Change in hair cycle and hair length in nude mice by administration of deuterium oxide

D2O increased hair length in Balb/c nu/nu (nude) mice in our previous study although it has an antimitotic effect in cells. To investigate the mechani sm of the effect on the hair length, we examined the change by the administ ration of D2O in the duration of the hair cycle and the proliferating activ ity of the hair matrix in relation with hair length in nude mice. The resul ts showed that 20 or 30% D2O administration did not change the gross struct ure of the hairs, the proliferative activity and keratinization of the hair matrix cells, but elongated the hair cycle. The duration of the hair cycle increased by the administration of D2O in a dose-dependent manner over the examined range and these effects were reversible by discontinuation of D2O . The change in the hair length correlated with the change in the hair-exis ting phase particularly. We also showed that the mast cell density in the s kin, which is related to the hair cycle, increased ill the deuterated mice at anagen VI stage which nearly corresponds to the hair-existing phase. The increase in the mast cell density may be related to the increase in the ha ir cycle duration. These findings indicate that the increase in hair length may be due to the increase in the duration of the hair cycle, in particular, an increase in t he hair-existing phase. This study thus suggests that D2O slows not only sh ort-term cycles such as circadian clock or ultradian clock, but also the ha ir cycle which is a long-term cycle.