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 3H]-thymidine ( 3H]TdR) suicide technique during recovery of bone marrow from the damage caused by a sublethal total body irradiation. In contrast with previous reports the 3H]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 3H]TdR. Following the depression occurring 26 hr after the irradiation with 2.6 Gy, the proportion of CFU-s killed by the 3H]TdR was permanently increased until 5-6 days after irradiation. Thereafter large differences in the 3H]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 3H]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.