Telogen mouse hairs treated with single doses of 2000 to 2500 rads showed persistent impairment of postirradiation growth as measured by determinations of overall hair length. Reductions in length of surviving hairs, compared to contralateral controls, averaged 27% for the hairs newly grown postirradiation. Subsequent hair generations showed no significant recovery: reductions in length of hairs from the same treatment sites averaged 33% 10 months after irradiation. Cyclic regrowth of hair in these areas was also unusually slow: after plucking, hair density was not fully restored in irradiated areas for 4 weeks or longer in contrast to the normal replacement time of 2 1/2 weeks observed in plucked control sites. This change was demonstrable up to 14 months postirradiation. Exposure of telogen hairs to a somewhat smaller dose of 1500 rads resulted in a 12% reduction of length in the generation of hairs growing immediately postirradiation, followed by complete recovery in the subsequent cycle of hair growth. The persistence of effects in the animals subjected to high doses of radiation may be the result of overall reduction in stem cell pool size since investigations with tritiated thymidine showed that cell generation cycles were almost identical in treatment and control sites but that the mitotic index was substantially reduced in the irradiated areas.

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