the delayed amplification of mutant hemopoietic cells. Whether this means erythrocyte‐committed cells are absent or are present but unable to respond to the appropriate cytokines is not possible to determine from the curr
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