14 4 2 ◊ N □ N 0 • 1::t,. □ a -2 • -4 JOURNAL OF COSMETIC SCIENCE a .A. .A. � � ·2 a 0 1:,. 1::t,. □ -2 0 Zl ■ 1::t,. ◊ X .A. ■ a c:i 0 x• 0 oo-00a □ 0 2 4 Figure 9. Two-dimensional projection of 44 Japanese females by the first two components (Zl and Z2) of PCA for the levels of eight lipids. Each point represents the values of Zl and Z2 for each individual. Symbols: 0: ages 1 to 10. D: ages 11 to 20. 6: ages 21 to 30. e: ages 31 to 40. ■: ages 41 ro 50 . .&: ages 51 to 60. T: ages 61 to 70. x: ages 71 to 81. sebaceous glands. Thus, the lipids in group A may originate from sebum penetrating into the inside of hair fibers within hair follicles where sebaceous glands are included histologically. Koch et al. (15) designated hair lipids within hair fibers as "internal sebum" in their paper. Although some FAs may originate from hair matrix cells, the existence of higher levels of unsaturated and branched FAs in hair lipids (7 ,18,28) suggests that most FAs result from decomposition of TGs or WEs as sebum. On the other hand, lipids in group B (CH and CERs) are thought to result from intrinsic constitutive lipids biosynthesized in hair matrix cells, based upon the fact that they are not synthesized in sebaceous glands (28). HCs (classified into group C) consisting of carbon number 22-44 has been reported in the analysis of hair lipids (29), but their origin remains unclear. It has been proposed by a study using Maple Syrup Urine Disease patients (6) that MEA (classified into group D) may result from an intermediary me­ tabolite of isoleucine within hair matrix cells. Therefore, according to their origins, group A can be designated as exogenous lipids and groups B and D as endogenous lipids. Changes in hair lipid composition with special reference to exogenous lipids with increasing age (Figure 9) could be explained in terms of increased sebum secretion during adolescence (30). The level of MEA did not correlate with both lipids in group B (CH and CERs), although these three lipids are endogenous. This suggests that the production of either CH or CERs may interact with each other during the biosynthetic process, while MEA may be regulated by another metabolism. The reason that there was a negative correlation between each level of exogenous lipids (group A) and endogenous ones (group B) might be due to the contribution of the CMC. Since endogenous lipids in group B are the main components of cell membrane lipids that form CMC (4,17), they can lead to the formation of stable and strong CMC. Therefore, stable and strong CMC may result in enhancing the barrier function (31,32),

HAIR LIPID COMPOSITION 15 which is attributable to preventing external materials from entering into hair fibers as is the case in skin. Consequently, higher levels of endogenous lipids in group B may result in diminished levels of exogenous ones due to their difficulty in penetration, while lower levels of endogenous lipids in group B may result in increased exogenous ones in hair fibers. Available studies dealing with lipids within human hair have hitherto used the term "internal lipids" for lipids within hair fibers (6, 7, 11-20). This term for lipids penetrat­ ing from a sebaceous gland into hair fibers may be not misdirected because these lipids are internalized. However, based upon the fact that they are not intrinsic internal lipids, they should be designated as exogenous lipids (group A), as revealed in this study. On the other hand, endogenous lipids consisting of CH and CERs (group B) and MEA (group D) should be designated as intrinsic internal lipids of human hair. While some F As originate from hair matrix cells, most F As may occur as a result of the decompo­ sition of TGs or WEs as sebum components. This is corroborated by the fact that there are higher levels of unsaturated and branched FAs in hair lipids (7,18,28) and that there is the positive correlation between FAs and the other exogenous lipids. Therefore, it is likely that a lipid class of FAs within hair fibers occurs mainly as exogenous lipids. CONCLUSION In summary, the lipid composition at the proximal root regions of human hair has been characterized for the first time. The hair lipids can be classified into four groups: group A (SQ, WEs, TGs, and FAs) group B (CH and CERs) group C (HCs) and group D (MEA). Since group A or groups B and D originate from sebum penetrating the hair shaft or from constitutive hair lipids biosynthesized in hair matrix cells, they were designated exogenous lipids or endogenous ones, respectively. The hair lipid composi­ tion among individuals was also characterized by a predominant negative correlation between lipids for groups A and B. This negative correlation suggests that the endog­ enous lipids in group B serve as a barrier against the penetration of exogenous lipids (group A). Although many researchers have hitherto used the term "internal lipids" for all hair lipids including exogenous lipids (group A in our study), only endogenous lipids consisting of CH and CERs (group B) and MEA (group D) should be designated as intrinsic internal lipids of human hair. ACKNOWLEDGMENTS We express our cordial gratitude to Dr. Yoshiaki Fujikura and Dr. Katsumi Kita for their discussions and encouragement of this study. Our sincere thanks are also due to Mr. Toshihiko Sakai and Ms. Kumi Sugino for their assistance in hair sample collections. REFERENCES (1) J. A. Swift and A. W. Holmes, Degradation of human hair by papain. Part 2: Some electron micro­ scope observations, Text. Res.]., 35, 1014-1019 (1965). (2) S. Naito, T. Takahashi, M. Hattori, and K. Arai, Histochemical observation of the cell membrane complex of hair, Sen-I Gakkaishi, 48, 420--426 (1992). (3) J. D. Leeder, The cell membrane complex and its influence on the properties of the wool fibre, Wool Sci. Rev., 63, 3-35 (1986). (4) A. Korner, S. Petrovic, and H. Hocker, Cell membrane lipids of wool and human form liposomes, Text. Res.]., 65, 56-58 (1995).