Non‐Circadian Rhythm In Proliferation of Haematopoietic Stem Cells

. The proportion of haematopoietic stem cells (CFU‐s) engaged in DNA synthesis was determined by means of the [ 3 H]‐thymidine ([ 3 H]TdR) suicide technique during recovery of bone marrow from the damage caused by a sublethal total body irradiation. In contrast with previous reports the [ 3 H]TdR suicide rate was not permanently increased. It was observed that CFU‐s passed through S phase in synchronous waves, following a dose of irradiation of 1.5 Gy. After a dose of 2.6 Gy, there was only one initial wave of increased CFU‐s sensitivity to the action of [ 3 H]TdR. Following the depression occurring 26 hr after the irradiation with 2.6 Gy, the proportion of CFU‐s killed by the [ 3 H]TdR was permanently increased until 5‐6 days after irradiation. Thereafter large differences in the [ 3 H]TdR suicide data were observed among individual mice. Evidence was obtained that individual mice, which had been irradiated by a dose of 2.6 Gy 8‐9 days before, had identical values of the CFU‐s [ 3 H]TdR suicide rate in the bone marrow from different bones of the lower extremities. the recurrence of the synchronous waves in CFU‐s passage through the cell cycle was recorded when the CFU‐s population regenerated to only about 10% of its normal value. These waves were obviously not related to a particular time of the day and, consequently, they did not represent the circadian rhythm. It is concluded that the synchronous waves in which CFU‐s proliferation occurred reflected the action of the control mechanism on CFU‐s proliferation. This mechanism should be endowed with an important systemic component besides locally operating factors.