Large numbers of primitive stem cells are active simultaneously in aggregated embryo chimeric mice.

The possibility has been repeatedly raised that erythropoiesis results from clonal succession--the differentiation of one or a very small number of the most primitive stem cells that are sequentially activated to proliferate forming clones of differentiated cells and then eventually decline, to be replaced by new stem cell clones. We studied this possibility in chimeric mice made by combining embryos from two different strains so that they would have two distinct stem cell populations, each of which produces a different hemoglobin type (d and s). These were compared with F1 hybrids in which every stem cell produces both types. We measured the percentage of type d in seven to ten serial samples of circulating reticulocytes taken at three- to seven-day intervals and found that the variability in percent of this hemoglobin was only slightly higher in the chimeric mice than in F1 controls; SD ranged from 2.7% to 5.5% in the chimeric mice and from 3.4% to 3.9% in the controls. Using the binomial formula, the numbers of new clones formed during the reticulocyte life span, approximately three days, ranged from 33 to 118 in the individual chimeric mice. However, these numbers are underestimates because estimated numbers of clones depend inversely on variabilities, and the calculations did not exclude the contribution of experimental error to the overall variability. Total percentages of type d hemoglobin were also measured in seven to nine successive serial samples at 60- to 136-day intervals. These gave mean values similar to measures of newly synthesized hemoglobin in the same mice, but SD were larger, ranging from 5.3% to 8.4%. This reflects experimental error, both because of excess day-to-day variability found in this type of measurement and because there could not be fewer primitive stem cells activated to form clones of erythrocytes during the 45-day erythrocyte life span than during the three-day life span of reticulocytes. Since most and maybe all of the variation between successive samples in the same chimeric mouse appear to result from experimental error, many or even all of the primitive stem cells may simultaneously contribute to erythropoiesis.