Human mixed cell colonies: unicellular or multicellular origin–analysis by G‐6‐PD

Summary. Marrow and peripheral blood cells from normal women heterozygous (Gd^B/Gd^A) at the X‐chromosome‐linked glucose‐6‐phosphate dehydrogenase (G‐6‐PD) locus were cultured at cell concentrations ranging from 2 × 10⁴/ml to 4 × 10⁵/ml to test formally the plating conditions necessary for reliable enumeration of multipotent stem cells (CFU‐mix). The culture system was rigorously tested by plating cells obtained after velocity sedimentation and the G‐6‐PD enzyme type of individual colonies was determined. At cell concentrations <7.5 × 10⁴/ml for marrow and < 1.25 × 10⁵/ml for peripheral blood, mixed‐cell colonies had either type A or type B enzyme, but not both. At higher cell concentrations, significant numbers of colonies showed both enzyme types and therefore arose from more than one cell. These studies demonstrate that enumeration of CFU‐mix by in vitro colony assay is accurate only at low cell concentrations. Studies of haematopoietic differentiation relying on in vitro colony assays of multipotent stem cells must be carefully analysed in light of these data.