Cooling the skin caused partial division cycle synchrony in proliferative cells of the mouse hair follicle. Synchrony was assayed in two ways:${}^{3}{\rm H}-{\rm TdR}$ incorporation into the skin and changes in radiosensitivity of the hair follicle cells as a function of time after cooling. Cooling arrested cells in mitosis. This was followed by a long G1 period (∼13 hr) and an S phase of about 10 hr. Half an hour after the end of cooling, cells arrested in mitosis were radiosensitive but this radiosensitivity was reduced if Colcemid was given at the end of cooling. Colcemid inhibits spindle formation and hence it is postulated that the radiosensitivity of mitotic cells may be related to the radiosensitivity of the spindle. Survival curves determined 3 and 7 hr after the end of cooling are described by mean D0 values of 114 and 412 rad, respectively: the control value is 197 rad.

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