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Abstract Mouse bone marrow cells in suspension were separated into a number of fractions on the basis of cell density by equilibrium density gradient centrifugation, or on the basis of cell size by velocity sedimentation. After each type of separation, the cells from the various fractions were assayed for their ability to form macroscopic spleen colonies in irradiated recipient mice, and for their ability to form colonies in a cell culture system. The results from either separation technique demonstrate that cells in some fractions formed more colonies in vivo than in the culture system, while cells in other fractions formed more colonies in culture than in the spleen. The results of control experiments indicate that this separation of the two types of colony‐forming cells was not an artifact of the separation procedures. From these experiments it was concluded that the population of cells which form colonies in culture under the conditions used is not identical to the population of cells detected by the spleen colony assay.
Worton et al. (Wed,) studied this question.