BARRIER FUNCTION AND HYDRATION 165 Finally, these studies also demonstrate the efficacy of an exogenous lipid mixture on skin moisturization, as assessed by capacitance. An optimal ratio of stratum corneum lipids increases water content in both damaged and normal human skin. Y2 contains 50% of polar lipids, which might retain more water and be superior to physiologic lipids or their mixtures in increasing skin capacitance. The absolute differences in skin hydration between human volunteers and mice are likely due to species differences. Ceramides and fatty acids alone, even in mixtures that disturb barrier recovery (8), will increase skin capacitance in damaged skin (15-18,24). Thus, stratum corneum hydration may be influenced by individual lipids, in contrast to the equimolar or optimized ratios required for barrier homeostasis. Further studies are needed to clarify the relationship of lipids for barrier recovery vs moisturization in this model. In any case, our results suggest that stratum corneum lipids might be useful not only, for skin barrier repair, but also as a skin moisturizer. This extract, and presumably other comparable, naturally occurring mix- tures, may be a cheaper, alternative source of physiological lipids for cosmetic products. ACKNOWLEDGMENT This work was supported by NIH grant AR19098. REFERENCES (1) P. M. Elias and K. R. Feingold, Lipids and the epidermal water barrier: Metabolism, regulation, and pathophysiology, Semin. Dermatol. 11, 176-182 (1992). (2) K. R. Feingold, The regulation and role of epidermal lipid synthesis, Adv. Lipid Res., 24, 57-79 (1991). (3) P.M. Elias, W. M. Holieran, G. K. Menon, R. Ghadially, M. L. Williams, and K. R. Feingold, Normal mechanism and pathophysiology of epidermal permeability barrier homeostasis, ½urr. Opin. Dermatol., 1, 231-237 (1993). (4) K. R. Feingold, M. Mao-Qiang, G. K. Menon, S. S. Cho, B. E. Brown, and P. M. Elias, Choles- terol synthesis is required for cutaneous barrier function in mice, J. ½lin. Invest. 86, 1738-1745 (1990). (5) W. M. Holieran, M. Mao-Qiang, W. N. Gao, G. K. Menon, P.M. Elias, and K. R. Feingold, Sphingolipids are required for mammalian barrier function: Inhibition of sphingolipid synthesis delays barrier recovery after acute perturbation, J. ½lin. Invest., 88, 1338-1345. (6) M. Mao-Qiang, P. M. Elias, and K. R. Feingold, Fatty acids are required for epidermal permeability barrier homeostasis, J. Clin. Invest., 92, 791-798 (1993). (7) G. K. Menon, K. R. Feingold, M. Mao-Qiang, M. Schaude, and P.M. Elias, Structural basis for the barrier abnormality following inhibition of HMG CoA reductase in murine epidermis, J. Invest. Dermatol., 98, 209-219 (1992). (8) M. Mao-Qiang, K. R. Feingold, and P. M. Elias, Exogenous lipids influence permeability barrier recovery in acetone-treated murine skin, Arch. Dermatol., 129, 728-738 (1993). (9) M. Mao-Qiang, B. E. Brown, S. Wu-Pong, K. R. Feingold, and P.M. Elias, Exogenous nonphys- iologic vs physiologic lipids: Divergent mechanisms for correction of permeability barrier dysfunction, Arch. Dermatol., 131, 809-816 (1995). (10) L. Yang, M. Mao-Qiang, M. Taljebini, P.M. Elias, and K. R. Feingold, Topical stratum corneum lipids accelerate barrier repair after tape stripping, solvent treatment and some but not all types of detergent treatment, Br. J. Dermatol., 133, 679-685 (1995). (11) M. W. Holieran, Y. Takagi, K. R. Feingold, G. K. Menon, G. Leglet, and P.M. Elias, Processing of epidermal glucosylceramides is required for optimal mammalian permeability barrier function, J. Clin. Invest., 91, 1656-1664 (1993). (12) G. K. Menon, K. R. Feingold, A. H. Moser, B. E. Brown, and P.M. Elias, De novo sterologenesis in the skin. II. Regulation by cutaneous barrier requirements, J. Lipid Res., 26, 418-427 (1985).

166 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (13) M. Mao-Qiang, K. R. Feingold, C. R. Thornfeldt, and P.M. Elias, Optimization of physiological lipid mixtures for barrier repair, J. Invest. Dermatol., 106(5), 1096-1101 (1996). (14) R. Ghadially, B. E. Brown, S. M. Sequeira-Martin, K. R. Feingold, and P. M. Elias, The aged epidermal permeability barrier. Structural, functional, and lipid biochemical abnormalities in humans and a senescent murine model, J. Clin. Invest., 95(5), 2281-2290 (1995). (15) G. Imokawa and M. Hattori, A possible function of structural lipids in the water-holding properties of the stratum corneum, J. Invest. Dermatol., 84, 282-284 (1985). (16) G. Imokawa, S. Akasaki, M. Hattori, and N. Yoshizuka, Selective recovery of deranged water- holding properties by stratum corneum lipids, J. Invest. Dermatol., 87, 758-761 (1986). (17) G. Imokawa, S. Akasaki, Y. Minematsu, and M. Yawai, Importance of intercellular lipids in water- retention properties of the stratum corneum: Induction and recovery study of surfactant dry skin, Arch. Dermatol. Res., 281, 45-51 (1989). (18) G. Imokawa H. Kuno, and M. Kawai, Stratum corneum lipids serve as a bound-water modulator, J. Invest. DermatoL, 96, 845-851 (1991). (19) N. Marsh, P. M. Elias, and W. M. Holieran, Glucosylceramides stimulate murine epidermal hyper- proliferation, J. Clin. Invest., 95, 2903-2909 (1995). (20) P. Bowser and M. Gray, Sphingomyelinase in pig and human epidermis, J. Invest. Dermatol., 70, 331-335 (1978). (21) R. Freinkel and T. Traczyk, The phospholipases A of epidermis, J. Invest. Dermatol., 74, 169-173 (1980). (22) R. Freinkel and T. Traczyk, Acid hydrolases of the epidermis: Subcellular localization and relationship to cornification, J. Invest. Dermatol., 780, 441-446 (1983). (23) S. Grayson, A. G. Johnson-Winegar, B. U. Wintroub, et al., Lamellar body-enriched fractions from neonatal mice: Preparative techniques and partial characterization, J. Invest. Der matol., 85, 289-294 (1985). (24) H. Tagami and K. Yoshikuni, Interrelationship between water-barrier and reservoir functions of pathologic stratum corneum, Arch. Dermatol., 121, 642-645 (1985).