Decrease in the ability of CFU-s in shielded marrow to be recruited into cell cycle after multiple irradiations:...

Nine doses of 1.5 Gy given to mice with one shielded leg result in very similar perturbations in shielded marrow (CFU-s kinetics whatever the source of radiation (X or gamma rays). At the time of the ninth irradiation, the size of the shielded CFU-s compartment is reduced to 75% of control level. After 15 min it decreases to 47% and, 1 day later, remains below the pre-ninth irradiation level (62% of control level) in spite of two significant peaks of CFU-s in DNA synthesis, at 1 and 8 hr after the ninth irradiation. For acceptable fitting to experimental data, computer simulations make it necessary to assume that a fraction of shielded marrow CFU-s is not capable of entering the cell cycle after the treatment. This is not explainable by defects in the stimulators of CFU-s proliferation secreted by shielded haemopoietic tissue because their production and their efficacy are demonstrated to be normal after the nine exposures. The incomplete recovery of the shielded CFU-s pool from proliferating CFU-s can be attributed to a loss in CFU-s by differentiation at birth.