MEASURING TURNOVER TIME 329 Table IV Effect of Finn Chamber Occlusion on Turnover Time of Stratum Corneum Untreated Occlusion* Subjects (days) (days) Ratio M, 23 11.2 17.8 1.59 M, 26 11.0 19.0 1.73 M, 28 13.8 45.8 3.32 M, 34 14.5 -- -- M, 37 13.8 37.5 2.72 M, 46 15.0 25.5 1.70 Mean ___ SD 13.2 4- 1.7 29.1 4- 12.2 2.21 4- 0.77 estimated by extrapolation. the fluorescence intensity decreased linearly with progressive tape strippings. Therefore, stratum corneum turnover can be determined in the above manner. The turnover time of the stratum corneum depends on environmental conditions. Normal routines (bathing, type of clothing worn, etc.) can markedly affect the rate of desquamation. In this paper, it was shown that skin protection by gauze prolonged turnover time by 75%, but gauze protection may not affect skin hydration. Therefore, this result seems to reflect turnover time in the absence of physical stimulation. How- ever, Finn chamber protection should increase skin hydration. Consequently, the turn- over time of stratum corneum may be prolonged further by Finn chamber occlusion than by gauze protection. There are conflicting reports regarding circadian variation in cell proliferation of human epidermis. It would have been very difficult to detect a circadian rhythm by normal fluorescence extinction. However, by this procedure, it was found that the desquamation rate was twice as fast in the daytime as at night. This result provides some support to the previous observation (6) that there is relative sup- pression of epidermal proliferative activity in human skin at night. left side of guinea pig back right side of guinea pig back •100- ._ o DC application • e background • 50- Vo• lr1=0.996 •o -- la. 0- 10 20 0 Days ø ø o DC apphcation 0 background 10 20 Days Figure 7. Decline of fluorescence intensity of depilated skin from back of guinea pig.

330 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table V Effects of Keratolytic Agents on Disappearance Time (days) of Dansyl Chloride on Guinea Pig Stratum Corneum WSP SA 6% Sulphur 6% Resorcinol 6% Guinea pig no. (days) (days) (days) (days) 1 11.3 3.6 10.1 8.5 2 10.3 3.1 9.2 8.6 3 10.9 2.0 9.1 10.8 4 11.2 3.0 8.9 8.0 5 10.9 2.8 9.7 7.2 Mean + SD 11.0 + 0.2 2.9 + 0.6** 9.4 + 0.5** 8.6 + 1.3' *, p 0.05 **, p 0.01 compared with WSP (paired t-test). WSP: white soft paraffin. SA: salicylic acid. The mode of action of keratolytic agents on stratum corneum has remained obscure. Salicylic acid, sulphur, and resorcinol all accelerated the desquamation rate of the stratum corneum however, different actions are suggested for the three. Sulphur seems to enhance the rate of desquamation by accelerating cell proliferation in the epidermis, judging from the thickened epidermis. On the other hand, the effect of salicylic acid may be due to a direct action on the intercellular cement substance of the corneocytes because the epidermis was not thickened or irritated. These results indicate that a comparator and a fluorometer are very useful for measuring the turnover time of stratum corneum and for evaluating the effects of keratolytics or cosmetics. .. ! ............ :u'iphu r 6% ..... ..:' !...... iR 'sor:c..inO[ '6,/_' ...... ,.. Figure 8. Histological study of skin from back of guinea pig treated with keratolytic agents for 2 weeks H&E.