DSC STUDIES OF SEBUM MODELS 223 Mp-4 due to saturated fatty acids plus triglycerides (DSC could not always differ- entiate these peaks). At skin temperature, these components are solid. 4. As the amount of saturated species increases, more of the solid phase is present in sebum relative to the liquid phase. Changes made to the constituents of sebum-carbon chain length, saturation, and re- placement of triglycerides with fatty acids, as happens in acne, affect Mp-3 and Mp-4 in the following way (these are the most relevant transitions to skin): 1. Increasing carbon chain length raises the Mp-3 and Mp-4 transition temperatures quite dramatically, i.e., 25 to 30 degrees above skin temperature, and so there is little chance of liquification of these solids at skin temperature. 2. Increasing the amount of saturated constituents raises the Mp-3 and Mp-4 transition temperature 10 to 15 degrees, and so there is little chance of these components melting these at skin temperature. 3. Increasing the proportion of triglycerides increases the Mp-4 transition temperature, which would be expected. The most important finding from this research is that sebum exists in multiple phases- liquids and solids--and that the degree of saturation controls the relative amounts of these phases. Whether the presence of an excessive amount of the solid phase of sebum in the follicle plays a role in the pathogenesis of ache still remains to be investigated. The literature reports indicate that comedonal plugs are enriched saturated species (10). Additionally, it is also known that lipids from skin of ache patients are deficient in polyunsaturated fatty acids such as linoleic and sebaleic acids (14). Both of these reports support the possibility that the sebum of ache patients is out of balance with regard to the relative amount of the solid versus the liquid phases. The presence of excessive amounts of the solid phase would render it difficult for the liquid phase to dissolve the solid phase, and could plausibly lead to altered sebum flow and blockage of the pilo- sebaceous duct as is known to occur in ache patients. In any case, our results suggest that there is a solid component in sebum at skin temperatures irrespective of its composition. REFERENCES (1) J. j. Leyden, New understandings of the pathogenesis ofacne,J. Am. Acad. Dermatol., S15-S25 (1995). (2) A. Jarrett, "Sebaceous Glands," in The Physiology and Pathophysiology of the Skin, A. Jarrett, D.C. Sinclair, and T.J. Ryan, Eds. (Academic Press, London, New York, 1986), Vol. 9. (3) W.J. Cunliffe and S. Shuster, Pathogenesis of ache, Lancet, 1, 685-687 (1969). (4) E.O. Butcher and A. Coonin, The physical properties of human sebum, J. Invest. Dermatol., 12, 249-254 (1949). (5) J. L. Burton, The physical properties of sebum in acne vulgaris, Clinical Science, 39, 757-767 (1970). (6) G. Miescher and A. Schoenberg, Unterchungen uber die funktion der talgdrusen, Bulletin der Schweiz- erischen Akademie der medizinischen Wissenschaften, 1, 101-112 (1944). (7) J. S. Strauss, P. E. Pochi and D. T. Downing, The sebaceous glands: Twenty five years of progress,J. Invest. Dermatol., 67, 90-97 (1976). (8) D.T. Downing, Variability of the chemical composition of human skin surface lipids, J. Invest. Dermatol., 53, 322-327 (1969). (9) N. Nicolaides and M. N. Ansari, The dienoic fatty acids of human surface lipid, Lipids, 4, 79-81 (1969). (10) N. Nicolaides, M. N. Ansari, H. C. Fu, and D. G. Lindsay, Lipid composition of coinedones compared with that of human skin surface in acne patients,J. Invest. Dermatol. 54, 487-495 (1970).

224 JOURNAL OF COSMETIC SCIENCE (11) (12) (13) (14) (15) K. M. Nordstrom, J. N. Labows, K.J. McGinley, and J.J. Leyden, Characterization of wax esters, triglycerides, and free fatty acids of follicular casts,J. Invest. DermatoL, 86, 700-705 (1986). A. Naik and R. H. Guy, "Infrared Spectroscopic and Differential Scanning Calorimetric Investigations of Stratum Comeurn Barrier Function," in Mechanisms of Transderma/Drug Delivery, R. O. Potts and R. H. Guy, Eds. (Marcel Dekker, New York, 1997), Volume 83, pp. 87-162. F. Faneka, K. Yamazaki, and M. Kobayashi, Vibrational spectroscopic study on polymorphism of erucic acid palmitoleic acid •y•-oq and •y-o• reversible solid state phase transitions, Spectrochimica Acta, 5A(8/9), 1589-1603 (1994). D. T. Downing, M. E. Stewart, P. W. Wertz, and J. S. Strauss, Essential fatty acids and acne, J. Am. Acad. DermatoL, 14, 221-225 (1986). P. Bore and N. Goetz, A physical method for qualitative examination of human sebum,J. Soc. Cosmet. Chem., 28, 317-328 (1977).