THE f NUMBER OF PRIMARY TRANSPLANTED SPLENIC COLONY‐FORMING CELLS

ABSTRACT The proportion of murine haemopoietic stem cells that settled in the spleen, after transplanting spleen cells into lethally‐irradiated recipient mice, was found by comparing the number of spleen colonies obtained by transplanting a whole spleen with an estimate of the total number of colony‐forming units (CFU) present in the intact spleen. the latter number was estimated assuming that endogenous spleen colonies were produced from surviving spleen‐derived CFU which exhibited the same survival parameters as transplanted CFU. Account was taken of the post‐irradiation loss of CFU from the spleen in the endogenous assay, which was found to be a reasonably constant factor for doses higher than about 100 rad X‐rays. The total measured number of CFU/spleen from transplantation was less than the number calculated in the intact spleen by a factor, the primary f number, of 0.03 ± 0.02.