222 JOURNAL OF COSMETIC SCIENCE bination peak of the unsaturated fatty acid and the unsaturated triglyceride, this result is difficult to resolve. CONCLUSIONS We conclude that under the experimental conditions explained earlier, the unsaturated lipids help dissolve the saturated lipids. As the proportion of the saturated, solid com- ponents (associated with the Mp-3 and Mp-4 peak) increase, it is more unlikely that the liquid components will be able to dissolve the solid under natural conditions of skin. Hence if P. a•cnes preferentially changes the relative percent of the longer chain, more saturated constituents, producing more solids, these will not be dissolved by the skin's natural liquid oils and may plug the pilosebaceous ducts. Some evidence suggests that comedonal material is relatively more saturated while sebum collected from the skin surface is more unsaturated (10). Additionally, it is well published that there is a deficiency in polyunsaturated fatty acids such as linoleic and sebaleic acids in acne (14). Converting triglycerides to free fatty acids by P, a•cnes may not have the largest conse- quences as far as the physical properties of sebum are concerned, since the saturated forms of both are still solids above skin temperature and the unsaturated species of both are still liquids below skin temperature. However if relatively more of the saturated fatty acid species are produced by bacterial hydrolysis, the balance between the liquid and solid phase will be altered. Actual sebum may lie somewhere in between all the model sebum that we have inves- tigated, and a portion of it may be solid at 32øC. Furthermore, the presence of the solid portion depends in a large part of the presence of the unsaturated or liquid portion of the sebum. The fact that sebum may exist in different phases has also been suggested by Burton (5). Butcher and Coonin (4) showed that while some components of forehead sebum started to solidify at 30øC, it completely solidified at 15ø-17øC. These studies are in agreement with our observations that sebum does not exist as one phase, but rather as a mixture of a solid and a liquid at skin temperature. DSC of scalp sebum has been performed by Bore and Goetz (15). Their results also confirm the presence of multiple phases in sebum. Their samples were also mixtures of solids and liquids at body tem- perature. They showed that as the percentage of the unsaturated portion increased, the viscosity of the sample decreased. We showed that as the unsaturated portion increased, the saturated portion's melting temperature decreased, which in turn may contribute to the decreased viscosity. SUMMARY The present research showed that: 1. Sebum is not one phase but may exist in multiple phases at skin temperature (32øC). 2. Two transitions are at very low temperatures, Mp-1 and Mp-2. These are attributed to the unsaturated portion of sebum, generally, Mp- 1 to the unsaturated triglycerides and fatty acids and Mp-2 to the unsaturated wax ester. At skin temperature, these compounds are liquid. 3. Two transitions occur at higher temperatures, Mp-3 due to saturated wax esters and

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).