Haemopoietic stem-cell kinetics during continuous irradiation.

Analysis of the kinetics of the biphasic response of haemopoietic stem cells to chronic low dose-rate irradiation indicates that a highly radiosensitive delay of entry into cell-cycle by Go cells may be responsible for the initial, apparently dose-rate-independent, phase of the dose-response curve. The second component of the curve appears to be the result of a combination of two factors: a relatively decreased differentiation demand on stem cells due to increased amplification in the transit-committed precursor cells, and a uniform shortening of the cell-cycle--including the S period--in the stem cells.