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Similarity in morphological appearance of cycling and resting hemopoietic stem cells
The morphological entity which has been identified previously as the most likely candidate for the pluripotential hemopoietic stem cell in mouse bone marrow, has been encountered in HSC concentrates of rat bone marrow. An identical correlation between percentage of HSC identified in electron microscopical preparations and percentages HSC calculated from results of spleen colony assays has been observed for HSC concentrates from normal rat marrow and from rat marrow in which the CFU-s had been induced into the cycling state by in vivo pretreatment with hydroxyurea. The ratio between counted HSC and calculated HSC was 0.5 in both cases and a similar ratio was calculated from previous experiments with mouse bone marrow. Evidence is provided that the deviation of this ratio from the expected value of 1 is not due to the use of erroneous f factors. This discrepancy can be explained by postulating a heterogeneity of spleen colony forming cells, in the sense that only 50 per cent of spleen colonies originates from pluripotential HSC and that the remainder arises from committed stem cells which have a different as yet not identified morphological appearance.
The morphological entity which has been identified previously as the most likely candidate for the pluripotential hemopoietic stem cell in mouse bone marrow, has been encountered in HSC concentrates of rat bone marrow. An identical correlation between percentage of HSC identified in electron microscopical preparations and percentages HSC calculated from results of spleen colony assays has been observed for HSC concentrates from normal rat marrow and from rat marrow in which the CFU-s had been induced into the cycling state by in vivo pretreatment with hydroxyurea. The ratio between counted HSC and calculated HSC was 0.5 in both cases and a similar ratio was calculated from previous experiments with mouse bone marrow. Evidence is provided that the deviation of this ratio from the expected value of 1 is not due to the use of erroneous f factors. This discrepancy can be explained by postulating a heterogeneity of spleen colony forming cells, in the sense that only 50 per cent of spleen colonies originates from pluripotential HSC and that the remainder arises from committed stem cells which have a different as yet not identified morphological appearance.